Combining multiple VHFs on single ant
 

I have a VHF installed in our cabin with only a remote speaker in cockpit. Instead of replacing the unit with a newer VHF with remote mic and/or station options, 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).

Is there a standard convention available which will allow both radios to share a common antenna without manual switching or other intervention?

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.

[ 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.

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.

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.[/quote]

Additional research lead me to Defender.com who offers the Shakespeare A2 vhf splitter. The device can be found at: http://www.defender.com/product.jsp?path=-1|344|295760|299311&id=162352

I think it is Shakesphere that makes an automatic switch for this
purpose. Look in the West marine cataloge.

Regards
Gary

On Thu, 24 Feb 2005 00:07:56 +0000, kailaniskipper
wrote:


I have a VHF installed in our cabin with only a remote speaker in
cockpit. Instead of replacing the unit with a newer VHF with remote mic
and/or station options, 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).

Is there a standard convention available which will allow both radios
to share a common antenna without manual switching or other
intervention?

On Thu, 24 Feb 2005 00:07:56 +0000, kailaniskipper wrote:


I have a VHF installed in our cabin with only a remote speaker in
cockpit. Instead of replacing the unit with a newer VHF with remote mic
and/or station options, 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).

Is there a standard convention available which will allow both radios
to share a common antenna without manual switching or other
intervention?

The 2005 BoatUs catalog is offering a Shakepeare device
called "Automatic Two-way Antenna Selector" for $80.

The description claims the switch senses transmit power, and
connects that radio to the antenna. Switch time is 100 ms. The device
draws 200 ma. Max RF power is 30W.

For less money ($43), there is a manual version...rated at 600W PEP.

Another device which may be of interest is a $90 box that measures
VHF forward power, and return loss (SWR). It also has a feature called
"radio receiver check mode"...whatever that means.

Norm B

"kailaniskipper" wrote in
message ...

Additional research lead me to Defender.com who offers the Shakespeare A2
vhf splitter. The device can be found at:
http://www.defender.com/product.jsp?path=-1|344|295760|299311&id=162352


--
kailaniskipper

Has one quirk you need to be aware of...using one radio effectively turns
off the other. You have to develop the habit of hitting transmit whenever
you change stations. Even then there is a chance that you are listening to
a dead radio.

Jim Donohue

Hooray for Gary!
I have one of those automatic switches on my boat. I have two VHF radios
connected to one antenna through the switch. It's all automatic, didn't cost
$4000-$8000 (!) and doesn't blow anything up!
Both of my radios are normally in the scan mode, one is connected to my ADF
(which is connected to the automatic switch), the other is my primary
communications radio. They each receive independently and simultaneously.
Keying the mike of one radio isolates the other radio while the mike is
being keyed to avoid damage.


"Gary Schafer" wrote in message
...
I think it is Shakesphere that makes an automatic switch for this
purpose. Look in the West marine cataloge.

Regards
Gary

On Thu, 24 Feb 2005 00:07:56 +0000, kailaniskipper
wrote:


I have a VHF installed in our cabin with only a remote speaker in
cockpit. Instead of replacing the unit with a newer VHF with remote mic
and/or station options, 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).

Is there a standard convention available which will allow both radios
to share a common antenna without manual switching or other
intervention?