kailaniskipper wrote in
:

I would like to simply add another standard VHF
radio at the helm and share the antenna (located at the top of a 55'
mast).


NO NO NO NO NO NO NO NO NO NO NO NO NO NO!!!!!

NEVER NEVER NEVER plug the transmitter of one radio into the receiver of
another, which is exactly what will happen if you try to do this,
destroying both of them.

Seen that, got the T-shirt.....(sigh)

We do share antennas between transmitters. But it takes expensive plumbing
and phasing networks called diplexers and the transmitters are not
frequency agile. Your local TV station actually has two transmitters, one
for the picture and one for the sound, on a single antenna with this
diplexer. Stop by the transmitter at the base of the tower and asked the
bored-to-death poor engineer to show it to you.

Radio repeaters use a duplexer, a complex set of very sharply tuned
cavities and associated plumbing that transmits on X and receives,
simultaneously on Y. At marine VHF frequencies, they are about 4' high x
3' square. Not a good idea in a boat. A friend of mine in the paging
business constructed such a duplexer so I could hook a cheap little ham VHF
transceiver used as a packet radio repeater up to his paging antenna 900'
above the building on 152.480 Mhz. The 152.48 transmitter was a 500 watt
Motorola monster. It ran for years with not a hint of interference, until
a direct lightning hit blew it all away in a flash. But, again, it was
only on 2 discrete frequencies, not a channelized band of frequencies.

More practical is your main transmitter on the mast and your secondary
transmitter on a stern rail-mounted fiberglass whip. This gives you the
backup antenna you need when dismasted or lightning struck. I'd even like
you to run the backup transceiver off the starting batteries separated from
the dead house batteries.