These things were called "ADSIDs", for Air-Delivered Seismic Intrusion Detector, containing VHF transmitters which were air-dropped along the Ho Chi Minh trail to monitor enemy movements as part of the large, sophisticated, complex Igloo White program.   The systems were FM in the 160 MHz range--sort of a terrestrial sonobuoy--with the antennas disguised to look like vegetation.  I suspect the frequency range was chosen so airborne sonobuoy receivers could be used.  EC-121 (Lockheed Constellation) aircraft with banks of receivers orbiting nearby relayed sensor data to a large processing center in Thailand where the data was fed to large IBM mainframes.   Attack aircraft were then dispatched to the trail according to patterns of sensor hits.  

Harris Corporation made the Igloo White sensor RF modules, and there was a display case with Igloo White components down the hall from my office there, its purpose now largely forgotten. 

There were also acoustic sensors called ACOUSIDs, and even seismic sensors made to look like bits of dog excrement known as--you guessed it--TURDSIDs.   A friend of mine got some of these in the late 70s as surplus and we decided to dissect one and test them.  The internal mercury button cells on some were still good, and when shaken vigorously, they would send out bursts of RF...

73,

John K9WT

I can’t locate the emails right now, but  there was recently some discussion of ‘vegetation – looking antennas’.

Here’s a scan from an article in ‘Vietnam’ magazine, December 2018.

-Hue Miller

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