I'm not sure what your question is but I think you are asking: If both an FM and an AM transmitter on the same channel are on, why is there interferance between the two? It is because they are both sending out the same frequency RF so either receiver on that channel will be effected. The FM receiver will not respond properly to the AM signal but it will respond but erratically. Likewise, if an FM signal is being transmitted on the same frequency that your AM reciever is tuned to, it will respond erratically but--it will respond, just not in a useable manner.