Passive Resonant Cavity Technical Surveillance Devices


Excerpt from the National Security Agency Museum Website

On August 4, 1945, Soviet school children gave a carving of the Great Seal of the United States to U.S. Ambassador Averell Harriman.  It hung in the ambassador's Moscow residential office until 1952 when the State Department discovered that it was 'bugged.'

The microphone hidden inside was passive and only activated when the Soviets wanted it to be.  They shot radio waves from a van parked outside into the ambassador's office and could then detect the changes of the microphone's diaphragm inside the resonant cavity.  When Soviets turned off the radio waves it was virtually impossible to detect the hidden 'bug.'  The Soviets were able to eavesdrop on the U.S. ambassador's conversations for six years.

The replica on display in the museum was molded from the original after it came to NSA for testing.  The exhibit can be opened to reveal a copy of the microphone and the resonant cavity inside.

Excerpt from Drawings and Photographs: Russian Resonant Cavity Microphone by the FBI Laboratory

This folder contains a scale drawing of the Russian resonant cavity microphone followed by photographs of the assembled unit and the unit taken apart.

There are also contained herein pictures of the wooden Great Seal of the United States in which the cavity microphone was concealed.  These pictures show the front of the seal and a close-up of the area where small pin holes permitted sounds to reach the microphone diaphragm.  These are followed by pictures of the interior hollowed-out portions which are exposed by removing the wooden back.  The resonant cavity microphone was concealed in the front carved portion of the seal, but as the photographs show, there is also a larger hollow.  The back portion of the seal contains a far more extensive hollow.  These additional hollows may have contained a previous device.


On a number of occasions over a period of more than a year, voices of United States' and British Embassy officials in Moscow have been heard by fellow employees of the Embassy staffs on high frequency radio receivers - indicating the presence of a clandestine listening device.  On September 11, 1952, the U.S. State Department advised that a technician assigned to the U.S. Embassy in Moscow heard the voice of the Ambassador on a receiver and after searching, located a new type microphone device concealed in a hollowed-out space within the wooden Great Seal of the United States in the library of the Ambassador's residence, Spaso House.  The device was removed and flown to Washington where it was shown to the President by the Secretary of State.  On the instructions of the President that immediate steps be taken to explore this device and develop suitable countermeasure equipment, the microphone was turned over to this Bureau to coordinate the technical study and develop countermeasure equipment.  The device was received on September 15, 1952, from Deputy Assistant Secretary of State Walter K. Scott, Mr. John W. Ford, Chief, Division of Security, and Mr. Fred C. Snider of the Security Division of the Department of State.  The wooden seal in which the microphone device was concealed was received in the Laboratory of this Bureau on November 12, 1952, from Mr. Robert C. Ecker, Division of Security, Department of State.  A scale drawing of this microphone device together with photographs of both the device and the seal are attached to this report.

Laboratory Examination

An examination was immediately started in this Laboratory.  A visual examination of its detail indicated that it probably functioned as a high frequency resonant cavity.  It therefore required no visible source of power, but would be energized by a remote transmitter.  The device would rebroadcast a signal to be received at a remote point and the rebroadcast signal would be modulated by sounds within the area wherein the device was located.  With equipment that was immediately available, the instrument was made operative and found to be resonant at a frequency of approximately 1700 megacycles.  Since the modulation is produced by a condenser-type microphone, the audio response was found to be excellent.

The operating frequency of 1700 megacycles within the general range of low radar frequencies and it became immediately obvious that extensive facilities and experience in this particular field would be necessary to design and construct the needed countermeasure equipment.  Assistance was therefore requested of the Naval Research Laboratory where an excellent and most exhaustive project has been completed.  The comprehensive report of the Naval Research Laboratory will be referred to hereinafter.

A physical examination of the microphone in the FBI Laboratory showed that the unit is approximately one inch in diameter and 3/4 inch long having extended from its circumference an antenna approximately nine inches in length.  The exact dimensions are set forth in the schematic drawing which is contained in Attachment #1 with accompanying photographs.  The entire unit has a weight of 1.1 ounces (31.386 grams).  It consists of a hollow cylinder machined from copper, the interior of which is polished and silver plated, the exterior being silver plated without polishing.  The silver plating is 0.0015 inches in thickness.  One end of this cylinder has exterior threads to receive a ring composed of bronze over which is fastened a diaphragm of silver plated metal foil.  Prior to the exploration of this device it was necessary to replace the original diaphragm which had become damaged.  The replacement diaphragm is of nickel 0.00025 inches thick, which has been silver plated on the inside.  By screwing the diaphragm ring on the cylinder, the foil can be stretched to ensure a flat diaphragm surface.  The end of the cylinder opposite the diaphragm has interior threads into which there is screwed a plate machined from solid copper and silver plated.  In the center of the interior surface of this plate there is machined a post on the end of which is a disk.  These are integral parts of the plate and all are silver plated.  Screwing in the plate adjusts not only the size of the cavity, but positions the disc close to the interior of the diaphragm to form the condenser microphone.  Two holes in the plate serve as a spanner wrench adjustment as well as an air escape to avoid compression within the cylinder and there is also a small hole drilled through the center of the disc, post and plate as will been seen in the photographs.  The surface of the disc is grooved.  The cylinder proper is completed with a slip-fit cap on each end which is composed of brass and silver plated.  The cap on the plate end is solid, whereas the cap over the diaphragm serves as a protective grill.  A hole through the circumference of the cylinder has a polystyrene plug to insulate the antenna from the cylinder.  The antenna is a silver plated brass rod projecting into the interior of the cylinder and threaded through to the polystyrene plug.  The antenna rod projecting into the cylinder has a brass plate attached.  It is interesting to note "N 11" is scratched into the circumference of the cylinder.

Examination of the Great Seal itself showed the Seal to be composed of two sections, front and back, both of hard maple.  Hard maple is available in the United States and most foreign countries having cool climate.  The exterior surface of the front portion of the Seal on which is carved the Great Seal of the United States is coated with a transparent finish similar to shellac.  Several wooden dowels, 3/8 inch in diameter, were located at various places in the Seal which were necessary to its construction.  One of these dowels was examined and found to be birch which is a wood widely used for dowels.  In its present condition the Seal has a diameter of approximately 22 inches and is approximately 3-3/4 inches thick.

The microphone device was found to have been placed in a circular-shaped hollow in the rear of the front section with another groove cut into the wood to accommodate its antenna.  A piece of plywood and cotton were used to secure the device on the sides, and a thick piece of walnut veneer was glued over it, thereby separating the device from the cavity in the rear section of the Seal.

The microphone was positioned in the Seal so that the diaphragm was located in the area of the depression under the eagle's bill.  The antenna extended downward.  Only a very thin layer of wood covered the diaphragm of the microphone and this thin layer of wood was penetrated by numerous pin holes which would normally escape detection, but which are emphasized in the photograph by shining a light through these holes from behind the Seal.

Into the rear plate of the Seal, which is composed of three boards, 1-5/16 inches thick, glued side by side, a hollow as been drilled and chiseled.  This hollow is generally rectangular in cross section, approximately 2 inches wide, 16-1/4 inches long and 1-1/8 inches deep.  The long dimension is vertical which the Seal is oriented in the proper position for mounting on a wall.  An extension of this recess projects sideways from the approximate center of the rectangular hollow.  This extension is roughly circular in shape, 3-3/8 inches in diameter, and the same depth as the main recess.  The rear surface of the front section of the Seal has been cut out to correspond with the 3-3/8 inches circular recess in the back.  However, only a shallow groove about one inch wide and one-quarter inch deep has been chiseled out to correspond to the 2 x 16-1/4 inches recess in the back.

Due to the absence of discernible discolorations of the surfaces of the wood of the Seal and of the recesses, the relative times in which the recesses were cut could not be determined.

The large recess in the Seal has the appearance of the work of a professional wood worker and would even then have required several hours to complete.  The smaller recess cut into the Seal for the antenna of the device is a crude, unfinished groove.  Cutting this groove and taking the steps necessary to install the device (cutting, gluing, and pegging in the plywood piece and gluing the walnut veneer) could have been accomplished in less than two hours.  To this time, however, must be added the time necessary to remove and replace the trim from around the Seal, separate and re-glue the back to the front, and finish the edge of the Seal to conceal the tampering marks.

Inset into the back section of the Seal were two nuts, apparently placed therein to secure two bolts from which the Seal was hung.  Examination of these nuts showed that they were 5/16 inch nuts having 20 "V" type threads per inch.  The physical dimensions of the nuts are similar to those of nuts manufactured in the United States.

With regard to the larger recess cut in the Great Seal, it is noted that this recess is considerably more extensive than required for the resonant cavity device which was actually found.  Accordingly, it appears probable that this larger recess has contained earlier microphone devices, either of a battery power radio transmitter type, or possibly of an earlier resonant cavity type similar to the present one, but operating on a lower frequency and therefore larger in physical size.  The geometry of the larger recess suggests the possibility of a radio antenna with the microphone device located in the center of the antenna rather than being located at the end of the antenna as is the case with the device which was actually found.

Countermeasure Equipment

The immediate, initial steps taken by the Naval Research Laboratory were directed toward producing one complete set of detection and jamming equipment for use by the State Department.  This objective was completed on October 3, 1952.  Subsequent work was then directed toward refining the design of the first equipment, exploring more fully the theoretical aspects of the microphone device, and checking such theory by experimental verification.  The detailed results incorporated in Naval Research Laboratory Report, #4087.

Briefly, it is noted that the resonant cavity microphone device is designed to be activated by a beam of radio waves directed toward it.  No batteries, wires, or other similar accessories are required at the microphone location..  Upon striking the microphone device, a portion of the impinging radio energy may be thought of as being continuously absorbed and then re-transmitted by the device; however, upon re-transmission, the radio energy sent out from the microphone device is modulated by, and thus carries with it any sounds which were present at the microphone location..  These sounds, therefore, may be recovered by receiving the re-transmitted signals on suitable radio receiving equipment, which may be located either in the immediate vicinity of the microphone device or which may be located at a distance.

Accordingly, the complete detection equipment requires:

  • A source of radio signals to illuminate the area under search with radio waves, thereby energizing any concealed devices of the cavity type.  (This source must be capable of being varied between the limits considered probable for the cavity microphone technique, in the present instance considered to be from approximately 65 to 3000 megacycles.
  • A source of sound within the area being searched to provide the necessary sound excitation of the microphone.
  • Suitable radio receiving equipment to detect the radio signal re-radiated, or scattered, by the microphone device.

When attempting to detect the presence of such microphone device without regard to whether the enemy may become aware of such detection processes, the complete detection equipment as described above would be used.  If, however, it were desired to detect the presence of such device without enemy knowledge of such detection, the source of sound (Item #2 above) would have to consist of conversation or other sounds normal to the area under search, and the source of radio waves (Item #1 above) would have to be that normally used by the enemy while operating the device for their own listening purposes, since any other source of radio waves could be subject to detection by the enemy.  Accordingly, only Item #3 of the detection equipment, namely, the receiver., would be actively used by the searchers under such conditions.

Procurement of Countermeasure Equipment

The critical element of the above detection equipment is the receiver (Item #3) and such a receiver has been designed in complete detail by the Naval Research Laboratory and fully described in Naval Research Laboratory Report, #4087.  A suitable sound source (Item #2) consisting of a simple tone generator, likewise has been incorporated in the cabinet containing a portion of the receiver equipment.  The information contained in Naval Research Laboratory Report, #4087, is set forth in sufficient detail to serve as the basis for construction or procurement of additional similar units.  The remaining element, namely, the source of radio signals, (Item #1) may be obtained through adaptation of commercially available signal generators or related equipment, or preferably, through the procurement of signal generators designed specifically for the present purpose.  In the initial detection equipment furnished to the State Department, the source of radio signals consisted of three separate components as follows:

  • A General Radio Type 1208A oscillator covering the range 65 to 500 megacycles.
  • A General Radio Type 1209A oscillator covering the range 250 to 920 megacycles.
  • An oscillator portion of a military radio receiver, type R-111B/APR-5A, providing signal over the range 1000 to 3100 megacycles.  (Although not immediately available for the initial equipment furnished to the State Department, it is noted that signal generators covering this general range are available from commercial sources.)

In the initial planning of this project the possibility was considered that it would be necessary to design and construct special transmitters for the illumination of devices of this type.  However, since suitable oscillators were found for this purpose and types other than those supplied to the State Department are commercially available, the time and expense to construct such transmitters was not considered justified.

Minimum construction or procurement costs of the receiver group equipment have been estimated to be approximately $1100 each.  Commercially available signal generators (3 General Radio units) covering the range 200 to 2000 megacycles are estimated to cost approximately $900 for the set of three.  These figures represent minimum costs and could easily double or even triple on urgent time procurement schedules.

It is understood that quantity procurement of the complete detection equipment is under study by a special committee.


With reference to the distance possible between the microphone device and the point of enemy listening, the maximum range is considered theoretically on pages 25 through 28 of Naval Research Laboratory Report, #4087.  As representative, the following specific examples are cited from that report:

For activating transmitter power of 1 watt, and assuming operation in free space (no walls or other obstruction whatever in near vicinity) with top quality receiving equipment, the figure of 591 feet is obtained for the range between the microphone device and the listening post (both the activating transmitter and the receiver assumed to be located at the receiving point).  For a transmitting power of 900 watts using high-gain antennas and again assuming optimum conditions, the figure of 15 miles range is obtained.

These ranges of course, are decreased very rapidly by the presence of intervening absorbing material such as wall structure, buildings, etc.  Low power tests using transmitter powers in the order of 4 watts or less are considered to be consistent with the theoretical development, when considered in light of equipment available and the presence of absorbing materials in the vicinity of tests conducted.  For example, using a power estimated to be approximately two watts, and a receiver known to be substantially less sensitive than the optimum considered in the theoretical development, an experimental operating range of approximately 100 feet was obtained in the vicinity of buildings, but with no walls directly intervening between the listening point and microphone device.  The presence around the microphone device of even the wood contained in the Great Seal was found experimentally to reduce the range very greatly.

Countermeasure Limitations

It is desired to here emphasize that the countermeasure equipment designed and constructed at the Naval Research Laboratory is for the specific purpose of detecting and locating the particular type of listening device submitted for study.  Effective as the resonant cavity microphone may be, it is but one of many possible listening devices.  Even the successful use of the countermeasure equipment in locating other similar microphones should not be accepted as assurance of security.  Some consideration might be given to protective security measures as a means of countering this and other types of listening devices.  It would be expected that any such measures would involve some expense and inconvenience.  One suggestion offered involves the use of a portable or demountable enclosure which could be quickly erected within a room.  Light weight acoustic wall tile or even a tent of heavy cloth would afford considerable protection against sounds within the enclosure reaching a microphone outside.  Additional protection would be obtained by incorporating a metal foil or screen to shield against radio frequency radiation from within the enclosure.  Another approach could involve small light weight headsets and microphones which could be interconnected and would permit two or more persons to engage in a discussion without producing the volume of sound of normal conversations.

The whole problem illustrates the difficulty of any attempt to counter any listening devices present without knowing the devices against which we are seeking protection.


The resonant cavity microphone has been found to give excellent audio response, but to have inherent disadvantages with respect to its location and operation.  It has the distinct advantage that no visible source of power is necessary for its operation.  It, of course, does not originate a signal but when excited by a remote transmitter, a modulated signal is rebroadcast which can be received at a remote point at the same or a different place from the energizing transmitter.

Interim countermeasures equipment consisting of transmitters and a receiver especially designed and built at the Naval Research Laboratory was turned over to the State Department on October 3, 1952, in an effort to permit the energizing and locating of any additional devices of this type.

Technical study continued and has resulted in the final detecting and receiving equipment built by the Naval Research Laboratory which was turned over to the State Department on November 19, 1952, and in the exhaustive Naval Research Report, #4087.

This equipment is designed for the specific purpose of detecting and locating listening equipment of the general type under study and is not to be considered as assurance of security against other types of microphones or listening devices.

The complete report consists of this report and two attachments:

  • A drawing and photographs of the resonant cavity microphone and the Seal in which it was concealed.
  • The complete and final report of the Naval Research Laboratory, #4087, "Final Report - Semi-active Listening Device."


Excerpt from Report on Research on EASYCHAIR, by the Dutch Radar Laboratory (NRP)

Technical Description

The passive modulator may be regarded in a broad sense, as a device which provides the following four functions:

  • Response to unmodulated RF energy.
  • Response to audio energy.
  • Modulation of the RF energy by the audio energy.
  • Radiation of the modulated RF energy.

When these devices are excited by RF energy at their resonant frequency, they radiate this energy in a manner similar to radar echo boxes and parasitic antenna elements.  The energy radiated by the passive modulator is amplitude modulated by audio energy impinging on the device.

The contemporary type of passive modulator, shown diagrammatically in Figure 1, consists of a resonant cavity of the re-entrant type.  The resonant frequency of the cavity is varied by the audio energy impinging on the unit.  When used with a fixed frequency transmitter, this results in a variation of the reactive loading on the antenna, with consequent changes in the magnitude of the reflected wave.

RF energy is coupled into the cavity by means of an antenna rod, one end projecting into the cavity, and the other end extending a half wave length beyond the outside surface of the cavity.  The cavity end of the rod is terminated in a capacitive probe which is tightly coupled to the mid-section of the re-entrant rod.

To determine the resonant frequency of a cavity of the re-entrant type, it is permissible to draw an analogy between the re-entrant cavity and a simple LC circuit.  The C is formed by the backing plate and the diaphragm, and is of such magnitude that the overall size of the cavity is small compared to the wavelength.  The equivalent lumped inductance may be calculated from the formula for inductance of a coaxial system:

The above relationships neglect the effect of the reactance reflected into the cavity by the tightly coupled antenna.

When the resonant frequency of the cavity and antenna system is known, a closer approximation of the equivalent lumped inductance is found by solving eq. (2) for L.

Measurement Standards

In order to evaluate the relative merit of passive modulator designs developed during the course of this project, a comparative measurement system was established.

For the purpose of studying passive modulators designed to operate in the frequency range of 1000 megacycles, the microwave signal generator, frequency range 1000 to 1300 megacycles, was used as the fixed power level source of RF at the selected frequency.  Audio excitation of the passive modulator at a fixed sound pressure level, was provided by a loudspeaker fixed to the jig in which the passive modulator was mounted.  A calibrated high gain receiver was used to detect the amplitude modulated RF energy radiated by the passive modulator.  A Ballantine AC vacuum tube voltmeter monitored the audio output level of this receiver.  A block diagram of the set-up is shown in Figure 2.

Experimental Results

A detailed study of the performance and characteristics of the contemporary passive modulator was made.  Physical and electrical characteristics were determined with a view to optimizing the present design.  In this design, a conducting diaphragm is used as one plate of a variable capacitor which varies the resonant frequency of the passive modulator cavity at the impinging audio rate.  Mechanically, this operation is analogous to the functioning of a condenser microphone.  The contemporary passive modulator design suffers from a lack of audio sensitivity, as does a conventional condenser microphone.  The displacement of the diaphragm, in both cases, due to the pressures exerted by the sound field, results in extremely minute variations in capacitance.  In the case of the contemporary design, the low audio sensitivity results in a very low percentage modulation of the radiated RF energy.  The present state of the art of condenser microphone design, however, indicates improvements which can be incorporated in the design of this type of passive modulator.

Measurements have been made of the sound pressure level, 8 to 10 feet distant from two persons speaking in normal, conversational level.  This sound pressure averaged 75 dB above the standard reference level of 0.0002 dynes per square centimeter, for a total of 3.95 milligrams average force acting over the 3.625 square centimeters of the passive modulator diaphragm area.

To enable a direct measurement of the rest capacitance of the passive modulator cavity to be made, the construction of one of the contemporary cavities was modified.  An annular sector of the re-entrant wall was removed, and replaced with a sector made of polystyrene.  In this manner, the re-entrant portion of the cavity was insulated from the rest of the cavity.  The spacing between the backing plate and the diaphragm measured before the alteration was 0.001 inches.  The altered cavity was readjusted for this spacing and a capacity of 10 picofarads was measured, using a Boonton Radio Corp. "Q" meter.  Calculation of the capacitance, using the areas of the plates and their spacing, and neglecting fringe effects, resulted in a value of 12.2 picofarads.

The displacement of the passive modulator diaphragm due to the force exerted by the sound field, results in a capacitance change in the order of .01 picofarads.  Since the change in capacitance varies the resonant frequency of the passive modulator cavity at the audio rate, the .01 picofarad capacitance change represents a change in resonant frequency of approximately 0.8 megacycles.  The "Q" of the passive modulator should be of the order of several thousand, in order to obtain an effective percentage modulation from this small change in resonant frequency.  Preliminary measurements have been made to determine the order of magnitude of "Q" for the contemporary passive modulator design.  An experimental measurement of passive modulator "Q" would best be made by measuring the response of the passive modulator as the frequency of the exciting RF energy was varied.  The passive modulator would remain tuned to a fixed frequency and the exciting RF would be audio modulated to enable measurement at the receiver.  Measurement by this method could not be made, however, since the audio modulated RF radiated by the passive modulator could not be distinguished from the audio modulated RF received directly from the transmitting unit.  The order of magnitude of passive modulator "Q" was determined by means of the experiments discussed below.

Figure 3 is a plot of relative receiver output against separation of diaphragm and the backing plate of the passive modulator.  The microwave signal generator, operating at a fixed frequency of 1002 megacycles, fed ten watts of RF power to a ten turn helical antenna.  The antenna was directed to activate the passive modulator spaced 15 feet distant.  A ten turn helical receiving antenna directed at the passive modulator, fed a Thompson Products Co. wavemeter used as a preselector and crystal detector for a high gain audio amplifier.  A Ballantine AC vacuum tube voltmeter served to measure the audio output of this receiver.  A loudspeaker operating at a fixed power level at a frequency of 2500 cps, supplied audio excitation of the passive modulator.  This unit was placed in the jig used to support the passive modulator.  The separation of diaphragm and backing plate, initially 0.00485 inches, was increased in 0.00008 inch increments, noting the receiver output for each increment, until receiver output had varied from noise level, through a peak and back to noise level.

The 0.00008 inch increments were determined by dividing the circumference adjustable rear section into 64 parts.  The adjustable rear section has 48 threads per inch.  Each division on the rear section then represents an advance of 0.000325 inches.  Advance of the threaded section was always in the same direction to minimize backlash.  By estimating the thread advance to one-fourth of a division, it was possible to measure an advance of 0.00008 inches.  This curve, as a first order approximation, can be viewed as the response of a passive modulator set at some resonant frequency, not audio excited, plotted against the frequency of incident unmodulated RF energy.  Figure 4 is a plot of relative audio output of the receiver against transmitter frequency.  During this experiment the passive modulator remained at a fixed setting.  The transmitter, passive modulator, and receiver set-up was identical to that used in the previous experiment, with the exception that the transmitter frequency was increased incrementally, starting with a frequency below cavity resonance, through resonance to a value above cavity resonance.

From the results shown graphically in Figure 4, the order of magnitude of passive modulator "Q" may be determined.  Using the relationship:

and taking the bandwidth to be the frequency separation of the two peaks on the curve, 20 megacycles, and fc = 1000 megacycles, "Q" is approximately equal to 50.  This value of "Q" represents that of the loaded passive modulator system.  Unloaded cavity "Q" is considerably reduced by the loading effect of the antenna on the passive modulator system.

Conclusions and Future Work

Analysis and measurement of the contemporary passive modulator will be continued.  Experimental evidence indicates that the operation of this passive modulator design can be improved.  By redesign, the "Q" of the device will be increased and the diaphragm construction will be improved, so that increased deflection will result from a given variation in sound level.  These improvements will increase the variation in reactive loading on the antenna of the passive modulator, with a consequent increase in the magnitude of change in the reflected wave.

The nickel foil diaphragm of one of the contemporary passive modulators has been replaced by a "Mylar" plastic diaphragm, 0.0007 inches thick, upon which an aluminum film has been evaporated.  This diaphragm is able to withstand extreme physical abuse, with no adverse effects.  Diaphragm materials of this type offer promise in passive modulator design.  The performance of this passive modulator unit will he compared with one of the nickel foil type, using the measurement set-up discussed previously in this report.  Experiments will be performed in an effort to optimize the method and degree of antenna coupling to the contemporary cavity during the next period.  Experiments to determine the relative merits of magnetic loop coupling as compared to capacitive probe coupling into the cavity will be conducted.

Excerpt from CIA Audio Countermeasures Activity Under E.O. 11905

We know that the opposition has the capability to build clandestine listening/transmitting devices which operate in the microwave region of the radio frequency spectrum.  As far back as the early 1950's the Soviets were actively using hostile devices which operate in the microwave region.  In 1952 a clandestine listening/transmitting device operating at 1.7 GHz was found to be concealed in a hand carved replica of the U.S. Great Seal presented as a gift to a U.S. Ambassador by the Soviets.  More recent "finds" have been made of hostile audio devices operating up to 1 GHz.  The recent disclosures in the public news media of the microwave signal flooding the American Embassy in Moscow is a clear example of the intent and practice of the Soviets to operate in the microwave region.

Excerpt from Husband, Lover, Spy by Janice Pennington

We had kept our radio on in the room.  Jerry [Mooney - a retired NSA cryptanalyst] laughed about it and told us that the radio being on would not deter anybody from listening to our conversation.  If we were being tailed, there were sophisticated ways of eavesdropping.  We could be in the middle of a rock concert and they could hear every word.

From an review of her book, Husband, Lover, Spy: a True Story

"Pennington, a hostess on the TV game show The Price Is Right, married German mountain-climber Friedrich ('Fritz') Stammberger in 1974.  The following year, while on an alleged climbing expedition where the borders of Russia, Afghanistan and Pakistan converge, he disappeared and was never heard from again.  Pennington made inquiries, but, she claims, the CIA informed her that her search compromised national security.  In 1980 she met Carlos de Abreu, a native of Mozambique, and four years later, when Fritz was legally declared dead, they married.  To exorcise her first husband's ghost, Pennington and de Abreu continued to investigate his fate.  Their quest ended in Moscow, where they found hazy information suggesting that Fritz had been a CIA spy, was dead and buried in Afghanistan.  The book is overlong, much of the suspense is artificially generated and the writing is compromised by sophomoric gush."

Excerpt from 'We Still Need Spies' in Newsweek, June 26, 1978

Two years ago, in response to U.S. protests, the Soviets apparently reduced their microwave bombardments of the Moscow embassy.  But eavesdropping continues.  Like its U.S. counterpart in Moscow, the Soviet Embassy in Washington bristles with mysterious antennas.  No one knows precisely what the embassy's electronic equipment picks up in the U.S. or relays back to the Kremlin.  But one high-level U.S. source maintains the Soviets used microwave gear during the 1973 Mideast war to listen in on White House conversations with the Pentagon, the State Department, and the CIA.

Excerpt from CIA-RDP78M02660R000800070028-4


The Soviets have been irradiating the U.S. Embassy, Moscow for more than 20 years.  The purpose of this irradiation has never been understood.  Approximately a decade ago medical research was instituted to determine whether the signals were harmful.  That research did not produce significant indications of hazard.

Signal strengths, from their inception until May of 1975, were very low - below the Soviet standard of 0.01 milliwatts per sq. cm. for an 8 hour day and therefore much less than the generally used 10.0 mW per sq. cm. industrial standard established in the U.S.

In May 1975, a new, much stronger signal was detected.  Though previous signals had not been considered a health hazard, the State Department felt that a possible hazard now existed.  As a result a medical scientist who had been following Soviet research in this area was dispatched to Moscow in July to make an on-site investigation.  His finding was that current medical research did not permit stating unequivocally that no hazard existed, but it did indicate that the probability was very low that a hazard existed from the levels then measured: 0.01 mW per sq. cm. for 8 to 10 hours per day.

During the next several months there were several changes in signal characteristics.  A second signal of similar characteristics to the first and an increase in signal duration to 19 to 20 hours per day were noted.  As a result, the medical scientist was again dispatched to Moscow.  This time he was accompanied by a physician with considerable experience in this area and an electronics expert.  Their findings were that while the power densities to which embassy staff were being subjected were not substantially different from those observed earlier in the year, the possibility of the existence of a health hazard was increased due to the increased exposure duration, certain changes in signal characteristics, and other medical considerations.

Based on the findings of this group the State Department instituted an effort to get the signal shut off and resolved to brief all embassy staff on the health hazard possibilities.

Excerpt from Spycatcher, by Peter Wright

Taylor and I divided up the technical work.  The Post Office pressed ahead with research into infrared detection.  I began using the resources of the Services Electronics Research Laboratory to develop new microphones and look into ways of getting sound reflections from office furniture.  I was already familiar with the technical principles of resonance from my antisubmarine work.  When sound waves impact with a taut surface such as a window or a filing cabinet, thousands of harmonics are created.  The knack is to detect the point at which there is a minimum distortion so that the sound waves can be picked up as intelligible speech.

One day in 1951 I received a telephone call from Taylor.  He sounded distinctly agitated.

"We've been beaten to it," he said breathlessly.  "Can we meet this afternoon?"

I met him later that day on a park bench opposite the Foreign Office.  He described how the British Air Attache in our Embassy in Moscow had been listening to the WHF [sic] receiver in his office which he used to monitor Russian military aircraft traffic.  Suddenly he heard the British Air Attache coming over his receiver loud and clear.  Realizing the Attache was being bugged in some way, he promptly reported the matter.  Taylor and I discussed what type of microphone might be involved and he arranged for a Diplomatic Wireless Service engineer named Don Bailey to investigate.  I briefed Bailey before he left for Moscow on how best to detect the device.  For the first time I began to realize just how bereft British Intelligence was of technical expertise.  They did not even posses the correct instruments, and I had to lend Bailey my own.  A thorough search was made of the Embassy but nothing was ever found.  The Russians had clearly been warned and turned the device off.

From questioning Bailey on his return it was clear to me that this was not a normal radio microphone, as there were strong radio signals which were plain carriers present when the device was operating.  I speculated that the Russians, like us, were experimenting with some kind of resonance device.  Within six months I was proved right.  Taylor summoned me down to St. James's Park for another urgent meeting.

He told me that the U.S. State Department sweepers had been routinely "sanitizing" the American Ambassador's office in Moscow in preparation for a visit by the U.S. Secretary of State.  They used a standard tunable signal generator to generate what is known as the "howl round effect," similar to the noise made when a radio station talks to someone on the telephone while his home radio or television is switched on.  The "howl round" detected a small device lodged in the Great Seal of the United States on the wall behind the Ambassador's desk.

The howl frequency was 1800 MH [sic], and the Americans had assumed that the operating frequency for the device must be the same.  But tests showed that the device was unstable and insensitive when operating at this frequency.  In desperation the Americans turned to the British for help in solving the riddle of how "the Thing," as it was called, worked.

Brundrett arranged for me to have a new, secure laboratory in a field at Great Baddow, and the Thing was solemnly brought up by Taylor and two Americans.  The device was wrapped in cotton wool inside a small wooden box that looked as if it had once held chess pieces.  It was about eight inches long, with an aerial on top which fed into a cavity.  Inside the cavity was a metal mushroom with a flat top which could be adjusted to give a variable capacity.  Behind the mushrooms was a thin gossamer diaphragm, to receive the speech, which appeared to have been pierced.  The Americans sheepishly explained that one of their scientists had accidentally put his finger through it.

The crisis could not have come at a worse time for me.  The antisubmarine-detection system was approaching its crucial trials and demanded long hours of attention.  But every night and each weekend I made my way across the fields at the back of the Marconi building to my deserted Nissen hut.  I worked flat out for ten weeks to solve the mystery.

First I had to repair the diaphragm.  The Thing bore the hallmarks of a piece of equipment which the Russians had rushed into service, presumably to ensure it was installed before the Secretary of State's visit.  They clearly had some kind of microscopic jig to install the diaphragm, because each time I used tweezers the thin film tore.  Eventually, through trial and error, I managed to lay the diaphragm on first and clamp it on afterward.  It wasn't perfect, but it worked.

Next I measured the length of the aerial to try to gauge the way it resonated.  It did appear that 1800 MH was the correct frequency.  But when I set the device up and made noises at it with an audio-signal generator, it was - just as the Americans had described - impossible to tune effectively.  But after four weekends I realized that we all been thinking about the Thing upside down.  We had all assumed that the metal plate needed to be opened right out to increase resonance, when in fact the closer the plate was to the mushroom the great the sensitivity.  I tightened the plate right up and tuned the radiating signal down to 800 megacycles.  The Thing began to emit a high-pitched tone.  I rang my father up in a state of great excitement.

"I've got the Thing working!"

"I know," he said, "and the howl is breaking my eardrums!"

I arranged to demonstrate the Thing to Taylor, and he traveled up with Colonel Cumming, Hugh Winterborn and the two American sweepers.  My father came along too, bringing another self-taught Marconi scientist named R. J. Kemp, who was now their Head of Research.  I had installed the device against the far wall of the hut and rigged up another monitor in the adjoining room so that the rounds of the audio generator could be heard as if operationally.

I turned the dials to 800 and began to explain the mystery.  The Americans looked aghast at the simplicity of it all.  Cumming and Winterborn were smug.  This was just after the calamity of the Burgess and Maclean affair.  The defection to the Soviet Union of these two well-born Foreign Office diplomats in 1951 caused outrage in the USA, and any small way in which British superiority could be demonstrated was, I soon realized, of crucial importance to them.  Kemp was very flattering, rightly judging that it would only be a matter of time before Marconi got a contract to develop one themselves.


Within eighteen months we were ready to demonstrate the first prototype, which was given the code name SATYR.  Kemp and I presented ourselves at the front door of MI5 headquarters at Leconfield House.  Hugh Winterborn met us and took us up to a spartan office on the fifth floor and introduced a tall, hunched man wearing a pin-striped suit and a lopsided smile.

"My name is Roger Hollis," he said, standing up from behind his desk and shaking my hand stiffly.  "I am afraid the Director General cannot be with us today for this demonstration, so I am standing in as his deputy."

Hollis did not encourage small talk.  His empty desk betrayed a man who believed in the swift dispatch of business.  I showed him the equipment without delay.  It comprised a suitcase filled with radio equipment for operating SATYR, and two aerials disguised as ordinary umbrellas which folded out to make a receiver and transmitter dish.  We set SATYR up in a MI5 flat on South Audley Street with the umbrellas in Hollis' office.  The test worked perfectly.  We heard everything from test speech to the turn of the key in the door.

"Wonderful, Peter," Hollis kept on saying, as we listened to the test.  "It's black magic."

Excerpt from Marty Kaiser's Website

One type of free-space transmitter, a type that has no battery, is the so-called "resonant cavity" transmitter.  The Great Seal of the United States in the Moscow Embassy concealed such a device.  As has been reported extensively in the media, a wooden wall plaque was presented as a gift along with the suggestion of mounting it on the wall behind the Ambassador's desk.  Many may recall the photograph of Ambassador Lodge pointing to a "bug" concealed in the back of the plaque.  The embarrassment caused by the detection of this transmitter motivated the intelligence community to spring into action and devices similar to it soon evolved.

The resonant cavity transmitter is an amazingly simple device technically known as a passive radiator;, i.e., one that lacks an internal source of energy.  In constructing this device, a layer of thin metalized material was stretched across a closed metal tube.  The specific size of the tube determined its resonant frequency.  A wire "tail", which functions as an antenna, is attached to the base of the cavity.  The cavity was then flooded with a beam of radio frequency energy from an external source (usually in the microwave region, 1 GHz and up).  The size of the cavity and the length of its antenna are carefully calculated so that a harmonic (multiple) of the inbound radio frequency energy that bathes the cavity is rebroadcast.  The metalized diaphragm acts as a transducer, and the audio range energy modulates the returned radio frequency signal that, in turn, is picked up by a receiver in the nearby listening post.  Do not assume these devices are the sole providence of Federal level agencies.

Excerpt from Odyssey of an Eavesdropper, by Marty L. Kaiser

The most famous listening device [Leon] Theremin invented was found in the U.S. embassy in Moscow, hidden inside a wooden carving of the Great Seal of the United States.  It was presented by a group of Russian schoolchildren to the U.S. Ambassador to Moscow, Averill Harriman, in 1946, shortly after the end of World War II in Europe, at a time when Stalin was consolidating his conquests within the Iron Curtain bloc.  The Great Seal bugging coincided with start of the Cold War in earnest.  The device, known as "The Thing" in intelligence circles, hung on a wall in the ambassador's private study.  It was far ahead of its time because it carried no wires, electrical circuits, or batteries to wear out.  Its inherent genius was its simplicity.

It worked by beaming an ultra-high frequency radio signal at the Great Seal from a van parked near the building.  The room conversation entered the carving in holes drilled below the bald eagle's beak.  The radio frequency signal from the van was picked up by a short antenna that entered the device, which looked like a small tunafish can.  On one side of the can, a resonant cavity held a thin metallic membrane that vibrated at a rate determined by the voices striking it.  The signal carrying the room conversation then returned to the van at three times the frequency of the original input signal, was detected, monitored, and recorded.  Soviet intelligence was able to record conversations for nearly six years until the device was discovered during a routine physical, not electrical, security check.


On one occasion, [Chris] Griffin asked if I could check Helen's [Bentley] campaign headquarters.  Again, I found nothing.  The phones were of a newer design that contained a CPU (central processing unit) to handle the switching.  I showed the staff how easy it was to bug those phones by simply pointing my 2010 Doppler Stethoscope at the CPU and listening to the conversation.  Look Ma, no hands, no wires.  The staff was amazed at the ease with which their phones could be compromised.

Here are some internal photos of a Martin Kaiser 2010 Doppler Stethoscope.  The Gunnplexer is a M/A-Com 87140-2, which puts out 25 mW (+10 VDC) at 10.25 GHz.  The MA86551 (or similar) horn antenna provides an additional 17 dB of gain.  This stethoscope model is meant to matched with the Martin Kaiser 1059 Preamplifier but the 2047 Ultrasonic/Contact Stethoscope will also work.

Excerpts from The Art of High-Tech Snooping, Time Magazine, April 20, 1987

Moreover, inspections of the new U.S. embassy building now under construction have turned up plenty of signs of bugs: cables seemingly unconnected to anything, odd indentations in wall panels, steel reinforcing rods so arranged as to convert structural pillars into antennas.


For a long time American experts have worried about mysterious low-level microwaves that have apparently been beamed at the embassy building.  One explanation involves a possible type of snooping that does not require hidden transmitters in the building.  Mysterious cavities along with configurations of steel rods and wire mesh have been found in the walls of the new embassy complex.  It is theoretically possible that the microwaves could somehow pick up the reverberations that emanate from within the walls of a building; a computer would then analyze those reverberations.

Excerpt from Bugging the Bedroom, Esquire Magazine, May 1966

It's an automobile spotlight.  But inside: a Doppler radar microphone hooked up to the car's radio.  The car is parked miles away from your house, but in line of sight of your window.  A narrow band signal is bounced off the windowpane, which vibrates as you talk inside.  The driver hears what you say.

Excerpt from How They're Watching Us, Popular Mechanics, July 1976

Early this year the United States protested against the Soviet practice of radiating the upper floors of our Moscow embassy with microwaves.  Though our diplomats didn't say so, it was thought that the Russians were either trying to reduce the effectiveness of antennas on the embassy roof (antennas for monitoring equipment) or were using microwaves in an attempt to intercept conversations.

Microwaves are short radio waves that travel by line of sight, like FM transmissions.  They are employed in long-distance telephone communications, in radar operations and in the latest type of home cooking oven.  They also are used in connection with resonators to eavesdrop on conversations in rooms that are in the line of sight of the listening post.  Resonators - small metal canisters or metal sheets - may be buried in the walls, ceiling or floor of a room.  Metal wastebaskets, air conditioning ducts or other metal devices also function as resonators.

A resonator vibration in response to pressure changes in the air, changes produced by sounds, include conversation.  When microwaves hit a resonator, they "pick up" the vibrations.  Reflected, the beam travels back to the receiver operated by the eavesdropper.  Electronic processing is then used to reproduce the spoken words.

Excerpt from Moscow Station, by Ronald Kessler

In September 1952, near the end of [George] Kennan's brief tour, two technicians arrived from Washington to check further for bugs.  Having found nothing, they asked the ambassador if he would go through the motions of dictating to his secretary in the den.  Perhaps the sound of his voice would activate the bugs.  As he later reported:

I droned on with the dictation [as] the technicians circulated through other parts of the building.  Suddenly, one of them appeared in the doorway of the study and implored me, by signs and whispers, to "keep on, keep on."  He then disappeared again but soon returned, accompanied by his colleague, and began to move about the room in which we were working.  Centering his attention finally on a corner of the room where there was a radio set on the table, just below a round wooden Great Seal of the United States that hung on the wall, he removed the seal, took up a mason's hammer, and began to hack to pieces the brick wall where the seal had been.  When this failed to satisfy him, he turned these destructive attentions on the seal itself.

In the seal the technicians found a cavity resonator that modulated microwaves beamed at it from the building across the street.  By converting the reflected beams back into sound waves, the Soviets could reproduce every sound in Kennan's office.

The day after finding the bug, Kennan noticed that his Soviet servants were unusually quiet, even hostile.  The tranquil mood of Spaso House had been shattered.  The thought of offending them weighed on him.  Perhaps, he thought, he should not have assisted the technicians after all.

Excerpt from The Age of Electronic Messages, by John Truxal

In the late 1960s, officials at the U.S. embassy in Moscow suddenly learned that the Russians had been using ingenious technology to listen to conversations in the ambassador's office.  On the office wall there was a plaque representing the American eagle.  The Russians had secretly hollowed out a cavity in the plaque and covered the front of the cavity with a membrane in such a way that it would not be noticed from inside the room.

Across the street, the Russians had a high-frequency radio with the beam focused on the cavity in the eagle.  Using the principle of resonance, they were able to listen to conversations with no "bug" or microphone in the ambassador's office, and indeed, no obvious way for the Americans to discover that the eavesdropping was going on.


Let us return to the opening paragraph of this chapter, where we mentioned the American eagle in the ambassador's office of the U.S. embassy in Moscow.  How did that system use resonance?

The below figure shows that the important parts were the cavity and the diaphragm.  The cavity was simply an empty space lined with metal.  This cavity was resonant for radio signals beamed at it.  The longer the distance D from the front of the cavity to the back, the lower the resonant frequency.

This resonance depending on size is just like sound signals resonating in organ pipes.  The long pipes resonate at low frequencies, the short at high frequencies.  A crystal glass partly filled with water acts the same way: the more water, the smaller the air cavity above the water, and the higher the frequency.  Thus, the cavity in the eagle was a resonant system, with the resonating frequency measuring D.

From across the street, the Russians beamed toward the eagle a radio signal with many different frequency components.  The echo coming back to them was strongest at the resonant frequency of the cavity.

During conversation in the ambassador's office, the speech sounds are really changes in air pressure. When

  • Air pressure rose, then
  • the diaphragm in the figure was pushed to the right, and
  • the resonant frequency of the cavity increased (smaller D), and
  • the radio echo that the Russians picked up across the street was at a higher frequency.

Thus, the frequency of the echo received across the street directly measured the changes in air pressure or sound in the room.  The Russians could listen to the conversation, but the only indication that the Americans had of this very elegant bugging was an extra, very weak radio signal, which probably could not be easily detected unless the frequency was known.

Excerpt from Spycraft, by Robert Wallace and H. Keith Melton

The Technical Services Staff was not a year old in 1952 when the CIA received information about an alarming audio discovery in Moscow.  During an electronic sweep, the countermeasures team discovered a device secreted in the wooden replica of the Great Seal of the United States that hung behind Ambassador George Kennan's desk in his residence at Spaso House, barely a mile from the Kremlin, at No. 10 Spasopeskovskaya Square.  The seal had been hanging there seven years, after a group of Soviet Young Pioneers presented it as a token of friendship on July 4, 1945, to then U.S. Ambassador, W. Averell Harriman.  The gift, presented by smiling children in neatly pressed uniforms, concealed a listening device that would baffle and frustrate the Agency for years.  "The Englishmen will die of envy," Valentin Berezhkov, Stalin's personal translator, whispered to Ambassador Harriman during presentation.


However, what was discovered hanging behind the Ambassador's desk in 1952 was revolutionary in the technology of listening devices.  Implanted in the middle of the carved wood of the Great Seal, cleverly hidden behind an air passage formed by the American eagle's nostril, was a device that was alarming as much for the technology it employed as the fact it had been active for more than half a decade.  Indeed, four American ambassadors - Averell Harriman, Walter Smith, Alan Kirk, and George Kennan - presumably had their secret conversations picked up by the bug.

Differing significantly in design and function from any piece of covert listening equipment previously known, the device was constructed of precision-tooled steel and comprised a long pencil-thin antenna with a short cylindrical top.  Agency engineers could not understand exactly how it worked.  The stand-alone unit, apparently, did not require a battery or an other visible power source.  It had no wires or tubes, nothing that identified the device as a piece of electronic equipment.  If the oddly shaped length of metal was transmitting conversation, then how was it doing it?

"The Thing," as it was soon dubbed, bounced among the Agency's lab, the FBI, and private contractors for evaluation and reverse engineering.  No one could offer anything beyond an educated guess to how The Thing worked, and somewhere in its travels from lab to lab it was damaged from either improper handling or shipping.

The Thing was eventually sent to Peter Wright, the principal scientist for MI5, the British intelligence service responsible for counterintelligence operations.  Wright worked for more than two months to solve the mystery before eventually coaxing it into operation.  He dubbed it a "passive cavity resonator."  The Thing, as Wright discovered, worked by reflecting radio waves, then picking those echoes up with a radio receiver.

To operate the device, the NKVD aimed a continuous 800 MHz radio signal at the seal from a listening post in the building across from Spaso House.  The Thing's thin diaphragm at the top, which Wright had repaired, vibrated with the sound of a voice.  Those vibrations were carried by an interior tuning post to the antenna.  Then, as the vibrations hit the antenna, they altered the reflected radio signal that bounced back to the listening post.  The Thing did not require internal power in the same way a mirror does not require power to reflect light.  The radio transmitter and receiver, code named LOSS (or REINDEER by the Russian techs), were a marvel of signal processing, considering the technology available at the time.

According to Wright's own account, once he understood the principle and made the device work, he took another eighteen months to create a similar system for British intelligence.  Called SATYR, his device featured aerials - transmitter and receiver - disguised as two proper British umbrellas.  SATYR proved to be a great success and Wright called it "black magic."  Then, as he observed, "the Americans promptly ordered twelve sets and rather cheekily copied the drawings and made twenty more."  The American version of the device, according to Wright, was called EASY CHAIR (also called MARK2 and MARK3).

Excerpt from Molehunt, by David Wise

But the CIA's technical boffins worked hardest of all at playing catch-up with the KGB: they were trying desperately to reproduce an unusual, highly sophisticated bug that the Soviets had used against the United States with devastating effect.  The bug employed a technology that had not been encountered before, and the CIA scientists were having trouble figuring it out.

In 1945, the Soviets had presented to Ambassador Averell Harriman in Moscow a carved replica of the Great Seal of the United States.  The hollow wooden seal had decorated the wall of four U.S. ambassadors before the listening device it concealed was discovered by the embassy's electronic sweepers in the early 1950s.

"We found it and we didn't know how it worked," [S. Peter] Karlow recalled.  "There was a passive device inside the seal, like a tadpole, with a little tail.  The Soviets had a microwave signal beamed at the embassy that caused the receptors inside the seal to resonate."  A human voice would affect the way the device resonated, allowing the words to be picked up.  "Technically it was a passive device, no current, no batteries, an infinite life expectancy."

The effort to copy the Soviet bug that had been discovered inside the Great Seal was given the code name EASY CHAIR by the CIA.  The actual research was being performed in a laboratory in the Netherlands in two super secret projects code-named MARK 2 and MARK 3.

Unknown to Karlow and the CIA, British intelligence had succeeded in replicating the Soviet bug, which MI5, the British internal security service, code-named SATYR.  In his book Spycatcher, former MI5 official Peter Wright said he first thought the device was activated at 1,800 megahertz, but then tuned it down to 800 MHz and it worked.  But, according to Karlow, the British did not share their secret with the CIA.

Karlow's work on EASY CHAIR was to have unexpected consequence.  When Anatoly Golitsin dug his package out of the snowbank in front of Frank Friberg's house in Helsinki, one of the papers it contained was a KGB technical document warning that the CIA was working on an eavesdropping system to match the Soviet bug.

Excerpt from The Ultimate Spy Book, by H. Keith Melton

In the early 1950s, a Soviet listening device was found in the American Embassy in Moscow.  This came to the attention of the world when it was displayed at the United Nations by the American ambassador in May, 1960.  It was a cylindrical metal object that had been hidden inside the wooden carving of the Great Seal of the United States -- the emblem on the wall over the ambassador's desk -- which had been presented to him by the Soviets.

The Great Seal features a bald eagle, beneath whose beak the Soviets had drilled holes to allow sound to reach the device.  At first, Western experts were baffled as to how the device, which became known as "The Thing" worked, because it had no batteries or electrical circuits.  Peter Wright of Britain's MI5 discovered the principle by which it operated.  MI5 later produced a copy of the device (codenamed SATYR) for use by both British and American intelligence.


A radio beam was aimed at the antenna from a source outside the building.  A sound that struck the diaphragm caused variations in the amount of space (and the capacitance) between it and the tuning post plate.  These variations altered the charge on the antenna, creating modulations in the reflected radio beam.  These were picked up and interpreted by the receiver.

Excerpt from a November 14, 2004 TSCM-L mailing list post, by James M. Atkinson of the Granite Island Group.

The audio transmitters you mention can be done by introducing a microphone into the sensor housing, or by taping or suspending a small piece of foil or metalized Mylar onto the microwave beam of the sensor.  A good example of this would be a PIR/Microwave sensor that is mounted on the wall of the office of an executive and is directed towards the window with drapes.  The eavesdropper places a very lightweight piece of foil inside of behind the drapes.  The air in the room slightly moves the foil which causes a very slight Doppler shift in the 10 GHz signal that can be picked up some distance away from the targeted area.  The critical parts of the equation is that the metallic foil has to be as thin and light as possible (minimal mass), should be in the main beam (easy enough), and should have sharp, almost saw tooth edges around the outside.

Excerpt from Don't Bug Me: The Latest High-Tech Spy Methods , by M. L. Shannon

Besides the microwave frequency bugs from an earlier section, there are other ways to use microwaves for surveillance.  The first method is to concentrate a microwave beam on something that vibrates from the sound in the room in which it is placed.  The reflected beam, coming back, is converted into sound, like the laser devices detailed in the next part.

In the U.S. Embassy in Moscow, the sculpture of the American eagle that the Soviets presented as a gift was made so it would act as a sounding board for reflecting microwaves.  Beware of bears bearing gifts.  In addition, the steel rebars (reinforcement bars) inside the concrete walls were arranged in such a way that they would also reflect the microwave beam, just like the eagle.

The second method, used both outside and inside, is to plant a small device called a resonator, which looks like several quarter-size metal disks with a small rod through the center, inside the area to be bugged.  The resonator may be inside a small metal cylinder.  A microwave transmitter is placed somewhere near the target on one side, and a receiver goes on the other side.  A concentrated beam from the transmitter is directed at the point where the resonator is hidden.

This device is vibrated by sounds in the room in which it is placed and modulates the microwave beam.  When the receiver picks it up, it can demodulate (recover) this sound.  This device is very expensive and therefore very unlikely to be encountered.

Another system that some scientists in Germany are working on will supposedly reflect the microwave beam from the changing density of the air - sound makes tiny compressions in air, and this system is supposed to convert this change in density into sound.

The principle that this works on is probably similar to one that produces laser holograms.  The laser beam is split into two smaller beams.  One of them is bounced off a mirror that changes the phase angle, and when the two beams are recombined, they create an image in space.  Unlike the lasers in the next part, microwaves penetrate walls and don't require a window.  Also, they are unaffected by the things that can interfere with lasers.

Excerpt from Mind Games, by Sharon Weinberger

Concerns about microwaves and mind control date to the 1960s, when the U.S. government discovered that its embassy in Moscow was being bombarded by low-level electromagnetic radiation.  In 1965, according to declassified Defense Department documents, the Pentagon, at the behest of the White House, launched Project PANDORA, top-secret research to explore the behavioral and biological effects of low-level microwaves.  For approximately four years, the Pentagon conducted secret research: zapping monkeys; exposing unwitting sailors to microwave radiation; and conducting a host of other unusual experiments (a sub-project of Project PANDORA was titled Project BIZARRE).  The results were mixed, and the program was plagued by disagreements and scientific squabbles.  The "Moscow signal," as it was called, was eventually attributed to eavesdropping, not mind control, and PANDORA ended in 1970.  And with it, the military's research into so-called non-thermal microwave effects seemed to die out, at least in the unclassified realm.

Excerpt from Mind Control, Playboy Magazine, January 1990

People in the intelligence community began asking the same questions in the early Sixties when the Soviets were bombarding the U.S. embassy in Moscow with low-intensity microwaves.  No official in Government has ever come up - publicly, at least - with the definitive explanation of what the Soviets were trying to do.  There were three theories.  First was the idea that the K.G.B. was activating its bugs in the embassy.  The second, and most likely, held that they were trying to jam super-secret U.S. listening devices in the embassy that were allowing the National Security Agency to pick up all sorts of secret Kremlin conversations.  The third suggested that the microwaves were somehow meant to affect the brains of the diplomats inside the embassy and alter their behavior.  That is the least likely of the three theories, but it was and is still seriously debated by U.S. scientists pondering the problem.

Excerpt from The Assassination Business: A History of State-Sponsored Murder, by Richard Belfield

The State Department also got in on the act, investigating the bizarre microwave signal being beamed at the U.S. embassy in Moscow.  This TOP SECRET investigation was called Project PANDORA and one of its aims was to see if this was a brain-programming weapon.  But the CIA and the Pentagon were impatient and could not be bothered to wait for the answer so they set up their own deep-black operation called Project BIZARRE, which assumed that the signal was a brainwashing weapon and asked whether they could build a similar one for the USA.

Excerpt from Remote Behavioral Influence Technology, by John J. McMurtrey

The microwave irradiation of the American Embassy in Moscow received little publicity until the winter of 1976 instillation of protective screening, but irradiation was known since 1953.  Original frequencies were 2.56-4.1 GHz with additional intermittent 0.6-9.5 GHz signals being permanent by 1975 in a wide band frequency hopping consistent pattern with one signal pulsating.  The irradiation was directional from nearby buildings and modulated.  Complaint to the Soviets had no avail, but the signals disappeared in January 1979 "reportedly as a result of a fire in one or more of the buildings."

A 9-11 GHz signal recurred in 1988.  Observed frequencies are basically within the microwave hearing spectrum, and pulsation is required.  Psychiatric cases occurred during the exposure period, though no epidemiological relationship was revealed with fully a quarter of the medical records unavailable, and comparison with other Soviet Bloc posts.  The CIA had Dr. Milton Zaret review medical Soviet microwave literature to determine the purpose of the irradiation.  He concluded the Russians "believed the beam would modify the behavior of the personnel."  In 1976 the post was declared unhealthful and pay raised 20%.

Excerpt from Theremin: Ether Music and Espionage, by Albert Glinsky

The residential nature of Spaso House limited access by Soviet technicians and thwarted attempts to install concealed surveillance devices.  The house and garden were surrounded on three sides by high brick walls, and an iron fence guarded by plainclothes Russian officers blocked the fourth side.  But Lavrently Beria was determined.  An engineer-magician would be required.

In one of his first dragnets in early '39 - just after the ouster of Yezhov - Beria had sanctioned the arrest of an engineer, Lev Sergeyevich Termen.  Nine months later, the same Termen had been delivered from Kolyma to the sanctuary of the Radio Street design bureau.  Beria knew that - after all, the institution of the sharashka was his creation.  After Sverdlovsk, Lev Sergeyevich had been reassigned to a sharashka a Kuchino, near Moscow, a facility for radio electronics and measuring devices.  There he had designed a "radio beacon whose signals helped locate missing submarines, aircraft, or secret cargo smuggled into the enemy's rear."  In the spring of '45, the Spaso House puzzle would be his next assignment.

Beria's demands were intimidating; there could be no wires, no traditional microphones, and the system had to be encased in something that would not call attention to itself.  For Lev Sergeyevich, the stakes were higher than ever.  Beria was no one to disappoint.  He always had his way, and failure for the inventor could mean a return to Kolyma, or worse.  But Lev Sergeyevich forged a working system.  The only remaining quandary was how to penetrate the ambassador's residence.  With his trademark sleight-of-hand, he soon found his answer in the archetype of the Trojan horse.

July 4, 1945.  The annual Independence Day reception at Spaso House was the one event of the year when Averell Harriman threw open the doors to his Russian hosts.  A delegation of Soviet boy scouts (Pioneers) presented the ambassador with a large wooden plaque - the carved relief of the Great Seal of the United States.  It was offered as "a gesture of friendship" and a token of fine Russian woodcarving.  Harriman thanked the scouts and hung the eagle emblem on the wall over his desk.  Lodged inside was the latest incarnation of Lev Sergeyevich's wizardry - a miniature apparatus bearing the hallmarks of his capacitive work from the space-control instrument to the burglar and fire alarm systems.

Set into a long, trench-like cavity gouged through an inner surface of the hollow plaque was a small metal cylinder, eleven-sixteenths of an inch deep and roughly the diameter of a quarter.  Attached to the cylinder was a nine-inch long protruding antenna tail.  The device was passive - it had no batteries or current, and its lifespan was indefinite.  Its presence went undetected by the routine X-ray screening of all objects entering Spaso House.  The device became active only when an external microwave beam of 330 MHz was directed at its antenna from a neighboring building, causing a metal plate inside the cylinder to resonate as a miniature tuned circuit.  The wood just behind the eagle's beak was thin enough to allow sound waves from human speech in the ambassador's office to filter through to a diaphragm that moved in response to the sounds.  The pattern of the diaphragm's vibrations caused fluctuations in the capacitance between the diaphragm itself and the plate of the tuned circuit that faced it, causing it to act as a microphone.  This produced corresponding modulations that were registered in the antenna - much like a broadcast transmitter - and reflected out to be picked up as words on a remote receiver.  Lev Sergeyevich was careful to select a bandwidth he knew was not under the control of American security.  With his experience in tuned circuits from his space-control instrument, and devices like the keyboard harmonium, he was the ideal specialist for the job.

Photo from April 24, 1947.  Which is within the operational time of "The Thing."

Excerpt from New Scientist, August 1, 1974

The vulgar lie detector continues to progress in its erosion of the dignity of man.  The American Civil Liberties Union - and more power to their corporate elbow - has reported to Congressional subcommittee that the Israeli Weizmann Institute has developed a "microwave respiration monitor" which can tell from half a mile away whether a person is telling the truth or not.  It is claimed by the Union that the apparatus is being tested from a hill overlooking the Allenby Bridge to check on the bona fides of Arabs approaching with intention to cross it.  A microwave signal is directed at the stomach of the would-be river crosser is held to reveal whether his abnormal breathing rate indicates that he is lying under interrogation by the military guardians of the bridge.

Excerpt from The Company We Keep: A Husband-and-Wife True-Life Spy Story, by Robert and Dayna Baer

Authors' Note:  The term parabolic mic substitutes for a device that is still classified.

In the second week of June, Charlie, the ex-Marine jet pilot, and I come down to Sarajevo from Tuzla to help Bob find an apartment for the parabolic mic.  ...  Charlie and I walk along the Miljacka River, noting buildings with a line of sight to the Hizballah safe house.  ...  I walk to the window of the bedroom facing the river.  It has an unobstructed view of the Hizballah safe house...


I slide my chair around so Dan has to look at me.  "This is god-damned bureaucratic terrorism.  We don't have cars.  We don't have a place to live, and on top of it I don't have a clue where we're going to put this damn ray gun."

In fact, it's not a ray gun.  It's a kind of parabolic microphone that sucks conversations out of the air at a long distance, even through the walls of buildings.  My plan is to find an apartment with a line-of-sight view of the Hizballah safe house, position the mic in the apartment's window so it can't be seen, and wait for the Hizballah operatives to blurt out something they shouldn't - a name, an address, or a telephone number.


The two are standing on the lawn in front of what looks like a giant amber parasol.  It's at least fifteen feet across.  I know it's a transmitter antenna, but why so big?  It could never be mistaken for the type of antenna you see relief groups here using.

Back at the apartment Charlie and Riley are sharing, we took great pains to keep the parabolic mic hidden, but this communications antenna might as well be a giant neon arrow pointing at the house I took so much pain to keep clean...


Charlie and I have learned to live with the parabolic mic, the two of us camping where we can.  ...  I notice that Bob's even more cautious these days because the parabolic mic has started to turn up a lot of good ideas - for example, the names of Hizballah operatives and even a couple of telephone numbers.  There have even been a couple of great intelligence reports.  Separately, with a telephoto lens we've been able to get plate numbers for their cars.


If I hadn't been standing in the window, I wouldn't have seen the guy standing by the river, taking pictures of our apartment building.  Instinctively I take a step back and to the side, into the dimness of the apartment.

It can't be a coincidence, I think.  There's no reason anyone would take a picture of our apartment.  I peek around the curtain for a second look.  He's still there, his camera pointed directly up at our building.  I turn around to look at the parabolic mic.  It's back away from the window, and behind a cloth hanging from the ceiling.  Even if he were level with our window, he couldn't see it.


Convinced, he goes back in to quickly put away the parabolic mic in its concealment device.  We both know if the Bosnians catch us with it, we'll be arrested.

On the street Charlie and I lock arms, a couple out for a walk.  Neither of us turns around to look at where I saw the photographer.  At the corner intersection we cross the river on the Cobanija bridge.  On the middle of the span, I turn to him so we'll look like we're deep in conversation...

That operation would have taken place in Sarajevo, Bosnia in the spring of 1996.  The target house was on the other side of the Miljacka River, near the infamous Cobanija bridge, from their second or third-floor apartment.

Dayna & Robert Baer Interview  Discussing their book and a bit out the operation of the "parabolic mic" which can hear through walls.  (YouTube)

Robert:  "... We were sent there for this reason, is Hizballah had attempted to kidnap and murder a CIA operative there.  He was pulled out in the middle of the night.  Dayna and I were brought in along with a larger team to put surveillance on Hizballah to figure out who they were, where their safe houses were, where they were living, and the rest of it.  And we had, what we're calling in the book, is a parabolic mic - a directional mic - that can read through walls.  It's a very amazing piece of equipment.  So you could listen into a Hizballah safehouse and get the translations and then we were going to use NATO troops to take down these safehouses.  That was the plan."

Dayna:  "But the parabolic mic it was setup actually across... in an apartment building.  It can read through walls, across the river, into another building on the other side of Miljacka River, which runs right through Sarajevo.  It takes in the information.  It was getting us names and addresses, and we were sort of coordinating that with taking photos of license plates.  The whole idea was to match it all up and try to run down who these people were.  Once we got this information, then what were we going to do with it?  We're in Sarajevo; it's not like you could call the DMV.  So, how do you get where did these people live and exactly who their names are?  That led into more of what Bob's job was, and that was to try to recruit somebody.  We targeted a local policeman who could help us possibly run traces on the information that we got."

Excerpt from Dancing with the Devil: Sex, Espionage, and the U.S. Marines, by Rodney Barker

Throughout the Cold War the Soviets had repeatedly demonstrated a bold proclivity toward the use of clandestine listening devices with legendary success.  They had given a hand-carved replica of the Great Seal to U.S. Ambassador Averell Harriman.  This masterpiece of art and ingenuity, constructed with a wireless resonant cavity, was given a place of honor on one of Harriman's walls, where it hung as an invisible witness to U.S. foreign policy in the making, monitoring the ambassador's conversations for several years, until located.  (Later a U.S. diplomat was quoted as saying they went to the middle of Red Square for their private conversations, while in the embassy "they spoke for the mikes.")

In the years since, the KGB had bombarded the embassy with microwaves to pick up the vibrations of voices on the window-panes; it had slipped an elaborate eavesdropping antenna in the embassy's chimney, stolen the embassy's electronic typewriters and rigged them to transmit every letter; and it had sprinkled a powder that was invisible to the human eye but glowed under certain lights as a way of tracing the travels of suspected agents.

Excerpt from The Master of Disguise, by Antonio J. Mendez

But the Tchaikovsky Street embassy was an ideal site for electronic eavesdropping.  Embassy and Agency security officers estimated that the KGB's ubiquitous local employees had seeded the entire building with hard-wire and wireless bugs.  The windows were silently scanned with microwaves that could reproduce the vibration of conversations into usable recordings at the numerous KGB listening posts ringing the embassy.  Between the overtly inquisitive UPDK local employees and the hidden bugs, Americans, from the ambassador to the lowest-ranking Marine security guard, were subject to audio surveillance during every moment they spent in the U.S. Mission, including their apartments.

Excerpt from Murray Associates, 2010

"This device is simple in concept but very complex in construction.  A remote transmitter sends a strong radio frequency signal aimed at the bug, with a directional antenna if possible.  A separate antenna is used to receive the signal which is reflected from the bug -- and everything else around it.  The trick here is to sense the reflectance variations caused by the bug and ignore other variations such as heating systems rattling ducts, etc.  In order to make the bug work, its antenna needed to be resonant near the incoming frequency with its resonant frequency changed by the movement of a diaphragm.  The diaphragm is of course moved by sound pressure.

The standard quarter wave antenna length explanation is probably not correct since the bug antenna did not appear to be connected to anything like a ground plane.  More likely it was a half wavelength at the excitation frequency.  To make all of this work, the resonant cavity under the diaphragm and bug antenna had to be carefully matched.  Diaphragm position had to be close to the the post for good sensitivity but not so close that it would touch the post as components aged.

One last problem in the operation was the excitation signal.  It didn't take a genius to discover the transmitted signal and subsequently the reflected signal.  It would be important in operation of the bugging system to turn it off when a sweep team was seen in the area.  The rumor is that this device was detected as the sweep technician dialed his receiver past the excitation frequency and heard voices.

This sort of bugs is not likely to be found in corporate or residential eavesdropping situations.  Lax access control, easily installed computer keystroke recorders, high tech baby monitors and cordless phones that broadcast conversations make the work of a modern day Theremin unnecessary."

Excerpt from Symptoms of the "Havana Syndrome", 2021

This is how KGB veterans describe the prelude to this operation:

"The successful operation to introduce Zlatoust into the American Embassy was preceded by a long and serious preparation: a specially organized event - the celebration of the 20th anniversary of the Artek camp, where the American and British diplomatic missions were invited in order to 'express gratitude from Soviet children for their help in the fight against fascism.' - a ceremony that was impossible to refuse to attend.

Thorough preparation - pioneer choir, lineup, orchestra, perfect cleanliness and order, special security measures, disguised as pioneer leaders, two battalions of NKVD fighters. And, finally, the gift itself with a 'surprise' - a unique work of art in the form of the US coat of arms (Great Seal) with a 'Theremin resonator' mounted inside."

The coat of arms with "Chrysostom" took its proper place - on the wall right in the office of the head of the American diplomatic mission.  It was here that the most frank conversations and extraordinary meetings were held - the Soviet leadership learned about the decisions made by the ambassador before the President of the United States himself.

Excerpt from The Big Ears of the USSR: The Top 5 Soviet Wiretaps During the Cold War

A Bug in the U.S. Coat of Arms

Soviet-American relations rapidly deteriorated after the war ended, so information about the enemy became particularly valuable to Moscow.  Russian intelligence therefore set its sights on the American Embassy in Moscow.  One particular trick brought it great success.

A bug called Chrysostom (also known as Golden Mouth) was delivered to the embassy already implanted in a gift from Soviet Pioneers - an elaborate replica of the Great Seal of the U.S. carved in wood.  The souvenir was so beautiful that the U.S. ambassador hung it on the wall of his office.  Thanks to Chrysostom, for seven years the Soviet government was able to learn about the ambassador's plans before they even reached the desk of the American president.

Chrysostom survived four ambassadors.  The furniture changed, but the bug kept its place on the wall.  The Americans finally discovered it in 1952, when its radio signal was modulated.  At first the Americans did not realize that the bug was hidden inside the eagle on the emblem.

It had no battery - it was just a resonance chamber with a flexible front wall acting like a diaphragm, which changed the dimensions of the chamber when sound waves hit it.  It was operated by a radio ray coming from a building across the street.

Excerpt from "Chrysostom" in the American Embassy - Masterpieces of Russian Espionage, by Oleg Kaptsov

This fantastic story began in late 1943, when Stalin was informed about the creation in the USSR of a unique eavesdropping device - a microwave resonator designed by Lev Termen.

The perpetual motion machine did not need batteries and operated in a completely passive mode - no magnetic fields, no own power sources - nothing that could unmask the device.  Placed inside an object, the "tadpole" was powered by microwave radiation from a remote source - the microwave wave generator itself could be anywhere within a radius of hundreds of meters.  Under the influence of the human voice, the nature of the oscillations of the resonating antenna changed - it was only necessary to accept the signal reflected by the "bug," record it on magnetic tape and decipher, restoring the original speech.

The spy system, codenamed "Zlatoust," consisted of three elements: a pulse generator, a resonator ("bug") and a receiver of reflected signals placed in the form of an isosceles triangle.  The generator and receiver could be located outside the listening facility, but the main problem was to install a "bug" in the office of the American ambassador.

The fire trick didn't work.  As practice has shown, with security, Americans had everything O.K.  Access to the secret premises of the Embassy was strictly limited.  None of the Soviet citizens and members of official delegations were allowed close to the upper floors of the building.

That's when the "Trojan horse" idea was born.

A rich collection of souvenirs made of wood, leather and ivory was urgently delivered to the reception room of the People's Commissar of Internal Affairs of Beria: a shield of a Scythian warrior made of a black alder, two-meter mammoth tusks, a telephone device of the Ericsson company, inlaid with ivory - a gift from the Swedish king Nicholas II, a luxurious basket for papers made entirely of an ivory foot...

Alas, none of the rare exhibits impressed the technical specialists of the NKVD - to install the "Zlatoust" required a very special souvenir made taking into account the technical characteristics of the listening device itself.  A souvenir that could not leave indifferent the American ambassador to the USSR, Averell Harriman.  An exceptional rarity that would be impossible to convey to anyone or "forget" in the far room of the Embassy.


A solemn line in the Artek camp, tied red ties and a line of young, ringing voices singing in English the anthem of the United States - the American ambassador was struck by a tear.  Touched by the warm reception, Harriman handed the pioneer organization a check for 10 thousand dollars.  The British ambassador who was present on the line also handed the pioneers a check for £5,000.  At the same moment, to the solemn sounds of music, four pioneers made a lacquered wooden shield with the coat of arms of the United States carved on it.

To the thunder of applause, Artek's director handed over to "our American friends" a certificate for a rare coat of arms signed by the All-Union elder Kalinin: sandalwood, boxwood, sequoia, elephant palm, Persian parrotia, red and black wood, black alder - the most rare breeds of wood and skillful hands of Soviet masters.  The gift was glorious.

"I can't take my eyes off this miracle!  Where should I hang it?"  A rare occasion when Harriman said aloud what he really thought.

"Hang over your head," Stalin's personal translator, Comrade Berezhkov, hinted unobtrusively to Harriman, "the British ambassador will burn with envy."

Trojan Passions or Operation Confession

The successful operation to introduce "Zlatoust" into the American Embassy was preceded by a long serious preparation: a specially organized event - the celebration of the 20th anniversary of the Artek camp, where the American and British diplomatic missions were invited to "express gratitude from Soviet children for helping in the fight against fascism" - a ceremony from which it was impossible to refuse.  Careful preparation - pioneer choir, ruler, orchestra, perfect cleanliness and order, special security measures, disguised as pioneers two battalions of NKVD fighters.  And, finally, the gift itself with a "surprise" is a unique work of art in the form of the U.S. coat of arms (Great Seal) with a Termen resonator mounted inside.

Operation "Confession" has Started!

As the analysis of the "bug" signals showed, the coat of arms with "Zlatoust" took its proper place - on the wall, right in the office of the head of the American diplomatic mission.  It was here that the most frank conversations and emergency meetings were held - the Soviet leadership learned about the decisions taken by the ambassador before the U.S. President himself.

On the upper floors of houses on the opposite side of the street, two NKVD safe houses appeared in front of the American Embassy - a generator and a receiver of reflected signals were installed there.  The espionage system worked like a clock: the Yankees said, Soviet spies recorded.  In the morning, wet linen was hung on the balconies of the apartments, "housewives" from the NKVD carefully shook the floorboards, literally throwing dust in the eyes of American counterintelligence.

For seven years, the Russian bug "stunned" worked in the interests of Russian intelligence.  During this time, "Zlatoust" survived four ambassadors - each time the new inhabitants of the cabinet sought to change all the furniture and interiors, only the wonderful coat of arms remained unchanged.

The Yankees learned about the existence of a "bug" in the building of the Embassy only in 1952 - according to the official version, radio technicians accidentally discovered the frequency on which Zlatoust worked on the air.  An urgent inspection of the Embassy's premises was carried out, the entire office of the head of the diplomatic mission was "shake off the top floor" - and they found...

At first, the Americans did not understand what kind of device was hidden inside the shield with the coat of arms.  Metal wire 9-inches long, hollow camera-resonator, elastic membrane... no batteries, radio components and any "nanotechnology."  A mistake?  Was the real bug hidden somewhere else?!

The British scientist Peter Wright helped the Americans understand the principles of Zlatoust's work - the acquaintance with Termen's microwave resonator shocked Western intelligence agencies, the experts themselves admitted that if not for the case, the "eternal bug" could still "stun" the symbol of American statehood at the U.S. Embassy in Moscow.

The Americans did not dare to disclose to the media the shocking fact about the discovery of a bug that more than seven years worked in the office of the head of the U.S. diplomatic mission.  Unpleasant information became public only in 1960 - the Yankees used Zlatoust as a counterargument in the international scandal associated with the downed American intelligence officer (Gary Powers) U-2.

After comprehensive studies of the coat of arms with the "secret," our Western friends tried to copy the "Zlatoust" - the CIA initiated the "EASY CHAIR" program, but could not achieve an acceptable quality of the reflected signal.  The British were more fortunate - created as part of the secret government program Satir, the resonator bug was able to transmit a signal at a distance of up to 30 yards.  Similar to the Soviet system.  The mystery of the Russian "Zlatoust" was not to the West's teeth.

One of the most successful Soviet intelligence operations during the Cold War alarmed Americans.  "Zlatoust" was only the beginning of the campaign to listen to the "enemy's stan" - much later, during the reconstruction of the U.S. Embassy building on Novinsky Boulevard in 1987, the Americans found that their apartments were literally teeming with all sorts of "bugs" and eavesdropping devices.  But an even more shocking case occurred on December 5, 1991 - on that day, the chairman of the Inter-Republican Security Service (ISB, successor to the KGB) Vadim Bakatin at an official meeting handed over to the American ambassador Robert Strauss 70 sheets with schemes for laying "bugs" in the buildings of the U.S. embassy complex in Moscow.  Eyewitnesses claim that at that moment the American simply lost the ability to speak - the first person of the state security service surrendered weapons to the enemy!  Finally, surprised the volume of all sorts of "bookmarks" - Soviet intelligence officers for years bugged the entire building along and across.

Excerpt from The Microwave Debate, by Nicholas H. Steneck

Figure 6
The suspected source of the Moscow signal.
From documents released under FOIA request, USSD MW II, no. 74 (Jun 19, 1973).

The origins of radiation problems in Moscow go back to 1952 when the U.S. embassy was moved from a site near the Kremlin to a newly renovated apartment building several miles away.  Shortly after the move routine radiation checks turned up unexpected readings.  One sweep made in advance of Vice-President Richard Nixon's trip to the Soviet Union in 1959 discovered high ionizing radiation levels in sections of the ambassador's apartments, including one of the rooms where Nixon was scheduled to sleep. Similar checks for nonionizing radiation conducted as early as 1953 detected the presence of a microwave signal apparently beamed at the embassy from a nearby building.

It is true that changes in the Moscow signal beginning as early as 1973 ultimately brought the signal problem to a climax.  In January 1973, after about a decade of irradiation by the original signal (called TUMS - technically unidentified Moscow signal), a second signal was picked up.  Dubbed MUTS (a variation of TUMS) this second signal was observed only until March 1973 and then disappeared.  It reappeared briefly in February 1974, by which time intelligence sources indicated that the Soviets were constructing special facilities on a building across the street from the embassy to house MUTS.  This construction project was carefully monitored and pictures of the additions regularly sent back to the United States (figure 6).  In May 197 5 MUTS reappeared on a regular basis and was joined in October 1975 by a second beam, MUTS-2, located on the top of a building south of the embassy.  This sequence of events led the State Department to send a special delegation to Moscow in early 1976 to negotiate with the Soviet Union in an effort to get MUTS turned off (TUMS was turned off on May 26, 1975).

The State Department's actions unquestionably were motivated by concern over the MUT's characteristics, not its power level.  MUTS, even when MUTS-1 and MUTS-2 were operating in tandem, was not appreciably more powerful than TUMS.  Both were regularly monitored in the low µW/cm2 range-on an average between 2 and 10 µW/cm2.  But MUTS was more complex than TUMS, and it is this fact that must have been the most troublesome.  The State Department, noted one secret cable, was "especially interested in changes" not only in "signal level" but in "composition or operating mode" as well.

Excerpt from Secret Memos Reveal Julian Assange's Escape Plans From Ecuador's Embassy, by BuzzFeed.News

Another option for exfiltrating Assange considered by embassy staff was to put him in a diplomatic bag, considered inviolable by treaty, but only if they solely contain documents relating to the normal practice of an embassy.

Officials dismissed this plan as they knew police outside the embassy had "advanced technology that can detect body heat."  Elsewhere, embassy staff had noted a police van across the road from the embassy was "bombarding" the building with microwaves, and was capable in the view of staff of collecting most if not all signals from within.


Alleged surveillance of Julian Assange while he was in the Ecuadorian embassy in London with a microwave or laser bounce listening device from a nearby hotel.

They apparently tried to install retroreflective stickers on the embassy's windows in order to get a better reflection for the laser.

Unrelated but highly interesting, is a CCC presentation discussing the sophisticated surveillance device inside the desktop phone of a journalist (Andy Müller-Maguhn) who was targeted for their support of WikiLeaks.  The phone, a CryptoPhone IP-19 based on the Snom 870 desktop phone, enables encrypted voice communications with other Cryptophones.

"In January 2018, Witness #2 said Morales urged them to 'place certain stickers on all the external windows of the embassy.  Specifically, he requested that I place them in the top left corner of all the windows.  The stickers were rather rigid.  They indicated that CCTV was in operation.'

'I found this strange because there had been a closed-circuit system for several years, and it didn't make sense to now have to advertise this on the windows of the embassy.  Nonetheless, during my visit to London I placed the stickers that had been supplied in the upper left-hand corner of the windows of the embassy.'

'Our American friends,' Morales said, had laser microphones that were directional and pointed at the windows.  They would 'capture all conversations.'  But according to Witness #2, since Assange used a white noise machine that 'produced a vibration in the window that stopped the sound being extracted via the laser microphone, which U.S. intelligence had installed outside.'

With the stickers placed in the upper left-hand side of each of the windows, they eliminated the vibration allowing the laser microphones to 'extract conversations.'  Witness #2 was upset Morales had not informed them of the real reason for the stickers."

Excerpt from Dr. Eric Haseltine - Moscow Station & Havana Syndrome, by Tim Ventura

"[Charles Gandy] took me aside and told me there were certain things as director I needed to pay attention too which, in his option, had fallen by the wayside over the years.  They have to do with a spycraft called "stand-off attacks."  And the stand-off attack is how to, without putting a bug or an implant in the computer or room, gather data and voice and other information from a distance remotely.


Although stand-off attacks can use implanted devices but they can also work without any implant at all.  For example, if you flood a room with a radar with a really high quality low-phase noise radar you can recover voice just from it's reflection off the human larynx.  And you can recover other kinds of signals too without having any implants.  The original stand-off attack was the Moscow embassy Great Seal bug of the late-1940s and early-1950s.  We called it 'The Thing.'"

This interview also includes Dr. Haseltine's thoughts on 'Havana Syndrome.'

Dr. Eric Haseltine is an author, futurist, and neuroscientist.  He has held several senior executive positions in private industry and the public sector.  He was the associate director and CTO for national intelligence at the Office of the Director of National Intelligence, the director of research at the National Security Agency, an executive vice president at Walt Disney Imagineering, and a director of engineering at Hughes Aircraft Company.

The Spy in Moscow Station: A Counterspy's Hunt for a Deadly Cold War Threat  by Dr. Eric Haseltine

Excerpt from Havana Tell You a Story, by I Spied Podcast

"Well Russia has a record of playing with microwaves, particualry in attacks on embassies, particularly using it as an [interrogation?] device either activating bugs or there was a theory... that I remember when I was working in the organization that you could basically throw a microwave through a building and the modulation of the waves from people speaking could be read at the other end."

Part 2 of our interview with CIA veteran Marc Polymeropoulos, where we dive into his life living with "Havana Syndrome," the mysterious illness/injury that U.S. spies and diplomats have been suffering.  Is it an attack or just a coincidence?  Marc gives us his theory and we also have chat about Ukraine, Putin and where that whole sorry mess might end.  Be sure to stand well back if you're microwaving popcorn this week!

Excerpt from the Secrets & Spies Podcast, Episode #44, "Havana Syndrome From a Technological and Medical Perspective" with Dr. Chris Gilbert and Dr. Eric Haseltine

"... The microwaves were really there, it started back in the 40s with an ingenious invention, which is you can basically, without putting any power at all in the bug, you can either put a bug in there that has no power or just use surfaces and electronics that are already in the room to recover voice from a room.  And it gets technical as to how you do that, but basically if you shine a radar at something with microwaves you can recover vibrations and you can recover changes in data in electionscs, so you can get both data and voice by flooding a room with microwaves and that's what the Russians were doning forever."

The Ending of The Conversation, by Francis Ford Coppola

How were they listening to Harry Caul, even after he tore his apartment apart - literally?

By flooding the room with a clean RF carrier, the retroreflections from the saxophone's dissimilar metals (or microvibrations) would be modulated by any nearby audio in the room.

The retroreflected RF carrier can then be received and demodulated to recover the target intelligence.

"The Thing" Block Diagram

From H. Keith Melton's CIA Special Weapons & Equipment


This is how I believe "The Thing" works.  It doesn't follow the description from Peter Wright exactly, but his book does contain numerous (intentional?) technical errors.  Spycatcher was ghostwritten by Paul Greengrass, and these could just be typographical errors.

From Electronic Design, Volume 14, Issues 14-17, by S. David Pursglove:

The diaphragm was 3 mils thick that covered the can opening.  The can measured 11/16-inch long, with an inside diameter of 0.775-inch.  The cavity inductance was 1/100 microhenry.  The entire unit, including the 9-inch antenna, weighed 1.1 ounces.  Silver-plated, high-Q resonant cavity.  330 MHz signal outside embassy in van to excite cavity.  Another van with an antenna aim at the cavity tune in on modulated signal.


A Continuous Wave (CW) RF carrier of around 800 MHz is transmitted to the cavity bug via a highly directional parabolic antenna.  This RF carrier needs to be extremely clean, with all the harmonics and spurs surpressed 80+ dBc.

The 800 MHz RF carrier enters the cavity via the 1/4-wavelength antenna probe (9.375 cm, in this case).  The high-Q, silver plated cavity is "tuned" via the adjustable "mushroom" (a 1/4-wavelength shorted stub) to parallel-resonante at an odd-harmonic frequency (3 times higher or 2400 MHz, helped by non-linearites in the threads?) than the transmitted RF carrier.  The use of higher frequencies will allow for a physically smaller cavity.  Also, a RF carrier which is 3 times higher can share an antenna which is for a frequency 3 times lower, due to the way the current is distributed in the antenna - or at least that's what I think.

But in this resonant cavity, one of the cavity's ends is replaced with a thin metal diaphragm.  The diaphragm may be made from metallized mylar, very thin copper sheet, or even gold leaf.  When sound waves hit this diaphragm, the cavity's resonant frequency will change ever so slightly.  This audio signal then frequency modulates the new, higher frequency return signal.

Design and Implementation of a Microwave Cavity Resonator for Modulated Backscattered Wave  by Ahmet Akkoç  (August 2015)  (6.3M PDF)

A thesis submitted to the Graduate School of Natural and Applied Sciences of Middle East Technical University.

In this study, the device called "Theremin's Bug" which is a passive microwave cavity resonator is analyzed in detail and a similar device is designed, simulated and implemented.  The device being the predecessor of modulated backscatter wave communication devices, RFID devices and microwave wireless sensors is invented by Russian inventor Leon Theremin in 1940s, and used as a covert spy device.  The structure is totally passive and contains no electronic component and no battery.  It uses the modulation of backscattering electromagnetic waves technique and resembles a passive microphone since it reflects the incoming wave by modulating with the ambient sound.  Since the device was a secret, there is no exact information about the technical details and principle of operation of the device.  All the information about the device is consisted of stories and tales.  And there are many technical inconsistencies about the device and working principle.  In this work, the device is studied analytically and a practical prototype is implemented.

The Thing - Creating a Model of The Great Seal Bug  Recreate your own at!

Jacek Lipkowski (SQ5BPF) builds his own operational Great Seal Bug!!!!!!!

Neil Smith (G4DBN) of Machining and Microwaves builds a working "The Thing" for the BBC!!!!!

3456 MHz Resonant Cavity Band Pass Filter

This is an example of a resonant cavity Band Pass Filter (BPF) using copper pipe end-caps and brass tuning screws.  Try replacing the PC board with a metal diaphragm, taking the RF output, as a 1/4-wave (6.5 cm) whip antenna, from a side wall, flooding it with a 1150 MHz CW RF carrier and monitoring 3450 MHz.  It might work.


Cavity Schematic

Experimental starting schematic for a homebrew Resonant Cavity Bug.  I don't have the proper test equipment or a machine shop to verify if it works, but it can be used as a starting block.

The output impedance, Zo, of this cavity is near the standard 50 ohms.  It is based on the following equation:

D1 = Cavity Diameter Outside
D2 = Cavity Diameter Inside (stub)
Zo = Characteristic (or surge) Impedance in ohms

Zo = 138 * log10 (D1 / D2)

Zo = 138 * 1og10 (0.875 / 0.375)

Zo = 50.78 ohms

To be effective, the tuning disc or slug must be placed near a high-voltage point (quarter-wavelength point) along the cavity.

A loose coupling (greater than or equal to 0.10") will have a higher insertion loss, but sharper bandpass (resonance).  Tighter coupling (approx. 0.05") will have a lower insertion loss, but wider bandpass.

In theory, to tune this device, you'll need to connect it to a RF signal generator operating at the frequency you wish to receive then adjust the cavity's tuning screw.  An example is next.  The '?'s mean that I have no idea if this is the correct procedure.

Set a RF signal generator to output a low power (100 mW?) RF signal at 2700 MHz with a FM modulation tone of 1000 Hz and 3000 Hz deviation?.  Connect this to the cavity's SMA antenna jack.  Adjust the Fine Tune screw until the cavity starts to 'howl.'?  That is, when the cavity reaches the proper resonant frequency (2700 MHz) the diaphragm should turn into a speaker, and you'll hear the modulating 1000 Hz audio tone.  Or at least I think so.

To use the bug, illuminate the cavity with a very strong & clean 900 MHz? CW RF carrier and receive the "resonanated" FM audio at 2700 MHz.  I think that might work.  Otherwise, illuminate it with a very strong & clean 2700 MHz CW RF carrier and try to recover the audio as a doppler shift (mix the outgoing frequency with the incoming frequency in a diode mixer?).

Since the cavity's antenna jack is at 50 ohms, standard antennas can be used for easier testing.


Paper Thin Bug?

A highly experimental idea is to use a Piezo speaker as a microphone and variable capacitance.  Combined with a small surface-mount inductor/capacitor tuned network and antenna, a theoretically "paper thin" bug is produced.

There are, however, numerous "bugs" to work out with this type of design though.  Piezo speakers have a fairly high capacitance (the one in the picture measured 0.067 µF @ 300 kHz).  When used in a passive-tuned circuit, this will result it a very low illuminating frequency.



Imagine using a circuit trace repair pen to "draw" the inductive and antenna elements.  This type of "bug" would be as thin as paper.


Scenes from the movie The Recruit.

Updates & Experiments

Further experimentation with homebrew microwave interferometer surveillance devices is proving to be very successful.  Microwave interferometers work by emitting an RF carrier and mixing the reflected (returned) signal with the initial RF carrier.  The Intermediate Frequency (IF) output is the difference in phase of the two signals.  If the reflector surface (say, a filing cabinet) is modulated by nearby audio, the returned RF carrier will also be modulated with that audio.  While the concept may sound complicated, most of the hardware you need (for experimenting) is found inside an automatic door opener.

Everyday automatic door openers utilize a 10 GHz "Gunnplexer" to generate this Doppler effect, which is used to detect the approaching people.  The IF output is a low frequency sine wave which is further amplified and rectified to control the door opening relay.



The beauty in using a microwave interferometer for surveillance work is the fact there is no need to plant any type of resonant cavity device.  Everyday objects can become the "resonator."  Peter Wright's Spycatcher also mentions MI5 used to develop specially-shaped everyday common objects like ashtrays, sculptures, ornaments, etc. to become resonators.  This enhanced the strength and audio quality of the reflected signal.

With the onslaught of vehicle radar systems, easily obtainable Gunn oscillators operating at around 77 GHz are becoming available.  At this high of frequency, and with a good parabolic dish, the RF beamwidth will be very narrow.  This makes receiving doppler audio reflections off individual objects - or even directly from a person's larynx - possible.

Sample Audio

This is an audio capture from a GBPPR Interferometric Surveillance Device aimed at a human larynx.

The words spoken are "Testing Testing 1 2 3" and the range was measured in inches.

Here is little information pamphlet on a similar commerical surveillance device called the 'Sabre' that uses remote RF energy (888 MHz) to 'illuminate' a remote transponder (125 kHz) which contains the target audio.  It is made by Security Research (Audiotel) in the U.K.

Here are some good millimeter wave RF application notes from QuinStar Technology.  Be sure to "read between the lines."    ;)

And here is something you don't see everyday...  A patent application explaining everything in exquisite detail:

This version appears to use a microwave interferometer consisting of a HP Model 83723B generating a synthesized 20 GHz RF signal source which then drives a HP Model 8349B 20 GHz RF amplifier.  This boosts the final RF output signal to around +20 dBm (100 mW).  The 20 GHz RF signal is also initially modulated with a 1 kHz sine wave using a HP Model 33120A audio signal generator.  The final modulated and amplified RF signal is then feed through a HP Model P752C-10 10 dB directional coupler to the Narda 639 waveguide 18 dB horn antenna.  The weak received (reflected) 20 GHz signal is then amplified approx. 14 dB using a MITEQ Model AMF-3D-000118000-33-10P low-noise amplifier.  This is then downconverted to a 1 GHz IF using a MITEQ Model SBE0440LW1 2nd harmonic mixer and a HP Model 8340A synthesized RF signal source to provides the mixer's local oscillator input.  The mixer's new IF output is amplified approx. 30 dB using a MITEQ Model 4D-00011800-33-10P RF amplifier and bandpass filtered using a 300 MHz wide Reactel Model 381-1390-50S11 filter.

A HP Model 8473C low-barrier Schottky diode detector demodulates the 1 GHz IF signal, recovering the final audio intelligence output.  The initial 1 kHz modulation tone allows a "lock-in amplifier" to be used during the final audio demodulation stage.  A lock-in amplifier, a Stanford Research Model SR830 in this case, tracks the phase of the input 1 kHz modulation tone and attempts to extract that same tone in the received audio.  This allows one to extract audio intelligence from any interfering noise.  The received 20 GHz signal would contain both amplitude and phase variations which contain the required target intelligence.  The Analog Devices AD630 datasheet contains an example lock-in amplifier schematic and sample oscilloscope photos.

Here is a fantastic email post by James M. Atkinson on the TSCM-L mailing list which describes operating similar devices in several real-world scenarios.  (Additional Info)

Here is our (fairly simple) homebrew lock-in amplifier project based around the Analog Devices AD630.

A laser version of the above type of surveillance device is described in this U.S patent application:

"A system for remotely detecting vocalizations of speech comprising: means for vibrating in response to the vocalizations of speech, said responsive vibrating means capable of being located on a throat region of a speaking person and capable of being reflective of impinging radiation; means for transmitting the radiation onto said responsive vibrating means and receiving the radiation from said responsive vibrating means, said transmitting and receiving radiation means capable of generating voltage output signals representative of the vocalizations; and means coupled to receive the voltage output signals from said transmitting and receiving radiation means for reproducing and transmitting the speech of the vocalizations from the output voltage signals."

Here are some audio extracts from an older model Decatur MV-715 Range Master X-band police radar configured to record the audio ouput straight from the speaker.

For further experimenting on using a Decatur Range Master radar to remotely intercept telephone audio, refer to this project:

Internal photos of a Decatur MV-715 & MV-724 radar gun.

General Notes / Links / Datasheets

Glottal Electromagnetic Micropower Sensors

Additional notes and links.

Related Books

Special Topics in Electromagnetics

(Excerpt from Chapter 4 - Biological Applications of Electromagnetic Waves)

Related Patents

Related Audio / Video

Other Related GBPPR Projects

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