VHF antena also used as FM antena?

[stone_love]

I've got a quick disconnect for my VHF in my boat, i'm also installing an FM radio/mp3 player.

Rather than install two antenas can i use my vhv antena as an fm antena?

[Tomfoolery]

Well, aside from the wavelength of marine VHF being about half that of FM radio, I suspect it would work at least to some degree, as almost anything will pick up FM radio if you're close to the transmitters. But I'd be concerned about someone forgetting that the antenna is on the FM set and keying the mic on the marine radio, frying the output stage.

I used a small rubberized omnidirectional antenna (bendy and hard to damage) a couple of boats ago, and it worked well enough. I kept it below decks, since it was a small trailer boat, and I didn't want more junk on the outside. I would do something like that before I'd try to retask a marine VHF antenna, but that's just me.

[Russ]

Sure can. If your antenna is on top of the mast, it will help with FM reception which is important to us.

I use an FM/VHF splitter like this one
http://www.wholesalemarine.com/p/SHA-43 ... rce=nextag

Another that I haven't tried, but looks cheaper.
http://www.overtons.com/modperl/product ... PING_91524

[Catigale]

I have the splitter on my and I receive FM from the Elizabeth Islands in MA very well -works great.

[Tomfoolery]

Wow! Who knew?

Looks like a good idea, and no chance of frying the marine transmitter, which would be my main concern.

[Nautek]

Splitters take some of the efficency away from your VHF
This may not be a problem if you dont need the range

On several boats i have owned i have run the antenna wire to a stauntion and in the case of the mac to the chain plate fitting and use the rigging as the antenna for the am fm radio

Works very well

I have attaches the wire to the chainplate inside the cabin

Maybe try this firse and if not happy go splitter

Allan

[vizwhiz]

CLEAR! (thump...)
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Just me reviving another old post...
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So one of the PO's had stuck one of those inexpensive rubber-covered flexy antenna's on the side of the cabin for the FM radio...ran the cable thru the head and just drilled a hole in the side of the cabin. The most unfortunate part of it is that the genny sheets always caught on the bottom part of it...had to remove it. Don't really mind that, was an eyesore on the inside and I didn't trust the installation to be waterproof.
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So this brings me back to the above subject...do I need a dedicated FM antenna for my radio, or is the splitter thing worthwhile? How about the idea of just attaching the FM antenna to the rigging as posted above? Experiences with both? I have the VHF cable up in the mast already, the antenna on the masthead...sure would be nice not to have to add another antenna.
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I checked the links, and they look legit...just a bit of a skeptic at the moment...I realize it isn't complicated, just don't like gimmicks.

[Sumner]

We just have a cheap antenna from I think Radio Shack stuck sideways in amongst the foam above the sink and it works fine. At home I can get stations over 100 miles away. Where you will be sailing about anything will get stations. I would not go without one though,

Sum

===================================
Our MacGregor 26-S

Our Endeavour 37

Our Trips to Utah, Idaho, Canada, Florida

Mac-Venture Links

[Russ]

I have a VHF/FM splitter. Been working fine for years. Single antenna on top of the mast.

Realize though that it DOES reduce the VHF quality. I have not noticed any significant degrade. If I were cruising offshore or in critical places where maximum VHF transmitting is required, I might not split it. But honesty 25 watts of VHF goes pretty far.

I had a little antenna in the cabin. Didn't work well because of boat moving and so low to the water line.

--Russ

[Catigale]

If you really want to pretend you are a blue water sailor...aka PBS...

Put a FM VHF splitter near the VHF with enough slack so that the direct antenna lead can be connected directly to the VHF to eliminate the losses. Then you can rebut PBS criticism of your setup and enjoy the other PBS while you sail too.

[mastreb]

So has anyone tested hooking up their VHF to the antenna stays? Yes, the stays aren't perfect harmonic lengths and there's an impedance mismatch, but they're really big antennas, and big tends to solve a lot of problems. An impedance match circuit would be simple, once impedance of the stays was measured, and the antennas are so large that mismatch losses may simply not matter at all. Because hooking a wire to the chainplates would be way easier than running coax through the mast, especially for those of us who trailer.

[Catigale]

You get NPR on the port stay, and Rush on the starboard of course...

[Russ]

So has anyone tested hooking up their VHF to the antenna stays? Yes, the stays aren't perfect harmonic lengths and there's an impedance mismatch, but they're really big antennas, and big tends to solve a lot of problems. An impedance match circuit would be simple, once impedance of the stays was measured, and the antennas are so large that mismatch losses may simply not matter at all. Because hooking a wire to the chainplates would be way easier than running coax through the mast, especially for those of us who trailer.

VHF uses a specific bandwidth. Radio guys can explain better, but the antenna must be tuned or matched to the physical length of the radio wave. I think that is approx 18" or a multiple of that for marine VHF. The stays are both the wrong length for the wave, but also weird shape as to force direction on it.
FM is the same. It must be tuned to the wavelength. Splitters do this somehow because marine and FM are close in wavelength.

Bigger is not always better. It may narrow your transmit beam too much. VHF/FM is line if sight. Generally taller is better. Why transmitters look for hilltops or tall places. Coast guard use tall towers for theirs.

Think flashlight. Bright focused beam may travel far, but must be accurate in its target. Widen the beam and it may not illuminate as brightly, but you can spread it out wider. 8db antenna is a focused beam, 3db is wider. Since VHF has a whopping 25watts, I feel the db gain is not as important. Masthead mount gets my vote everytime.

--Russ

[mastreb]

Hi Russ,

Sure, for standard reception you need to be at wavelength or a harmonic such as half or quarter. Every conductor is an antenna of some efficiency. But every bit of math I've ever seen for antennas is about making them shorter, not longer than wavelength. The stays are longer than the VHF 160MHz full wavelength of 6.5 feet--in fact, they're just about 3 times longer. If basic antenna theory applies, 3X wavelength should make the stays a +9dB monopole receiver. But there's impedance and back-reflection to work about. I've got an oscilloscope I could test with, but that's a lot harder than just touching the antenna to the chainplate and seeing what happens.

Matt

[Nautek]

I wonder if it matters too much for the car radio
As mentioned before i have hooked mine to the chain plate fitting and i have no problem picking up FM or AM stations
Car radios are pretty forgiving and a wire coat hanger will work as an antenna

Allan