Car Mast Advice

[Howard]

Hi all ,

I've just bought a little boat , and am about to fit a radio to it.

I've bought an aerial , but it has an 'earthing ring' on the back it says to make sure that there is a good contact between the bare metal on the inside of the wing of the car and the 'earthing ring' ...

Now ...

The boat is made of fibreglass , which doesn't conduct electricity , so is the aerial going to work and let me listen to FM stations or not ?

Many thanks

H

 

[GordonTarling]

I think you'll need a fibreglass aerial for that.

Seriously, a car aerial comprises two parts - the mast which you can see and the bodywork. In order for it to work properly on fibreglass, you may need to connect some metal (foil will do) to the earthy side of the aerial. Best thing to do is give it a try and see.

 

[Privatedoc]

It just needs a ground plane....perhaps a 6" diameter bit of tinfoil or similar to act as a counterpoise to the mast. often the simple thing to do is bolt the aerial to a metal part of the boat, eg a rail or similar. That way it acts as a receiving dipole

(you can see I'm a radio ham ;-) )

John MM1ANP (callsign)

 

[BTB 500]

My caravan has glassfibre sides (well actually a ply/foam/grp sandwich), and uses an FM aerial something like this:

Caravan FM Leisure Radio Aerial from Towsure

Works very well.

 

[jimti]

you can get aerials with built in ground plane as well if all else fails.
Vehicle Aerial Mounts - Artificial Ground Plane (EGP) - Products for Sale

 

[BTB 500]

you can get aerials with built in ground plane as well if all else fails.

Those are surely for transmitter aerials though (if that makes any difference)?

From the OP I'm assuming Howard just wants to listen to normal FM broadcasts.

 

[jimti]

Those are surely for transmitter aerials though (if that makes any difference)?

From the OP I'm assuming Howard just wants to listen to normal FM broadcasts.

Ah, yes you are right, he is probably going to be pottering up and down the Thames. Thinking about it he won't be fitting a maritime cb radio

 

[Howard]

Well , i've bought the aerial already

One of these ...

White flexible Marine AM/FM radio aerial Boat Yacht RIB on eBay (end time 05-Sep-10 04:22:46 BST)

But the instructions clearly say to make sure it has the 'earthing ring' on metal ?

I was thinking of connecting the earthing ring to the neg terminal of the battery , as a car body is effectively connected to the neg pole isn't it ?

Are you thinking i can just attach a steel plate inside the bodywork , would i need to connect it to the neg pole ?

 

[jimti]

Are you thinking i can just attach a steel plate inside the bodywork , would i need to connect it to the neg pole ?

you don't need to conect it to ther battery, it is just working like the aerial it self does but the other side of the radio wave

 

[Howard]

Great !

So how big does it need to be ( the plate ) ?

 

[jimti]

the bigger the better, but a 6 inch diameter plate should be enough tbh

 

[Howard]

Can it really be a few layers of tinfoil ( folded over and over to form a plate ) ?

 

[jimti]

any metal will do it, I have never used tin foil except to block a signal,
look at this page Ground Plane
don't take too much notice of measurements they are more important when trying to transmit, but it shows what you are after.. Is tin foil a ferrous metal?

 

[Howard]

No , i don't think it is .... i can come up with a large square of steel though.

Thanks I'd not have figured it out myself.

 

[GordonTarling]

It doesn't have to be ferrous metal, just as long as it conducts electrickery.

 

[Ted]

Just to put my oar in (joke)
The reason that they say to ensure that a good connection is required is to prevent interference - usually from ignition borne sources (does your tub have a diesel engine?)

A simple earthy/common connection is not enough, not because of the current (miniscule/zero) but because of the RF.
The best way to acheive this in a car is to ensure that the aerial has a REALLY good earth, and that the radio itself is also earthed (don't know if you've ever heard the interference on a set that has been earthed via the aerial.

My advice (fwiw) is as described, to put a piece of aluminium foil as the earth for the aerial. Then if possible, bond this to the largest metal part of the boat - (I don't know if this is possible) Then make sure that this is well bonded to the radio, and 'earth'.

Just remember - this is just about minimising interference NOT about getting a signal.

PS for bonding, dont just use a piece of wire - us a proper earthing strap (not a high current engine to chassis one). These should be available from any ice installer.

 

[Howard]

Blimey ... all getting a little complex now ....

It's outboard powered , the largest metal part of the boat is the front rail ( stainless ) , getting an earthing strap that would reach from the aerial mount to the rail might be a challenge.