Originally Posted by gemiller
You will generally find that a second comm antenna should be mounted on the opposite side of the fuselage from the first comm antenna. If that is not possible, 4ft separation is what I would recommend. The comm radios have very sharp filters so the only real risk is jamming the receive comm with the transmit comm when they are tuned with little frequency separation ( a few MHz). I have seen dual comm antennas spaced much closer than 4 feet and I suspect for those installations the user rarely, if ever, tuned the two radios to nearly the same frequency.
My experience comes from 40 years of designing antennas for aircraft at a large aircraft manufacturing company.
Once upon a time I had an FAA repair station. A 206 Stationair drove up and asked me to check out a comm problem. It was the stock factory set-up with 2 rather robust Cessna ARC transceivers - usually got about 12 watts of transmitter power on the meter. Well, I unknowingly pulled the wrong comm cable and measured about 3 watts on the wrong antenna! Two resonant antennas can exhibit a surprising amount of coupling even at 4 feet.
So, the ~4 feet of separation of the comm antennas on nearly all Cessnas is marginally close .
I like one one the top, one on the bottom as well.