
Wartime Radio: The Secret Listeners (1979) [video] - noyesno
http://www.eafa.org.uk/catalogue/5108
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aurizon
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.

~~~
stevetrewick
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'

[https://www.cia.gov/library/center-for-the-study-of-
intellig...](https://www.cia.gov/library/center-for-the-study-of-
intelligence/kent-csi/vol7no4/pdf/v07i4a08p.pdf)

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luxpir
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.

~~~
Steltek
Forget encryption, below the noise floor data modes like JT65 must be
ridiculously hard to detect.

Of course, what really renders this all moot is the Internet. That giant blind
drop in the sky (cloud?).

~~~
superuser2
It's not hard to imagine ISPs being ordered to shut down in a total war
scenario. Or at least block encrypted traffic.

There is no plug to pull to shut down radio communications, and active jamming
of frequency-hopping systems is hard.

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xwat
Lovely video, thanks. I would also recommend to watch Secret History of
Sillicon Valley -
[https://www.youtube.com/watch?v=ZTC_RxWN_xo](https://www.youtube.com/watch?v=ZTC_RxWN_xo)
\- to anyone interested in similar topics.

