For as long as small, hidden radio transmitters have existed, people have wanted a technology to detect them. One of the more effective ways to find hidden electronics is the nonlinear junction detector, which illuminates the area under investigation with high-frequency radio waves. Any P-N semiconductor junctions in the area will emit radio waves at harmonic frequencies of the original wave, due to their non-linear electronic response. If, however, you suspect that the electronics might be connected to a dangerous device, you’ll want a way to detect them from a distance. One solution is harmonic radar (also known as nonlinear radar), such as this phased-array system, which detects and localizes the harmonic response to a radio wave.
One basic problem is that semiconductor devices are very rarely connected to antennas optimized for the transmission of whatever harmonic you’re looking for, so the amount of electromagnetic radiation they emit is extremely low. To generate a detectable signal, a high-power transmitter and a very high-gain receiver are necessary. Since semiconductor junctions emit stronger lower harmonics, this system transmits in the 3-3.2 GHz range and only receives the 6-6.4 GHz second harmonic; to avoid false positives, the transmitter provides 28.8 decibels of self-generated harmonic suppression. To localize a stronger illumination signal to a particular point, both the transmit and receive channels use beam-steering antenna arrays.
In testing, the system was able to easily detect several cameras, an infrared sensor, a drone, a walkie-talkie, and a touch sensor, all while they were completely unpowered, at a range up to about ten meters. Concealing the devices in a desk drawer increased the ranging error, but only by about ten percent. Even in the worst-case scenario, when the system was detecting multiple devices in the same scene, the ranging error never got worse than about 0.7 meters, and the angular error was never worse than about one degree.
For a refresher on the principles of the technology, we’ve covered nonlinear junction detectors before. While the complexity of this system seems to put it beyond the reach of amateurs, we’ve seen some equally impressive homemade radar systems before.
There is an urban legend that during the construction of the American embassy in Moscow, for logistical reasons local contractors were used to do the concrete work.
The KGB allegedly snuck hundreds of thousands of small diodes with their legs soldered into a loop into the concrete pours, thus scattering them all over the building and introducing so much background response that bug-finding methods like this would be ineffective.
Dunno whether that’s really true or not, but it makes for a great spy-vs-spy legend.
Sounds like a fun folk tale.
This article is super cool. I wish it were approachable at home. I could see some alternate use cases for devices like these!
Not a folk tale. A low-cost “solution” they implemented for counter-counterintelligence.
Real story.
ERP of 74 dBm is about 25kW. I’m sure the transmit duration is minuscule, but it sounds like a difficult device to construct without using restricted ITAR/EAR items.