[stackj]

Mine is not pressurized but I do have a marine radio installed that I use for the Coast Guard Auxiliary.

In my case, I had my avionics shop install an antenna on the belly and submitted the appropriate Form 337 for that installation.

My radio plugs into the cigarette lighter, so there was no installation paperwork required for that (It is considered portable).

With your pressurized model, you will have to work with someone who is authorized to penetrate the pressure hull if you plan to run an additional antenna cable through it. This requires either instructions from the manufacturer for hull penetration or a DER/FAA approval.

Some of my friends who also fly for the CGAux use an internal antenna and just place it a window. We seem to have pretty good luck with that arrangement.

If your marine radio is 12 volts, you can get a power supply that will still plug into your cigarette lighter and yield 12 volts for the radio. My friends frequently just use a simple motorcycle battery to provide power for the marine radio. (I would recommend a Jel-Cell as they will not leak.) They just have to make sure they remove it and charge it often enough to support their mission requirements. (a six hour mission for us requires 6 hours of receive time and about 10 to 15 minutes of transmit time.)