Bodypilot, AM and FM radios are functionally identical, the difference is how the transmitter information is sent to the receiver. Kind of like a language. This answer can get very technical, but to make it simple, it is just two different ways of accomplishing the same. Both types have their pro`s and con`s. Because the F.C.C. regulates radio emissions in the U.S., the transmitter has to have a sticker attached that means the radio is "accepted" for use in the U.S., the so called "Gold-Sticker". If the radio does not have it, you may want to have a certified radio shop check it and attach to it the acceptance-sticker, this for a price ( $$ ). The AM radio will do the job the same way as an FM radio, but older AM radios may not be "legal" anymore. Most FM radios are OK F.C.C. wise. The best thing to do is to buy a new radio, AM or FM, or make sure the used radio is Gold-Sticker`ed. JT