Mainly used in command vehicles (ACV and possibly LCV) where you had multiple sets and needed to prevent breakthrough from one into the other[1].
Basically it's a tuneable notch filter, so only good for blocking a single interfering signal.[2]
There's some information (not a lot) in Wireless for the Warrior Volume 2.
Chris.
[1] e.g. keeping (or reducing) the output of your ET-4336 out of your WS19 front end. (A difficult job, given the relative power levels involved.)
[2] Modern practice is to use an "acceptor" unit ("SURF" - Selector Unit Radio Frequency) to reduce mutual interference, and low-pass filters to keep harmonics from the HF transmitter(s) out of the VHF receivers.
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