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Broadcasting, Russian Style

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Why I'm going to jail for being excessively innovative.

I've been reading web pages about some RF devices that are interesting. The Russians bugged The Great Seal of The United States at the Moscow embassy some years ago. You can read about it here, here, and here. The method they chose was rather impressive. The design was innovative, simple, and involved no active electronics. Using sound waves striking a metal diaphragm, they were able to modulate a reflected signal. The metal diaphragm formed one half of a capacitor. As the capacitor changed in value, the resonant circuit (resonant cavity) it was attached to changed frequency. When "lit" with several watts of RF, it created an echo much like RADAR, which instead of changing frequency in response to change in direction (Doppler shift), changed frequency based on noise in the room. When researching bugs and other covert listening devices, the term "non-linear junction detector" is often cited as a way to detect bugs.

From my radio hobby, I know that non-linear junctions are the bane of radio communications. Interference to repeaters may often be found as a simple rusty bolt or a rusted tin roof anywhere in the vicinity of any of the antennas radiating signals. The product of the mixing signals is the interference. Notice the words "mixing" and "product". Any time that a signal is intentionally "mixed", several "products" result. The most common device for "mixing" RF is  -- suspenseful pause -- a "diode" "mixer".  A "diode" is by definition a non-linear junction. And interestingly enough, a diode may be made out of an evacuated glass tube.

Now, what if you take two carriers in the same band and cause them to mix together and create a signal on a known, secondary frequency?  This is astoundingly easy to do at microwave frequencies, where one carrier and another may easily be offset by a few tens of megahertz easily, yet still be within the resonance regions of the antenna attached.  By taking a signal at 10,100 MHz and another signal at 10,200 MHz, the resulting product, or heterodynes will be at 20,300 MHz, 10,100 MHz, 10,200 MHz, and 100MHz. It is necessary post-mixer to employ a filter to remove the unwanted frequencies present.  At 10 and 20GHz, this filter may be implemented in microstrip. In the same fashion, the antenna itself may be made out of microstrip, simply etched on to a piece of PC board. Or it may be constructed using conductive tape attached to points on a PC board. Mixing carriers presents it's own problems. Most mixers are low-level; to create a large post-mixer signal, the mixer must be selected or engineered to for the levels present.

By making a small PC-board which contains resonant antennas at microwave frequencies, a feed-horn of sorts may be created. It is possible to transmit two signals at one time, using antenna or signal polarities which are at opposites of each other. These opposing signals may be either horizontally- and vertically-polarized, or left-hand and right-hand circularly polarized. It is necessary to use signals at the peak of perpendicularity or orthogonality to prevent cross-talk between the mixer inputs which would otherwise subtract the product from the output of the mixer. By using vacuum tubes as a mixing device, a low-power heterodyne may be generated using some fraction of the power directed into the input of the microwave antennas. The result is a remotely-located transmitter on the frequency of interest, without presence of any transmitting equipment at that location. This has the benefit of confounding direction finding equipment. One may further obscure the location of the microwave transmitting equipment through the use of plane reflectors or metallic billboards. It is possible to push this one step further and remove the transmitter another step by using an on-channel active repeater to hide the location of the actual transmitter itself. By using the above methods and suggestions, it's possible to create a throw-away device which may be replaced or placed in a different spot each time, yet still radiate on the frequency of interest. And that's why I'm going to jail for being excessively innovative....


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