P.a. stands for public address,you need a pa horn for this. F.m. is for frequency modulation alot of ten meter radios offer this now it's mostly line of site communication if you have somebody else with fm though it sounds alot better. A.m. is amplitude modulation it's the standard way cb's have communicated since the beginning.I hope this helps.Good luck. Dan
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P.a. stands for public address,you need a pa horn for this. F.m. is for frequency modulation alot of ten meter radios offer this now it's mostly line of site communication if you have somebody else with fm though it sounds alot better. A.m. is amplitude modulation it's the standard way cb's have communicated since the beginning.I hope this helps.Good luck. Dan