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At THIS site is a price list of various items including two that would be of interest in combining an AM and FM low power transmitters.
The AFBC1 is similar to a 3 way splitter, but is specifically for the AM and FM bands. Simply use it in the opposite direction..ie instead of two antennas combining to feed one coax to a receiver, tie the AM and FM TX’s into AFBC1 and the combined signals will output on the single output port..feeding your common wire antenna.
The AFMBS1 is similar in function to the combiner, with the exception that it actually separates the two bands. Connection would be the same as with the AFBC1.
Using these would be the way to go to combine your AM and FM TX’s into a single antenna.