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wrote:
I have a 19' Proline CC and will add a VHF radio w/antenna after
Christmas. I'm thinking about putting a 36" antenna on the CC
horizontal rail but read that you need to have 3 feet between the
radio and the antenna. Is this true?

There are different antennas and some of them require at least
3 feet of cable between antenna and radio. Those antennas are mostly
collinear dipoles. Gain is from 3 to 6dB over a lambda/4 ground
plane antenna. The three feet of cable are necessary to maintain the
impedance. But you can coil it up. The coil must not touch any metal.
Best results are achieved with a cable coil of roughly 8" diameter
and three turns not farther from the antenna feed point than 3".
This is called a "current balun". It prevents the cable from
radiating and thus from disturbing the radiation pattern of the
antenna.

36" is slightly longer than lambda/2 on the VHF marine band and so
you got no ground plane antenna but probably a vertical dipole with
a little bit less than 3dB gain. In this case: Yes, the three feet
of cable are necessary.
Advantage of this kind of antenna is that you need no ground plane
under the antenna to make up for the other half of the dipole.

It is also adviseable to install the radio farther from the antenna
to avoid backfeeding the radiation into the transceiver. And who likes
to fry his brains with radiation anyway? 25 Watts is a lot more than
a cell phone emits.
Put the antenna as high as possible to achieve the best coverage.

Kind regards, Eike