[pps43]

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]

Interesting, some I've some googling to do!

[jacquesm]

Any radio that uses a mixer is also a transmitter.

[pps43]

Only 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]

It 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]

Note 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]

A 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).

[pps43]

Initial 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.

[jacquesm]

'not much' == 'some'.

If you've never built a radio and tried to shield this unintended export than I can totally see how you might think this is just a matter of careful design and more shielding but it really is a lot harder than that and you will simply never reach zero to the point where even an ordinary spectrum analyzer hooked up to the input of your radio will not show the oscillator frequency as a nice fat peak.

[mhh__]

As invented by Peter Wright of Spycatcher fame

[panpanna]

If 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.