This is a simple, little battery operated device to "jam" those poorly designed UHF repeater systems which are commonly used by the security guards (Rent-A-Cops) at shopping malls. The vast majority of these UHF repeater systems operate between 450 MHz and 470 MHz and use the "plus 5 MHz offset access system". What this means is, the repeater's INPUT frequency is exactly 5 MHz higher than the repeater's OUTPUT frequency. This device uses this fact to mix (with a Mini-Circuits ASK-1) the repeater's own OUTPUT frequency with a 5 MHz clock oscillator signal to generate a new signal, which will be equal to the repeater's INPUT frequency.
When this device is placed near the repeater's antenna system, the repeater will essentially "jam" itself. The next time the repeater is accessed, the repeater's output signal will "mix up" to hold the repeater open - rendering it unusable. This is a very good prank to play on those asshole mall security guards who are always hassling you.
The main drawback is that the jammer needs to be placed fairly close (within about 50 feet) to the repeater system's transmit & receive antenna. Most repeater systems will use the same antenna for both transmitting and receiving, but on some systems, the antennas are separated to improve coverage. You may need to throw the jammer device on a rooftop, or in some nearby trees (best) to be effective. Practice makes perfect! For the antennas, nothing fancy is needed. 6 inch pieces of #30 gauge wire make good, easily hidden, 1/4-wavelength antennas at around 460 MHz.
Repeater systems which do not use the "5 MHz offset system" are also susceptible to this type of jamming. You'll just need to generate a clock signal which is equal the repeater's required offset frequency. For example, 2-meter amateur radio repeater systems normally use a 600 kHz input offset. To generate a 600 kHz clock signal, take a 6 MHz clock oscillator and divide its output by 10 with a 74LS90 counter. Everything else remains the same, with only the antennas changed to 19 inch long pieces of wire.
This type of mixing jammer is not effective against repeater systems which output (transmit) a different Private Line (PL) tone than the one required for the input (receive). This is done, in fact, to prevent this type of jamming.
Datasheets & Notes
- 5 MHz Full Size (TTL/CMOS) Clock Oscillator - Mouser Stock No. 520-TCF500
- 6 MHz Full Size (TTL/CMOS) Clock Oscillator - Mouser Stock No. 520-TCF600
- 74LS90 (DIP14) Decade Counter - Mouser Stock No. 526-NTE74LS90
- UHF repeater systems which operate between 470 MHz and 512 MHz tend to use a 3 MHz input offset.
- Mini-Circuits ASK-1 Mixer (220k PDF)