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I expected to see something along the lines of vans with directional antennas driving around Moscow listening for spurious emissions from local oscillators of s
by pps43 6y ago
I expected to see something along the lines of vans with directional antennas driving around Moscow listening for spurious emissions from local oscillators of superheterodyne receivers 465 kHz away from the number station frequency, as in Operation RAFTER.
- saffronique 6y agoInteresting, some I've some googling to do!
- jacquesm 6y agoAny radio that uses a mixer is also a transmitter.
- pps43 6y agoOnly if it's poorly designed (leaks local oscillator frequency back into antenna) and not shielded. Modern receivers use quadrature sampling detectors rather than traditional superheterodyne. In that setup any leakage would be on the same frequency and harder to detect.
- jacquesm 6y agoIt is as good as impossible to run an oscillator based receiver that is also connected to an antenna that does not radiate. That's nothing to do with poor design, it's just physics. Zero coupling does not exist in practice. By design the mixer stage sits pretty close to the initial amplifier and it will result in some of the oscillator energy making it back to the antenna circuitry. FWIW I built a ton of transmitters and radio gear in my teens, it is pretty easy to take a theoretical stance here and declare that anything that leaks is not designed properly but that's about as 'true Scotsman' as you could get.
- giantrobot 6y agoNote also with a highly directional high gain receiver that tiny bit of radiating energy is very detectable. It's just going to feed into background noise for most receivers so no one cares. But it will be detectable by a motivated hunter with the right equipment.
- pps43 6y agoA directional antenna has to be much larger than wavelength. Won't fit in a van if we're talking about HF (with wavelengths measured in tens of meters).
- giantrobot 6y agoNot[0] really[1]. Loop antennas (active or passive) have good gain perpendicular to the loop. It would be easy to mount on the inside of a wood paneled moving van with some nice receiving equipment. You park the van so the side faces the target's apartment and voilà you can listen to the mixer of their superhet radio. If your searching for receivers a couple vans driving around could triangulate and detected signals. Just getting the right apartment building would be enough for the KGB to turn the place over. [0] http://www.kr1st.com/swlloop.htm http://www.kr1st.com/swlloop.htm [1] https://www.fmuser.org/fm-receiver/receiver-antenna/DE31MS-loop-active-shortwave-antenna-BNIB.html https://www.fmuser.org/fm-receiver/receiver-antenna/DE31MS-l...
- pps43 6y agoYou can make a small (relative to wavelength) loop antenna that has a sharp minimum, but it will be very inefficient. You want your directional antenna to have a sharp maximum (high gain) so that it can increase signal to noise ratio.
- pps43 6y agoInitial amplifier does not let much energy flow backwards. More importantly, there's a lot of background noise on HF bands that will mask that weak signal.
- mhh__ 6y agoAs invented by Peter Wright of Spycatcher fame
- panpanna 6y agoIf your have not read it yet, I highly recommend this book! Peter started as a radio engineer so there first part of the book is slightly technical. Last chapters are more psychological, also very interesting.