I recently read this book, quite a story, and it had some radio content.

One of the author's friends had a job installing upgraded comms gear in helicopters because VC or NVA were intercepting or at least detecting helicopter comms with an ordinary FM radio. 

What was not clear was whether that meant a broadcast radio or an FM field radio of some kind. 

I believe helos were equipped with low band FM gear and that either:

1)captured or interoperable sets were used or

2)harmonics of the low band FM gear were detected on a relatively empty FM broadcast band.

Option 1 would give legit intercept capability, which could be defeated by encryption.  Option 2 might give enough early warning capability to either disappear or assume a defensive posture.  Presence of signal, encrypted or not, might give the necessary warning.

Encryption would require the ground pounders to be encrypted also. 

Anyone know anything that might flesh out the story?

RF