[Sean Williams]

Hi Colin,

This is inserted in the antenna lead before a receiver - it is another tuned circuit which helps keep unwanted signals out of the RF section of the receiver.

Quite a useful bit of kit, and will also possibly contain a quantity of amplification, which is sometimes also helpful on the higher frequencies.

The protection unit function comes into its own when you are trying to operate a receiver on a multi-transmitter site - the extra rejection of out of band signals really saves the receiver, especially if there are several tens of volts of stray RF floating around.