FM radio antenna

[Stemar]

I just acquired a cheap second hand car type cd/radio for Jissel and need an aerial. Handy nearby is the base of a metal stanchion insulated by plastic grommets from the guard wires. Jissel's plastic, so it seems that said stanchion is an insulated stick of just about the right length...

Is it likely to work?

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[claudio]

Yes, It should work OK. You must have seen cars with wire coat hangers where there was once an aerial /forums/images/icons/smile.gif



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[Talbot]

You may well need to extend the ground. A normal car aerial extends its range cause it is connected to the earth of the car, you may have to do the same for an aerial onboard.

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[muchy_]

Yep, sounds better than the setup I've got at the moment.

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For God's sake woman, it's a cabin not a bedroom.

 

[Stemar]

Re: extend the ground

How do I do that?

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[Rick]

Aerial design seems to be some sort of black magic to me - I tried all sorts of options, and eventually discovered best reception by attaching the centre, but not earth connector to a chain plate - entire rig now seems to be aerial, and reception fantastic.

One caveat - mounting automotive style radios can be the start of electrolysis problems - buy one of those plastic boxes, and mount the radio in it so you have no chance of creating a battery circuit around the boat.

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[CharlotteRusse]

Re: extend the ground

It may be worth just trying it first (if that is easy enough). A car antenna does have the advantage of the metal car body forming a ground plane but you may get away without having to copper plate your boat! You have quite a good ground plane a metre or so away after all! (I'm thinking of something wet and salty).

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[sailorman]

how about a "splitter" on the v.h.f antenna cost around 30 quid

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[Talbot]

Re: extend the ground

tis tru that you have a very good ground plane under the boat, but you need to connect to it. Assuming you have an inboard engine, then I would merely connect the earth of the aerial to the engine earth, and that should be sufficient. Of course if you have an SSB, connect the aerial earth to the SSB ground plate!

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[kds]

mmmm ... Had a friend who had the whole thing blown up by a lightning stike no-one else noticed - but we do it with our VHF ariels, don't we ?
Ken

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[snowleopard]

i get pretty fair reception with a yard of wire behind the headlining!

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[Stemar]

Thanks to all.

Given that I paid £1 at a boat jumble for the radio, a £30 splitter is a non-starter. I think plan A must be the stanchion, with plan B to add an earth direct to the anode if necessary.

I've got several metres of marine vhf co-ax. I presume that would do the trick for Solent pottering


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[MainlySteam]

If you are just connecting to the stanchion or whatever with no groundplane to connect the antenna end of the coax braid to then do not use coax. You will be better off using a single wire from the centre pin of the radio antenna outlet to the stanchion, remembering that wire will also be part of the antenna so should be kept clear of other wiring etc.

The reason is that the coax if it is not terminated to a groundplane at the antenna end it acts as a capacitor - capacitors short circuit radio frequency and that short circuit will be back to the radio's chassis resulting in signal loss.

If there is no groundplane as above, you may, or may not, get some improvement in signal (but for FM I very much doubt it, AM perhaps) if you run another wire from an existing screw on the radios chassis to something in contact with the sea nearby. But I would not worry unless the single wire to the stanchion alone does not give you the results you want - it probably will. Connecting the chassis to ground in this way may open up opportunities for electrolytic corrosion, depending on how your boat is wired, especially should you get a fault in the negative of the DC return to the switchboard/batteries.

In the end you will probably get equal results as the stanchion just by running the bit of wire I mention inside the headlining or whatever without bothering with connecting it to the stanchion. For FM only you will find it need not be very long (I run a marine VHF - a bit higher in frequency than broadcast FM - on receive in my office with just about 500mm of wire on the centre pin of the antenna connector at the back of the set and can hear marine VHF shore stations 40-50 miles away).

John

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