PCS-Electronics

is there a device to put in someones car, at any frequency, so that when they are near, you will hear a different sound on your radio?. say, to monitor someones whereabouts?

ok, lets say 102

im not sure, 100-108?

it should be on fm.

I would like know what kind of transmitter could be placed in a vehicle
that could be heard from a radio in another vehicle when nearby or approaching, thus alerting someone to someones presence.

thanks