Is the RF ground in the C. Crane FM Transmitter-2 separate/filtered from the power or audio grounds?
I have a C. Crane FM Transmitter-2. I want to put it in a weather proof box up high outside. It would be easiest if I could run a single cable to it. I am thinking either using a coax cable with audio/DC combined or a balanced audio cable with an arrangement similar to phantom power. Either way, I end up tying the power ground and the audio ground together and they get tied to the shield of the cable.
I have thought about the RF ground a bit. If the short antenna on the FM Transmitter-2 wants something to push against, RF could end up on the shield of the combined audio/power cable. I could either suppress that with chokes as close to the FM Transmitter-2 as possible, or I could see if some length of coax between the board I used to split the audio from the power and the chokes gives me more range.
In the C. Crane manual for the FM Transmitter-2 they suggest putting the transmitter over a metal ground plane to make it work better. That makes me think they are counting on capacitive RF coupling between the FM Transmitter-2 and the metal surface. That got me thinking about how I could really work to get the shield of the coax to act as the other half of a dipole for the FM Transmitter-2.
I would make a circuit board the size of the FM Transmitter-2. One side would be a ground plane. The other side would have the capacitors and inductors to split audio from power on the single cable up, as well as a jack for the audio to the FM Transmitter-2 and a cable to run power to the DC jack on the FM Transmitter-2. The ground plane on the circuit board would be tied to the shield on the cable feeding the FM Transmitter-2, it would be tied to the DC ground, and the audio ground. The goal would be to get as much capacitive coupling between the board and the FM Transmitter-2. Then I would put chokes on the cable at the distance from the board that gives me the best signal. As a first guess I would think around a ¼ wavelength of the frequency I am transmitting on.
Then if I got complaints or an inspection, I could pull a unmodified FM Transmitter-2 from the box; all I would have to do is unplug the audio cable from the board, and the power cable from the FM Transmitter-2.
Part of figuring all that out depends on what the FM Transmitter-2 does with RF and ground.