Great find! THX and HLX callsigns. Imagine that. Mine's much more cumbersome. Hoping for something a little sleeker for the full licence.
Encryption would pretty much scupper this kind of listening nowadays. We're not allowed to use it, as licensed amateurs, but I'd put most amounts of money on the military using it. You could at least pinpoint the origins of signals, which could be useful.
My father, in WW2, was in the signal corps, and in England in those days all receivers were licensed.
Nazi spies had radios that were licensed, as they did not want their radios detected by their IF(Intermediate Frequency) radiation. Superheterodyne radios tuned to various frequencies by means of a low power variable frequency oscillator that was mixed with of off air frequency to make the IF output of the sum and difference
which were selected by tuned IF transformers and amplifiers.
They would listen for these signals and drive to maximize them to see where they were tuned. Usually the spies also had a legal radio operating in the hopes that their common IF signals would be lost in the general hubub of radio noise.
My Dad listened for the rejected signal - which was usually in the clear, although faint, with vans with sensitive regenerative receivers in them.
Once a suspicious address was found, that was handed over to a special squad who set a watch on that house, followed people and listed for transmissions. These transmissions were usually brief and once a spy was found he was analyzed in detail by groups of people who would follow him/her in a special way that passed the tracking off to another as they walked off to avoid the subject getting to know he was followed by seeing the same guy behind him all the time.
I was only vaguely aware that receiver detection was a thing but had no idea how it worked. Googling around based on some of your keywords opened it up and also produced this declassified CIA gem 'Agent Hazard in the Super-het'
Lovely video, thanks. I would also recommend to watch Secret History of Sillicon Valley - https://www.youtube.com/watch?v=ZTC_RxWN_xo - to anyone interested in similar topics.
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[ 2.8 ms ] story [ 27.7 ms ] threadEncryption would pretty much scupper this kind of listening nowadays. We're not allowed to use it, as licensed amateurs, but I'd put most amounts of money on the military using it. You could at least pinpoint the origins of signals, which could be useful.
Of course, what really renders this all moot is the Internet. That giant blind drop in the sky (cloud?).
There is no plug to pull to shut down radio communications, and active jamming of frequency-hopping systems is hard.
And data modes must be fantastic for the military, agreed. I don't know if any encrypted streams have been detected by amateurs, do you?
My Dad listened for the rejected signal - which was usually in the clear, although faint, with vans with sensitive regenerative receivers in them.
Once a suspicious address was found, that was handed over to a special squad who set a watch on that house, followed people and listed for transmissions. These transmissions were usually brief and once a spy was found he was analyzed in detail by groups of people who would follow him/her in a special way that passed the tracking off to another as they walked off to avoid the subject getting to know he was followed by seeing the same guy behind him all the time.
https://www.cia.gov/library/center-for-the-study-of-intellig...