Interesting. At that time it wasn't easy to build a receiver so sensitive that it could detect from distance a receiver local oscillator, which radiates only by

Interesting. At that time it wasn't easy to build a receiver so sensitive that it could detect from distance a receiver local oscillator, which radiates only by mistake (bad design, insufficient screening, ...) as it's not connected to any antenna. However I don't understand this: "By accident, one such receiver for MI5 mobile radio transmissions was being monitored when a passing transmitter produced a powerful signal which overloaded the receiver, producing an audible change in the received signal. The agency realized that they could identify the actual frequency being monitored if they produced their own transmissions and listened for the change in the superhet tone." Why would they have to do that? The monitored LO frequency already tells which frequency the receive is tuned on, by being 455 (sometimes 460) KHz below or above (usually above) the one is being received. If you know that frequency you already know which one the other receiver is tuned on. What am I missing? BTW, those values are just industry standards. When building a radio one can use whichever IF frequency they want: either standard 455KHz or 10.7MHz but also very different ones if one has the necessary parts to build filters (coils, xtals and xtal/saw filters, etc). Very old TV sets are great sources of parts that can be used: IF coils (TV IF is usually around 39 MHz), SAW filters, demodulator chips and complete tuners that could be used to build VHF/UHF receivers. For example: http://www.rfcandy.biz/communication/sprec.html http://www.rfcandy.biz/communication/sprec.html https://mekweb.eu/?lang=en&q=download-details&f=84 https://mekweb.eu/?lang=en&q=download-details&f=84