I’ll point out that a common method of detecting bugs at the time was to set up a radio receiver with a speaker and then sweep the frequency, if you managed to hit you would get a feedback sound between the speaker and bug. These oddball modulation schemes would prevent that from working.

I like it a lot that many of techniques have a hybrid analog/digital structure that would involve sample-and-hold, sweeps and comparators like the Triple Pulse scheme.

Today I can’t believe you wouldn’t use some digital solution but at that point in time you’d be lucky to be able to use a small IC.

Also of interest in this domain.

The great seal bug(The Thing)

https://www.youtube.com/watch?v=NLDpWrwijE8 (Machining and Microwaves)

What I like about this specific video is that the guy actually builds one. And there is a world of difference between a popular article on how the thing worked. and the subtle genius engineering required to get it to actually work.

Today with spread spectrum, it's probably much easier to hide a covert radio signal.

it would be interesting to see what the waterfall charts of these looked like, and I can't tell if there is enough info in the article to produce a gnuradio flowgraph for any of them. it could be a fun retro spy tech project.

In practice most audio channels are low-pass filtered and bandwidth limited, so I'm guessing that these modulation techniques are not going to work. Also, we have digital methods now.

Oh crap... the Scanlock Mark VB receiver shown on that picture is really similar to the Autolock 7 receiver I snatched at a flea market for a song many years ago, and after finding absolutely nothing about it online sold it on Ebay for like 3 songs. Had I known it was a bug finding device I could have donated it to the Cryptomuseum.