VHF - FM/AM antenna splitters - SailNet Community

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  #1  
Old 05-24-2007
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VHF - FM/AM antenna splitters

I'm installing a new FM/AM/CD player in the boat. It was suggested that I use a splitter to allow my VHF antenna (Morad VHF 156) to do double duty for the FM/AM. It would be nice not to add another antenna but, given the difference in frequencies, I'm wondering just how well a signal splitter will actually work. Has anyone gone down this road before?
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Old 05-24-2007
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Bad idea... VHF antennas aren't generally all that useful for the FM/AM bands and transmitting on the VHF might fry the stereo...
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Old 05-24-2007
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i've used this splitter on two boats, both AM/FM and VHF attached. I mostly bay sailed then and was rarley out of sight of land, never the less poor FM/AM reception. And i was never satisfied with the VHF transmitting power. I'm aware of the line of sight restrictions of VHF, but still my handheld VHF transmitted better.

Only recently did i become aware that the splitter "probably" limited one or both radios. But ya i never went up the mast or dropped the mast to install that second line.
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Old 05-24-2007
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Windborn-
A separate AM/FM whip antenna will be way cheaper, way more reliable (no way you can transmit into the stereo with the VHF), much better way to go.
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Old 05-25-2007
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Quote:
Originally Posted by RickBowman
If you split a signal, guess what happens? Signal loss. I left my VHF wire alone, and installed a dedicated AM/FM dipole antenna. It cost about $10.00 from an audio electronics car stereo store. I needed to install a 6 foot interference/shielded cable between the radio and the antenna as there is sufficient electronic signal/noise where the radio is installed from other electronic circuits of 12 volt and 110 volt. Then I routed the cable on top of the aluminum fuel tank where it would be out of the way.
In case your wondering, AM/FM signals go through fiberglass like it isn't even there. Great reception!

So if I'm reading this correct, your laying the am/fm antenna on top of the Aluminum fuel tank ?
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Would a cheap car antenna work, with the shielded cable ?
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Suspicians confirmed

Thanks for the responses folks.

There was something about the splitter thing that just didn't feel right. Interestingly, I was told to go that route by "experts" at two well known marine supply houses (to remain unnamed). Kind of makes you wonder why these splitters are being marketed by antenna manufactureres in the first place. The one from Shakespeare (the recommended product) goes for about $50.

Maybe there's more to this than meets the eye, but I'm going to go the cheap dipole route for the new stereo, and run it along the hull behind the lockers.
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Old 05-25-2007
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Quote:
Originally Posted by sailingdog
Bad idea... VHF antennas aren't generally all that useful for the FM/AM bands and transmitting on the VHF might fry the stereo...
I have an PL 259 "T" splitter on my antenna going to both the VHF and the FM/AM car stereo in the old boat. While you are correct that sending 25 watts into the antenna lead of the stereo is bad policy, if you remember to shut the stereo off when activity using the base unit VHF (which I rarely transmit on, anyway), it's not a problem.

I liked it because it worked and I found it that way. I also like it because during lightning storms I can undo one RG 58/U lead and be done with it. The stereo is 30 year old Radio Shack quality, so I don't care, really...if I install a stereo on the new boat I will do a separate antenna installation.
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Valiente, given the way that "power" switches don't actually shut internal power, and the way that RF power being fed into a receiver might fry solid state parts regardless of whether a radio is on or off, a splitter could be problematic in any case. As long as it works--no problem. If it somehow fails...we all know how Mr. Murphy loves to show up on boats.

I'm not sure which splitters they are referring to or why they are recommending them. I've seen splitters designed to allow you to use a VHF with the standard "car stereo antenna" that's already installed in a car (or boat) and the purpose of that is to save you the bother of installing a real VHF antenna. And the other way around, a splitter designed to let the car radio take advantage of the existing VHF antenna that's way up the mast.

"Splitting" does not mean splitting the signal in two though. It means that some frequencies (marine) are allowed to to "this way" while others (consumer AM/FM bands) are allowed to go "that way" so there is a small loss--nothing near half the signal--on each side. These are totally different from, say, a cableTV splitter that splits one signal into two halves.

The thing is, a VHF antenna is literally tuned to be resonant near the 160Mhz frequency, while an AM "car" radio performs best with an antenna tuned for 6-16KHz and an FM "car" radio works at some 80-110MHz. Trying to make the antenna from one work for ANY of the others--is a compromise far worse than the loss from the way splitters work. While the extra height at the masthead will give the FM signals more range, the wrong antenna will kill that extra range. Net gain? Who knows, how important is the local news and traffic? And on the AM side, if you tune to what used to be called a "clear channel carrier" (nothing to do with the corporation of the similar name today) you can pick 'em up on a coat hanger from a thousand miles away anyhow.

Easier to get a "car" antenna for the "car" radio", lay it up out of sight under the deck someplace, and take the cheap and simple route on this one.

(And yes, I know marine VHF's are also FM radios...I'm just using AM/FM as conveniently incorrect names for the car stereo's bands.)
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I can't really argue your well-reasoned points, HS: it was as I found it. I worked with CB radios and ham receivers as a teen (just prior to discovering rum and wenching) and I know it's poor form. But it never failed to work, and I always got 5 x 5 out of the ICOM M45, which is now on the new(er) boat. I put an old Navinco VHF from the '80s for the "borrower", who never uses anything but a hand-held anyway. I suppose I could just unplug the stereo from the connector, and plug in an FM dipole and leave it lying under the settee. It would likely work even better.
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