Bill Cheek's Scanner Performance Frequently Asked Questions Nov 18, 1998 SCANPERF.FAQ 11/18/1998 This article/file is for the benefit of all hobbyists. (c) 1995-98 (All Rights Reserved) by: Bill Cheek and -COMMtronics Engineering This file is for personal use only and may NOT be placed on a CD-ROM nor any other media that conveys, stores, or transports for any monetary cost including shipping & handling, without expressed permission of the author. This file can only be given away, absolutely free of charge, no strings attached. If given away, or displayed on Web sites, it MUST be intact, word-for-word with no editing, additions or deletions, except as may be authorized in writing by the author. -Bill Cheek- Author NOTE: This Frequently-Asked-Questions (FAQ) file or article is one of a series of FAQs that I regularly publish. The complete list of FAQs is given below. If this one doesn't answer your questions, try one or more of the following additional FAQs: 1. Scanner Modifications FAQ: SCANMODS.FAQ 2. Scanner Data Decoder FAQ: SCANDATA.FAQ 3. Scanner Performance FAQ: SCANPERF.FAQ <--- This FAQ 4. Scanner S-Meter FAQ: SCANSMTR.FAQ 5. Scanner Radios FAQ: SCANRADS.FAQ 6. Administrative FAQ: ADMIN.FAQ You can download the latest of any of my FAQs directly from my FTP site as follows: ftp://ftp.cts.com/pub/bcheek/faqs/ ============================================= Bill Cheek's Scanner Performance Frequently Asked Questions File ============================================= Contents ========= 01: What is the most important ingredient to enjoyable scanning? 02: So what do I need to know about antennas? 03: So what do I need to know about coaxial cable and feedline? 04: How can I improve my reception capabilities? 05: Can my scanner be interfaced to a computer for remote and automated control, as well as for programming of the memory channels by the computer? Can the interface acquire data from the scanner and send it to the computer? 06: Will a preamplifier be helpful to pulling in weak signals? 07: Can my scanner be tuned/adjusted for improved performance? 08: What effect does an earth ground have on my station performance? 09: What else affects the performance of my station? 10: What is a "decibel" (dB) and what does it mean? Questions and Answers ===================== 01: What is the most important ingredient to enjoyable scanning? There are three vital ingredients to successful scanning: A. The antenna B. The coaxial cable (feedline) C. The scanner Let's put it into perspective. If I had $500 to put into a scanner monitoring post, I would invest $400 of it in the antenna and coaxial cable system, and $100 in the scanner. Don't take that too literally, but give the principle some deep thought. The antenna and feedline make all the difference; not so much the scanner. But a lot depends on your interests. A $500 scanner and a 12" piece of wire sticking out the antenna jack will pick up a lot of local signals and you might well be "happy" with that situation. If you want to "hear all there is to hear" and more, then your first concern is the antenna and feedline. So long as you have a scanner in the first place and so long as it does what you want it to do, then the antenna and feedline are the most important elements of your station. ================================================================== 02: So what do I need to know about antennas? You are kind of fortunate, unless you're a ham, SWL'er, or a CB'er. These guys need to know a lot more about antennas than a scannist. Still, there are some crucial things to know. Antennas seem unimportant to the scannist because there are few from which to choose, and anything from a bailing wire to a rubber duckie to a piece of aluminum seems to work just fine. And it does, but Catch-22 is the "why". One of the characteristics of scanner signals (30 MHz VHF and up) is that they're "line of sight" for the most part. And line of sight means strictly a local geographical area. So it's easy to be deluded into thinking you are hearing all there is to hear. And so your antenna seems good enough and not worth any further ado. Again, this is okay if your interests are casual and limited to local signals. But here, in as few words as I can put them, are the hard core, brass tacks facts about antennas as related to scanning. The Scanning Spectrum ===================== Scannists typically are interested in all signals between 30 MHz and 1300 MHz or higher. Antennas typically are "resonant" and very narrow banded. That's why you see "antenna farms" around serious ham shacks and SWL posts. It takes a separate antenna for each narrow interest, to put it mildly. In the scanning spectrum of 30-MHz to 1300-MHz, there are no less than two dozen separate "interests", so imagine having 24 or more antennas on your house. Fortunately, most scannists get by with a single antenna that has a special design to make it cover that full spectrum. That coverage comes at the expense of performance, however. Here's why: Antenna Characteristics ======================= Antennas do to radio waves what glass and reflective surfaces do to light: they capture and focus radio waves. A telescope captures and focuses light with a comparable result in antennas that we call GAIN. Like in telescopes, the higher the power or GAIN, the narrower the field of view. In a word, look though a window glass for a wide field, or through a telescope for a narrow field. The field of view in a telescope can be roughly compared to the spectral bandwidth of an antenna. The wider the bandwidth of an antenna, the lower its GAIN or power. Scanner Antennas ================ Antennas for scanners are wide banded at the expense of no GAIN. This applies to all scanner antennas, regardless of what they are called. The most commonly used antennas for scanners are "discones" for base stations and "rubber duckies" for handhelds. Both are very widebanded and very low GAIN. But they work. Sort of..... In addition to discone antennas, there are a few other kinds from various manufacturers, but you have to go by their trade names because there isn't a good generic word for them like the "discone". An example of the discone is Radio Shack's #20-043. Examples of "other types" are Radio Shack's #20-176 and #20-014. Discone Antennas ================ In addition to physical shape, discone antennas are characterized by an extremely wide bandwidth and a relatively "flat" GAIN of about 0-dBi, in other words, no GAIN, but a bandwidth of several octaves. "No gain" doesn't mean the antenna won't work. All it means, to use an optical comparison, is a view similar to a 3-dimensional, 360-degree "fish-eye" lens...that sees all directions within a sphere equally well. The human eye, in comparison, has some gain since it sees roughly only a quarter of a sphere with much better resolution of distant objects than a "fisheye" lens. Well, a discone antenna receives on a par with a theoretical "point" antenna that is given to receive equally well from all directions within its sphere. This point antenna is called an "isotropic" source, and does not exist in reality. Its reference GAIN is stated to be 0-dBi (zero decibels with respect to isotroptic). All GAIN figures of antennas should have a reference (gain over what?), and the theoretical "isotropic" antenna makes one such good reference. Thus, a gain antenna might be stated to have 5-dBi GAIN, meaning 5 decibels of gain over an isotropic antenna. (See discussion ahead on "decibels". Discone antennas have 0-dBi (or less) gain. Other Scanner Antennas ====================== Some antennas, like Radio Shack's #20-176 and #20-014 are actually several optimized antennas packed into one. #20-014, for example, combines three antennas; a short UHF stub, a longer VHF-Hi element, and the longest VHF-Lo element. The three optimized centers appear to be around 450 MHz, 160 MHz, and 50 MHz. Considering that any antenna has "overlap" on either side of its optimal center frequency, this #20-176 antenna can be said to cover 30 - 1300 MHz. What they don't tell you about these "combination" antennas, is that there are DEEP nulls or pits across their spectral bandwidth. For instance, the #20-176 antenna might actually perform worse on the military aero band (225-400 MHz) than a "flat" discone antenna, but it may beat the tar out of a discone on the 150 MHz Land Mobile band. Antenna Specialists, Inc., Channel Master, and Austin Antenna are makers of various styles of these "other" antennas. In general, these "other" antennas cost a lot more than discones, but do not provide overall better performance.....just on selected bands, where GAIN might be upwards of +3dBi, and in the off-bands, it might be as low as -6dBi. This isn't a fraud or even a fault; it's just a fact that scannists tend to monitor four key bands: 30-50 MHz 108-176 MHz 400-520 MHz 800-900 MHz So if a manufacturer can optimize an antenna for these four bands and prove that it outperforms a discone in those bands, then you might well hear a lot of hype, hyperbole, and hoopla about it. If you are interested in what goes on between these primary interest bands, say 72-76 MHz, 178-400 MHz, 520-800 MHz, and 900-1300 MHz, then these "other antennas" might well prove disappointing. Directional Beam Antennas ========================= Scannists typically want to hear all there is to hear in a 360-degree circle (azimuth). So the discones and "others" as discussed above are the predominant choices. These antennas are called "omnidirectional" because they receive equally well from 360 points around the compass. There are, however, other kinds of antennas that work much like a telescope, where they capture lots more signal from one direction and much less from all others....exactly like a telescope! These antennas are called directional beams, and resemble outdoor TV antennas in appearance and in how they work. Probably the premier directional beam antenna for the scannist is called a "log periodic". It is very wide-banded, as a scannist needs, but it also offers two other hot parameters: GAIN and REJECTION. We have already discussed GAIN and compared it to a telescope. REJECTION is also an important parameter. Not only can you use a telescope to see better what you want to see, but you can also use it to NOT SEE what you don't want to see! This is a rare application for telescopes, but a neat trick in scanning where nearby strong transmitters often interfere with more interesting, but weak and distant signals! How often have we complained of "pager interference" and intermod? Well, you can use a directional beam antenna to REJECT undesired signals from one direction, often without degrading a desired signal from another direction, depending on how you point it. Log-periodic antennas are expensive and sometimes mechanical nightmares to erect and keep erected. There is an alternative. Think about it: TV ANTENNAS!!! Why not? TV antennas are low cost, highly directional beams, designed for 54-88 MHz, 88-108 MHz, 174-216 MHz, and 470-890 MHz. Thanks to "overlap" effects, they also work fine between 30-54 MHz, 108-174 MHz, 216-470 MHz, and 900-1300 MHz; a lot better than discones or "other" antennas. TV antennas employ a combination of log-periodic, yagi, and folded dipole arrays to achieve high GAIN; excellent null rejection, and light-weight. The primary Catch-22 in using a TV antenna as a scanner antenna is to mount it in the vertical plane, and NOT horizontal, as TV antennas are deployed. Here is a diagram: NORMAL TV ANTENNA MOUNT FOR SCANNING boom | ------O------ | elements | elements | extra | regular | boom | mast boom --> O=========X mount | | 2-3 ft | | | | | | | mast | | | | | Turned 90-degrees! The subject of antennas can go on and on, but for the scannist, it needn't get too deep. If you want some plain, solid advice that involves least time, trouble, and cost, then put up a discone antenna for base stations. Radio Shack's new #20-043 appears to be a good one. Two known good discones include the Icom AH-7000A and the Diamond D-130J, both available from radio stores everywhere. If you have specific needs, then an "other" antenna might be the ticket for you - also sold by radio stores everywhere. If you just have to reach out to the hinterlands and boondocks, then you have no choice but to put up a directional beam. A TV beam antenna offers the most bang for the buck and still keeps it simple. For handheld scanners, you may as well stick to the supplied rubber duckie antenna, at least when you're hoofing it about town. I hear about so called "hot-shot" special design rubber ducks, but just between you, me, and the doorpost, save your money. There are limits to what can be received at tabletop or waist-high level, no matter how good the antenna. But see Question #4! =================================================================== 03: So what do I need to know about coaxial cable and feedline? A. Don't use RG-58, RG-59, RG-8x, or RG-8M under any circumstances. These cables are too lossy. Up to 90% and more of the signal can be lost in these cables before the signal ever gets to the scanner. You don't need such losses! B. Don't feel like you have to use the buck-a-foot cables, either. There is a point of diminishing returns where cost escalates out of proportion to any savings of loss. The compleat scannist might opt for Andrew LDF2-50A hardline coax, but at over two bucks a foot and $40 or more for each connector, there is usually no point in such extravagance. If there is a point to it, you'll know. If there is any doubt, then there isn't. C. Most scannists do just fine with RG-6 satellite cable. It's cheap; it's low-loss; and it's well shielded. Furthermore, RG-6 takes cheap connectors, even the ideal gold-plated ones are cheap enough. Just put a gold-plated Type F connector on each end and then get the appropriate Type F-to-whatever adapter to mate with the antenna and the back of the scanner. D. "Purists" may want to consider the 70-cents-a-foot Belden 9913 coaxial cable. It's extremely low loss and connectors for it aren't all that costly. 9913 can be worth it, but the difference between it and RG-6 isn't all that marked for most scannists. E. Make sure all connectors are properly installed. Connectors are the weakest link in any radio station. In summary, coaxial cable is a strong point of focus for the serious scannist because it is the single place where for a modest expense and a little time, losses can be converted to gains. To recover 2 or 3 dB of losses in an old cable is like adding 25% to your range of coverage! If you want a rule of thumb, use RG-6 Satellite cable with gold-plated connectors and adapters to keep the losses to an acceptable minimum. See next question for more info on this subject. =================================================================== 04: How can I improve my reception capabilities? Easy - maybe. Just increase the height of your antenna, no matter what kind it is. Each 10-ft increment of added height can yield an apparent "gain" of up to 3-dB. For scanner signals, 3-dB can be all the difference between good reception and none at all! There is no substitute for height of the antenna when trying to receive signals outside your immediate geographical area of more than a few miles. VHF and UHF signals weaken dramatically as they travel along the surface of the earth (groundwave or plane-earth wave); much more so than the spacewave that travels a couple of wavelengths or more above the earth's surface. Antennas at low heights intercept the spacewave of only very close transmitters, and the groundwave of all others. Yes, the quality and type of antenna makes a difference to a station's reception capabilities, but the single most important factor is the height of the antenna, regardless of its quality and type. Next to antenna height, the quality of the coax cable is an important variable. Just about any coax is fine for 10-ft runs or less. If over 20-25 ft, then use RG-6 satellite cable unless you want to go whole-hog on Belden 9913 or Andrew LDF-2A hardline coax. In no case, should you use RG-58 or RG-59 cable except on very short runs, and not even then???? Signal loss in those two cables makes the difference between hearing and not hearing, compared to RG-6 satellite cable. Do NOT pay any mind to the prevailing opinion in some quarters that 75-ohm coax is too lossy because of the impedance mismatch. That's foolishness. 75-ohm coax connected to a 50-ohm load results in a 1.5 : 1 VSWR, for a 4% signal loss (about 0.1-dB), hardly worth a bead of sweat compared to the natural 25%-50% losses in RG-58 and RG-8 cables. Antenna height and good quality coax cable will help you hear all there is to hear from your location. ============================================================== 05: Can my scanner be interfaced to a computer for remote and automated control, as well as for programming of the memory channels by the computer? Can the interface acquire data from the scanner and send it to the computer? In a word, yes. Many scanners "can" be interfaced to a computer for one-way control and automated programming of the memory channels. These scanners include: PRO-2004 PRO-2005 PRO-2006 PRO-2035 PRO-2042 PRO-2041 PRO-2022 PRO-34 PRO-37 PRO-39 PRO-43 PRO-26 PRO-64 Probably quite a few others are possible. There are three scanners that are capable not only of the one- way mode of remote control and automated programming, but also of a "data acquisition" mode where the scanner sends its display (channel, freq, bank, mode, delay, lockout) and other data to the computer for automated logging and data processing. These scanners are the PRO-2004, PRO-2005, and PRO-2006. Several companies make scanner/computer interfaces, including Optoelectronics and COMMtronics Engineering. Each interface has its own unique characteristics, advantages, and limitations, but all are "good" in the sense of freeing the operator from much of the labor and drudgery of scanning. The Optoelectronics interfaces are generally limited to the PRO-2005/6 & PRO-2035/2042. A "roll your own" computer interface was published in the World Scanner Report, V2N1-V2N6. Another more powerful one was published in my 3rd book, THE ULTIMATE SCANNER (Cheek3). COMMtronics Engineering's CE-232 Scanner/Computer Interface is in its fifth year and continues to be a front-runner and state of the art. For more info: http://ourworld.compuserve.com/homepages/bcheek/ ftp://ftp.cts.com/pub/bcheek/ce-232 John Montalbano KA2PYJ, seems to have designed a nice one-way interface and control software. I can't vouch for it, but go to the following URL and see for yourself. http://www.qsl.net/ka2pyj/ ============================================================== 06: Will a preamplifier be helpful to pulling in weak signals? Theoretically, yes. And yes, according to some scannists who laid out their money on the hope and promise. In actual practice, no. At least, not like you'd think or hope. Yes, a properly designed preamp can make all the difference in the world, so long as it's mounted correctly and used with discretion. But that's the rub. Preamplifiers are a lot like antennas, where performance can be utterly awesome over a narrow bandwidth. Specific uses for preamps include satellite monitoring where the signals are very weak in the first place. Satellite aficionados use antennas specifically designed for the bands of interest, and low noise, high gain preamps designed for those bands. That's the rub for the casual scannist. A preamp designed for 30 MHz - 1300 MHz simply cannot perform well because it isn't and can't be optimized. It may have high gain, but it will also have high noise, resulting in little or no change in the overall Signal-to-Noise ratio, and sometimes even degraded! There are a number of complex reasons for this, and we won't get into them in this FAQ. Your best bet is to avoid wasting your money on preamps until you have a specific application and are willing to make some compromises with your station setup and the convenience aspects. Most scanners are designed for all the gain the front end can handle anyway, and the addition of a preamp will cause all sorts of havoc and consternation. For example, I had one fellow call me over to his station to demonstrate his hot-dawg preamp and how much it enhanced his station. Hah! In short order, we determined that he was hearing things that weren't really there. The preamp ruined the dynamic range and noise figure of his otherwise decent scanner, and allowed signals from other bands to come in as if they were on the displayed frequency. BEWARE video and TV preamps! These animals are designed for strong signals in the first place, since TV pictures become noise-free (speckle-free) only with signals approaching 1000-microvolts. Note that a scanner signal should be noise-free at 1-microvolt! TV and video preamps just don't deliver for the discerning scannist. There may be exceptions. There always are..... If you have a well designed receiver with a hot front end and if you use a GaAsFET preamp, you may very well hear some signals better with it than without. You will also hear more pseudosignals and interference. So it pays to save your money until you know for sure what you're dealing with and how to apply more esoteric information than can be presented here. ============================================================== 07: Can my scanner be tuned/adjusted for improved performance? Perhaps, but probably not by you. Most scanners have a dozen or more internal adjustments, that if any one is mistweaked, the performance can be drastically degraded. DO NOT ADJUST ANYTHING unless you know what you are doing and have the proper equipment at the ready! I have known some scannists to put their ear close to the speaker and then tweak things one way and another in hopes of getting signals to pop out of the noise. Forget about it; it ain't gonna happen; not unless something is wrong in the first place, but it will take a lot more than an ear to make any adjustment "right". For one thing, the latest crop of scanners have fewer adjustments and those are very critical, usually computer tweaked at the factory. Unlike older equipment, modern adjustments don't drift out of "whack". They're good for the life of the scanner. The last scanner I know of that had any drift and misadjustment problems was the legendary PRO-2004, a hybrid of old and new technologies with some factory errors thrown in for good measure. I can usually get better performance out of any PRO-2004 that comes across my bench, but this example is the exception to the rule. Don't tweak your scanners unless you know how and have the right test equipment. ==================================================================== 08: What effect does an earth ground have on my station performance? For above 30 MHz? None. Or nearly none, anyway. The farther below 30 MHz you go, the more effect grounding (or lack of) has on performance of the station. By and large, scannists ground their antennas and chassis of all equipment in the station for the sake of personal safety and some protection against lightning. Safety is enhanced in the event a 110-vac wire ever touches the chassis of the equipment because a fuse or circuit breaker will blow, thereby protecting the operator from shock. Lightning protection is enhanced by virtue of Ohm's Law. Electrical damage is caused by the square of the current multiplied by the resistance of the path. Well, an ungrounded antenna might actually have a 50 ohm path to ground without you knowing it. So here comes a strike and 1000-amperes flow through that 50 ohms. Guess what? Fifty million watts of power is dissipated in a microsecond or two, with the effect of a monstrous flash-bang bomb and resultant terror and havoc. Conversely, say you have a zero-ohm path to ground, thanks to good grounding, then that 1000-amperes times zero ohms equals zero watts. No damage! Well, there is no such thing as a zero-ohm path to ground, but the lower the resistance to ground from the tip of the antenna, the less chance there is of damage to your station. A number of years ago, I had a 100-ft communications tower in Missouri where I saw lightning strike it many times. Never was there any damage to the station, even though one time I was on the microphone when a bolt hit. Balls of St. Elmo's Fire erupted out of the equipment rack and skittered over the desk to the floor where they danced around and dissipated. There were scorch marks across the surface of my desk, but the station equipment was unscathed!!! When I finally took the station down to move to Colorado, I found where the coaxial cable wouldn't pull out of the tower. Curious, I followed it along its length and found several spots where surges had erupted through the outer insulation and welded the copper shields to the tower legs!!! The lightning had improved its own ground!! Some time later, I learned that professional installations purposefully grounded the shields of their coax cable at 10-ft or 20-ft intervals down the tower. Can you say I was a "believer"? A "good" hobbyist grounding system consists of two or more 10-ft 1" dia copper water pipes driven into the ground. They "drive" fairly easily with the ground end sharpened somewhat and a cap placed over the driven end. I drill 1/8" holes at 6" intervals down the pipes so as to let me water the soil at regular intervals to keep conductivity high and resistance low. NEVER ever use salt and other compounds or mixtures in a vain attempt to "improve" the ground. Any improvement will only be temporary until corrosion sets in. Corrosion is a high resistance to the point of even being a good insulator. You don't want that. Copper water pipes are superior to normal ground rods by virtue of their diameter and cross-sectional area that contacts the soil. Contact area of a ground rod is equal to circumference of the rod multiplied by the length. The circumference of a 5/8" ground rod is 1.9635 inches whereas for a 1" copper pipe, it's 60% greater at 3.1416 inches. This means at least 60% lower resistance to earth! The two or more pipes should be interconnected with #6 solid copper wire or 2" wide solid flat copper ribbon conductor. Mechanical clamping is preferable to soldering for reasons we'll not get into here. A single #6 solid copper wire should enter the station by the most direct route with no sharp bends or kinks, and be ran along the length of the station desk or console in the rear. Then the chassis of each equipment can be connected to this copper bus by short lengths of #14 solid copper jumpers with alligator clips on one end instead of permanent connections. The other ends of the jumpers can go to case or chassis screws of each equipment to complete the station grounds. A single #6 solid copper wire should also go from the ground rods up the tower or mast and be securely bolted to not only the mast, but also the grounded portion of the antenna mount. This grounding system will not intrinsically protect you against the effects of lightning, (nothing will do that), but it can reduce chances of harm and damage. And in some cases, it can enhance performance of the station. =================================================================== 09: What else affects the performance of my station? That's it. I think we've covered it all in a nutshell. Oh sure, you can find all sorts of goodies to add to the scanning post, but the things that distinguish the men from the boys are pretty much limited to: antennas; feedline; preamps (or lack of); a properly tuned and operated scanner; and a decent earth ground. The rest is incidental and subject to your unique interests and preferences. I will suggest that computer automation offers the power and the force to ease a lot of the drudgery so as to free you up to attend to the antenna, feedline, and earth ground details. A directional beam antenna offers the added power of REJECTION to your arsenal for pulling in those elusive, distant signals. Concluding with the fact that there is no substitute for height of the antenna, you really do have it all now, in terms of what makes or breaks a radio station. ============================================================= 10: What is a "decibel" (dB) and what does it mean? A decibel is one-tenth of a bel, named after Alexander Graham Bell, inventor of the telephone, and an early pioneer in the study of power ratios. The "bel" unit is rarely used now, but the "decibel" is as common as fleas on a junkyard dog, so let's take a look into what it means and how to use it. A decibel is a ratio; nothing more; nothing less; an expression of one power level to another. Mathematically, the decibel is expressed as: dB = 10 Log (P1/P2) where: dB = decibels Log = logarithm to the Base 10 P1 = the first level of power (watts) P2 = the second level of power (watts) For example, let's say that a power smplifier has a 1-watt input and a 10-watt output. What is the gain of that amplifier in decidels? dB = 10 Log (10/1) = 10 Log 10 = 10 (1) {The logarithm of 10 in the Base 10 is "1"} = 10 db Gain As another example, suppose an antenna has 3 dB gain. What is the ratio of the input signal to the output? (Input is the free space wave power and Output is the power into the coax.) 3 dB = 10 Log Pr (where Pr is the Power Ratio) .3 = Log Pr (divided both sides by 10, so now we know that 0.3 is the logarithm, so next we take the antilog of 0.3) 10E(0.3) = 1.995 (10 raised to an exponent of .3 = 1.995) In this example, we find that 3 dB of gain is equal to a ratio of double or "2" (rounded off). In fact, an antenna with 3 dB gain doubles the signal power into the coax. Now let's look at one more: Suppose we have an antenna with 6 dB gain. What is the ratio of the input signal to the output? 6 dB = 10 Log Pr (where Pr is the Power Ratio) .6 = Log Pr (divided both sides by 10, so now we know that 0.6 is the logarithm, so next we take the antilog of 0.6) 10E(0.6) = 3.981 (10 raised to an exponent of .6 = 3.981) In this example, we find that 6 dB of gain is equal to a ratio of quadruple or "4" (rounded off). In fact, an antenna with 6 dB gain quadruples the signal power into the coax. Here we have established a powerful layman's rule of thumb for working with decibels. Every increment or change of 3 db is a factor of DOUBLE or ONE-HALF, depending on whether we're talking GAIN or LOSS. And 10 dB is a factor of TEN (gain) or ONE-TENTH (loss). Say your coax has a 10 dB loss. That means down at the receiver, the signal is 10% of what it was at the input to the coax! If it has a 12 dB loss, then the signal at the receiver is 1/16 of the input. A 1 dB loss is about 20%. 2 dB loss is about 37%. 3 dB is a 50% loss. 6 dB is a 75% loss and 10 dB is a 90% loss. Get the picture? Nope, not yet you don't...... Read on, MacDuff...... Polarity. There is a difference between +3 dB and -3 dB. The first is gain while the second is loss. OR IS IT? You have to be careful how you work, talk, and think with signed decibel numbers. Look, a -3 dB loss is really a +3 dB gain, and a +3 dB loss is really a -3 dB gain. Remember my amplifier example way up above? Well it had a 10 dB gain. What I didn't tell you there is that it also has a -10 dB loss. So it can get confusing, to say the least. But it needn't. Suppose your net worth is ten bucks. Then your net obligations are exactly -$10. On the other hand, if your net worth is -$10, then you owe ten bucks. So a lot depends on how you express what you're talking about. By and large, we want to stick to talking about GAIN where a positive number means a multiplication and a negative number means a fraction. Usually, we talk GAIN in antennas and amplifiers, but we talk LOSS in coax cable, attenuators and radio paths. So you are going to see positive and negative numbers everywhere when working with decibels. Get used to it. But there is some sense... Ferinstance, say a 20-mile radio path has a loss of 120 dB. That's saying the same thing as a GAIN of -120 dB. Common sense says to refer to loss for things that normally have a loss. And refer to gain for things that normally have a multiplication factor. Antennas and amplifiers are usually multipliers. In a word, "loss" almost always means LOSS. But GAIN can mean a multipler or a loss, depending. You'll quickly get used to it if you relate to money, net worth, and net obligations. It will make sense after a while. You'd ordinarily not say, "I'm worth -$10." It makes more sense to say you owe ten bucks. Likewise, you'd not usually say you owed -$10 when you are worth ten bucks. ============================================================== That's it for now. I will keep this FAQ updated. If you see things that need to be added to it, by all means, let me know. If this FAQ fails to address your question(s), then hit me with 'em, but be specific. And be detailed with symptoms and observations if you need tech support on mods or troubleshooting and diagnosis. Use e-mail if you can. Copyrighted (c) 1998 (All rights reserved) Revised 11/18/98 ------------------------------------------------ -- Bill Cheek - President/CEO/Publisher \ / COMMtronics Engineering/World Scanner Report \ / Box 262478 ~ San Diego ~ CA ~ 92196-2478 \ / Voice: 619-578-9247 (1:30-5:30pm, PST, weekdays \/ Fax: 619-578-9247 (any time) /\ E-mail: bcheek@cts.com / \ ftp://ftp.cts.com/pub/bcheek / \ http://ourworld.compuserve.com/homepages/bcheek / \ ------------------------------------------------ -- ========================END OF FILE===========================