A question concerning VHF antennas...

[chrisr]

hi all

here is a question for anyone who knows about vhf antenna

like most boats, our vhf antenna is on the masthead. this raises the question what to do if the reason we are using the vhf is that we have been dismasted

we have a hand-held, but their power is limited

i'm wondering if it is feasible to have a second VHF antenna installed eg on the targa bar (which would probably survive any dismasting) ? what would be the affect of having both antennas connected to the VHF at the same time ?

thanks in advance

cheers,

[Jim Cate]

Chris, most folks, myself included, that have a backup VHF antenna simply leave it disconnected until needed, or perhaps use it for their AIS or even the FM receiver. Ours sits on the solar arch, off to the side to minimize shading, and has been tested for both VHF and AIS use. The end of the cable sits next to the VHF unit and the splitter for the AIS. A very quick switchover should it be needed.

Jim

[ozsailer]

Gday Chris, while a dismasting is relatively rare having a spare antenna and cable is not a bad idea. Simply disconnect the old antenna cable to the VHF and connect the new one. Cable tie the new antenna at you highest or most convenient point. Radio should be up and going as soon as you turn it on. You may not have the former distance to transmit but you are going to be transmitting a fair way. Just look at ll thoese fishing boats who are so low to the water and their antenna height and they work very well.




Greg H
Lagoon 440
Oz

[Pete7]

Like Jim, our AIS antenna will reach the VHF just by unplugging the cable and connecting it to the VHF. We also carry one of those emergency antennas which we inherited from the previous owner, but without a SWR meter can't tell if its any good. Might be better than nothing though.

Pete

[Sailorman4009]

I believe in redundancy in critical systems. I have a vhf antenna on the radar post at my stern connected to a gas trap lightning arrester. That device is grounded to the bonding system. The output of it then goes to a separate used ($25.00) vhf radio I got a a parts meet. It is totally independent from the main vhf and mast mounted antenna which also goes through a gas trap.

These gas traps are critical to protecting your radio from lightning and if the mast antenna is struck the main vhf may be toast. If dismasted/lightning struck I simply use the backup system.

If all else fails then the handheld.

[chrisr]

thanks guys...all interesting comments

it was whilst thinking about the idea of a spare antenna that i started wondering if it was possible to connect to two antennas at once...to save having to make the connection if dismasted

certainly mounting a second antenna (eg on the targa) and running the cable to the vhf, but not connecting, is a good option and worth considering. on the other hand, there is a bit of cabinetry to remove before getting to the back of our vhf, and i'm not sure doing that after dismasting is going to be much fun !

cheers,

[Apollo366]

To connect two antennas to the same radio would take a fancy electronics box of some kind, my only thought is a Ham Radio shop might know of one and be able to advise you.

I also have a second VHF antenna, but it's because I have little faith in the coax in my mast, it's very corroded down at the foot of the mast in the bilge.

I just bought a very good quality antenna and base mount from West and mounted it on the solar panel arch and ran the cable down to the back of my electrical panel where the other cable has a joint from moving the VHF to the cockpit where I can use it and hear it much easier.

PS, most VHF radios can be removed from the front, usually a pop off bezel around it, ask a radio guy if you can't get it to go.

[RobertoD]

Sailing mainly on Lake Ontario and the Great Lakes I have a simpler back-up. A small, compact emergency antenna, with a length of antenna cable connected, so should the mast come down, or the main VHF antenna become compromised (hit by a Canada Goose!!) I can take the emergency antenna as high as I can to keep VHF operational. Should the main VHF be damaged, the VHF Handheld is a back-up, but limited range. Plus further backup - I have an inexpensive (but very Good) BAOFENG UV-5R programable hand-held SW radio, with VHF channels programmed for easy selection, with a spare extended Baofeng antenna giving more range than the handheld VHF. Simple and inexpensive, but adequate for my needs and location.

[Lake-Effect]

Lots of good info here. The ultimate backup is of course complete redundancy - separate radio and antenna system, an insulated pole to get the spare antenna as high as possible, separate battery even.

I've made a simple emergency antenna out of RG58, cut for CH16, following guidelines like this thread. It had a measured SWR of just under 3:1 on CH16, which could have been improved by better test setup and tweaking the arm lengths. Again some sort of stick or pole to raise it would help.

[Riddle]

Connecting two 50 ohm VHF antenna’s to one VHF radio at the same time is not a good idea, it will reduce output power, and may even damage the radio.
The simplest way is to unplug and replug the connectors.
Have a spare emergency VHF antenna with cable and plug ready.
Or use the AIS antenna (if there is a separate AIS antenna).
If you like a better solution, there exist “coaxial cable antenna selector switches” of 50 ohm.
Indeed in specialized hamshops or internet.

[ithet]

I have read that you can create an emergency antenna by splitting 18" of the outer cover off the co-ax end by and then pulling the core out through the screen which is pulled out and taped to the co-ax below.

[DefinitelyMe]

We are also proponents of having a second antenna, in our case mounted on the stern rail. We use it for AIS transmission and can easily plug it into the radio if necessary. It's also cheaper to buy a second antenna than a splitter, so it's win-win.

Before the days of AIS we kept a second antenna down below with about 20' of cable and the PL259 connector already installed. Had to use it once when the masthead antenna failed coming into Bermuda. Two minutes had us up and running again, with the antenna zip-tied to the binnacle.

I don't see any reason to have a second one installed permanently unless you're actively using it. Just one more thing to wear out and need replacement.

One of these days I'll get around to buying the connectors to plug the hand-held into the masthead antenna, so that in the event of a complete power failure or whatever we can significantly improve range on the handheld.

[ferrailleur]

just a stupid follow up?
If you dismast in 30 knts wind.... wouldn't you have any pressing priorities to save you and the ship other than fumbling with a VHF connector??

[Adelie]

Mucking around with the vhf is something you do after you've stabilized the boat.
If the boat is in such extremis that you need to immediately send a Mayday, whip out the handheld. If you are worried about the range of the rubber duck antenna get a telescoping 1/4 or 1/2 wave antenna. I got mine from a Ham radio store.

[S/V Illusion]

Actually, connecting a radio to two separate antennas is a workable solution if fed in phase (in simple terms) meaning same antenna, same length/type coax, same impedance, etc... They will also work out of phase or dissimilar polarization and the power each 'sees' would be approx. half what the radio transmits with no negative effect on receive.


The problem arises when one antenna is damaged, shorted, etc... which will effect the entire antenna system including both antennas so it's not a good idea in a dismasting to rely on that as a redundancy.