VHF/FM same coax?

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icurnow

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I intend to fit both fixed VHF (perhaps ICOM) and FM radio/CD from a car into my Hunter Horizon. If anyone can answer following questions much appreciated:

1. Can one masthead aerial receive both signals and be split with Y adaptor in cabin?
2. If using two aerials should I run two seperate lengths of COAX and associated holes in coach roof, OR, can I join signals at masthead with Y adapter and then split again behing radios in cabin - hence only needing one main run of coax?

Thanks in advance Ian Curnow
 
Re: Yes and no

First things.

Do NOT put a 'Y' adaptor in. If you do you will probably blow the front end of the FM receiver and you may also blow the transmitter circuitry in the VHF.

What you need, if you wish to use one aerial is a thing called a 'diplexer' specifically designed for this task. I think they are available in America as I have seen some discussion on forums from over there.

Much easier and less dangerous, is to run the VHF aerial as normal. Then, for the FM radio, have a wire about 4 feet long between the headlining and deck (if GRP). connected to the radio.

If you wish it is possible to fit a normal car aerial for the FM radio but this should not be close to and run parallel with the VHF aerial.

Iain
 
Fit a standard VHF antenna at the masthead and run a single good quality coax down into the boat.

You can buy splitters which are designed for exactly this use. They fit into the coax between the VHF and the antenna and provide a normal FM radio antenna plug on a short lead. Importantly, they direct the transmitted signal from the VHF, up to the antenna. If you use a Y-splitter, as soon as you key the microphone on the VHF you'll destroy your FM radio.

Alex


Alex Rogers
<A target="_blank" HREF=http://www.YachtsAtSea.com> www.YachtsAtSea.com </A>
 
WOW... what a helpful and quick responce from you all. Very impressed. Thanks you for your advice, which is very much appreciated

Ian
 
In addition to the already posted advice, I would like to add my 2 cents worth.
I think that VHF needs 50 ohm cable and FM needs 75 ohm cable for correct matching of antenna to receiver. Splitting any received signal will reduce the signal strength at each receiver by 3dB (half power!). As previously stated the first time you transmit you'll wrech your FM receiver ... at least.
Much cleaner and reliable to fit two separate antennas and feeders.

Good luck.

Alan.
Nettuno, Italy.

Alan Cloke
 
For my two penny worth.Make the effort and install 2 aerials;] do not get involved with di-plexers.the signal dribbling out from di-plexers is rubbish you will end up be able to listen to only strong local rubbish radio station and be totally unable to listen to very interesting/intelligent but further away type transmissions.
Ps Do ALL local stations transmit unadulterated puerile crap or is it just the lot near me........................???

Oooh look its still not dark and its nearly 5pm
 
Sorry, not true. I mostly listen to Five Live in Holland at weekends. With a car aerial inside the boat and in the cockpit the reception was OK but subject to interference. Since fitting a splitter gizmo in the VHF aerial the reception is much stronger, less interference and better clarity. Radio Four is also better and was clear even in the Baltic. No detectable difference in VHF transmission or reception. All local FM transmissions are fine.
 
if you go to west marines website you will find the splitter in there its made by shakespear i believe, anyway had one fitted on my boat for two years without problem, the only reason i fitted one is it cuts down on the potential leak points in the coachroof, of which there are already enough !!!!!........keith
 
Its questionable whether an FM antenna at the masthead is any benefit.

Unlike a car, a boat is not metal, and a suitable antenna stuck to the inside of the boat (hidden behind the bunks) is just as good. You can buy these aerials and I used them on a previous boat.

I currently have a multi-band aerial at the masthead, with a dipelxor at the chart table and its useless, as its cut for 30Mhz. The manufacturer admitted as much once I tackled him. I get a better signal out of the coax between the radio and the diplexer if I disconnect it.

Also Marine Band is 150Mhz (and usually 50ohm) and FM is 100Mhz (and usually 75ohm). The aerial will be cut for Marine Band. Also, unless the diplexer is amplified, the loss in the mast cable is usually greater than the gain in the aerial. AM radio, of course, doesnt rely on the external aerial at all.
 
Umm I though Five was on AM only.And of course you can pick up BBC 1/2/3/4 whatever.... it is pumping out ZILLONS of watts from loads of locations.Try listening to a weak london station (ie Jazz FM) when out of local area with di-plexer.No signal at all, at least you will get something with separate antenna.Think you lose 3db or maybe loads more with any non amplified splitter.Plus all any added preamp will do is boost local interference as well as required station.

Oooh look its still not dark and its nearly 5pm
 
A basic splitter would drop at least 3dB as you suggest - but then that would also fry the input of the FM radio - so the Shakespeare device can't be one of those.

In theory the splitter would work perfectly if it were a high-pass/low-pass filter. Each signal could to transmitted to the relevant receiver with no attenuation - but given how close the frequencies are I don't know if it would be practical to implement such filters without active components - which would require a power supply.
 
Unlikely to be able to pick up any UK FM stations in Holland, although on one occasion we did, just for a morning.

In fact we have two radios on board, one with the masthead antenna and splitter, the other with a car aerial inside the boat. The masthead one always receives better, FM or AM.
 
Poss that height of di-plexed ant is better at receiving line of site(sort of) vhf signals.ooh and may involve tropospheric ? ducting Innit guv ????.........

Oooh look its still not dark and its nearly 5pm
 
Vyv 5 Live is AM medium wave 909 or 693 (as I'm sure you know). Reception isn't influenced by your FM aerial.

<P ID="edit"><FONT SIZE=-1>Edited by JohnM on 18/01/2003 01:33 (server time).</FONT></P>
 
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