Isolators are only used on the tx side.

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An isolator is generally a 3 port circulator where one port is terminated. In some systems, such as radar, a circulator can be used to allow a transmitter and receiver to share the same antenna. It is possible to have more than 3 ports on a circulator

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You typically don’t see a circulator in “LMR.”

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You don’t need to tune into any frequency in particular. As mentioned, simply insert a power meter in the line and put a load on the other port. If you want to measure the level at a given frequency, then you’ll need additional equipment.

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As far as what to do… you can trap/stub it, add band pass filtering, or ignore it (if you have equipment that tolerates it.) Why would we do something about it? Because hams are cheap. BpBr duplexers are used, with the owners not understanding that they really aren’t “band pass” as they think they are. Or, notch only duplexers are used, or crappy rf decks. Insert a lot of reasons.

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An actual band pass filter, given adequate separation (i.e., FM broadcast/2m) can kill “all” the signal/voltage from the broadcast transmitter. Only passing the desired band, i.e., 2m, or even a narrower portion. Alternatively, you can trap the broadcast station, cleaning the line for all the downstream users, then add a band pass too, shielding your receiver from all kinds of unwanted signals.

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Matt

AL0R

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