Re: Racal Preselection & Protection unit
The MA197B was originally designed for the Royal Navy for "Common Antenna Working" ie to permit a single wideband antenna to feed a number of preselector / receivers, tuned to different frequencies. Typical use would be on board a ship, where transmitters were very close to receiving antennas, hence the "protection" part of the title. The various units would be daisy-chained together.
At the frequency to which the preselector is tuned the input impedance is 50 ohms. At all other frequencies it is very high. Thus all other frequencies will be reflected from the first preselector, and passed on via the daisy chain with quite low loss.
Bill
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