Aerial fitting into a non-metallic wing...

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Aerial fitting into a non-metallic wing...

Postby ChevetteCharlie » 10 Apr 2006, 15:01

I know one of you educated chaps will have the answer to this...

Does a car aerial have to ground?
i.e., does the serrated part of the fitting have to be in electrical contact with the body, hence effectively connected to ground/negative to work properly?

So, if you fit one to a fibreglass wing will it need to be earthed in some other way? A wire back to a decent ground on the metal part of the car for instance?
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Postby Miss Icey » 10 Apr 2006, 15:07

Wilson wrote:Nope, genrally not, the outer casing is just a shield, its usually grounded just to help reduce interferance. Its only the core thats important really


Wilson wrote:if you do ground it, it only need to be at one end, i.e onto the stereo!
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Postby Doive » 10 Apr 2006, 15:32

Probably best to ground it anyway, otherwise you tend to get a floating earth and poor signal at the antenna. Basically it will seek to reduce the gain seen by the aerial, and probably reduce the SNR as well. You may see no difference at all. Plug it in first to the stereo before fitting to the car and see what it goes like, then if it's a bit poor run a ground from a good earth.
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Postby ChevetteCharlie » 10 Apr 2006, 16:17

Thanks chaps, I knew you'd know!
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Postby Neilyman » 10 Apr 2006, 17:44

Yep, it does need grounding at the wing, which is why it has pointy`teeth` on the coller that goes under the wing. Without that earth, as Doive said + lots of buzz, click, pop, farts etc, & multipath distortion....!, but that`s another story :)

Cut out a peice of tin etc, as big as you need, drill big hole though it for aerial, small hole in another place to bolt a ring terminal to that is connected to a peice of wire, grounded at t`other end, the thicker the better!
Clamp said bit of tin between wing & teeth bit of aerial & clamp it up tight.
Jobs a good `un :)

This info gleened from years of fitting aerials for a Lotus Main dealer. Oh!, & the odd Reliant or 2 ;)
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Postby ChevetteCharlie » 11 Apr 2006, 09:15

Ace, that's exactly the method I had in mind.

I didn't want to cloud the issue but I did want to get the theory.
The aerial is actually fitted to my boat. Of course, there's no negative grounding to the boat hull anywhere (that's a thread for a whole other forum!), so I just have to run a lead back to the negative busbar.

Thanks again everyone! 8)
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Postby Doive » 11 Apr 2006, 11:08

You have a negative busbar on your boat?! Is it running at 33kV or something? That must generate a heck of a spark!

Sorry, I'm in one of those moods :lol:
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Postby chris(not) » 11 Apr 2006, 12:38

Busbars are used for large currents- doesn't matter what the voltage is, that only determines the spacing / mounting of the bars.

(I think you knew that anyway, Doive ;-)

For the boat aerial, especially if you want FM reception, the ground plane idea (sheet of tin / ally /metal anyway) is good. If you can make it large compared with the wavelength (3 metres at 100MHz), so much the better, but that sounds unlikely. In any case, the usual rules of mounting the aerial as far away from any interference source apply. Hopefully if it's a boat, you've got a diesel engine which only has the alternator / dynamo to make electrical noises. Otherwise, suppress everything like for a car. Probably the best ground reference you'll have is the engine block, but if it's more than a metre or so away from the aerial, the ground wire you use (or negative busbar) won't look like a decent connection at FM radio requencies anyway, though it may help for LW / MW.

Anorak off....

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Postby Doive » 11 Apr 2006, 13:42

chris(not) wrote:Busbars are used for large currents- doesn't matter what the voltage is, that only determines the spacing / mounting of the bars.

You're right I did know that, it's what I do for a living! However, the busses I deal with are typically 33kV and above, so that's really a busbar to me.
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Postby chris(not) » 12 Apr 2006, 13:10

Thought so :)

Last time I used busbars was in a 4V 12,000 Amp superconducting magnet supply- about six laminations 150 x 6 mm each ISTR.

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