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Car Radio Antenna problem

penneck
Grafter
Posts: 655
Thanks: 8
Registered: 03-08-2007

Car Radio Antenna problem

When I bought my car some years ago, I changed the radio to fit one that could pick up MW and LW as well as VHF. This was about 9 years ago so before DAB in cars. I wanted LW so I could listen to cricket Test Matches which, in those days, were only transmitted on Radio 4 LW (or TV - not a good idea if you are driving). Sometime since last year the radio has just about stopped receiving MW and LW (it is very faint, and if the engine is running, the static obliterates what little signal there is), but VHF is still good. While the car was parked at home, I was able to disconnect the antenna lead and use a bit of wire as a temporary arial. This picked up on everything so it doesn't appear to be a fault with the radio. I didn't try running the engine so I don't know if the static was still a problem with the wire antenna
My questions to any knowledgeable radio engineers/car electronics experts among you are:-
1. what fault in the antenna/antenna lead would cause this loss of MW/LW?
2. what do I do to improve the suppression to stop the static?
I don't want to spend a lot fixing the car cos I'm changing it in October - it is a 16-year old Vectra so long in the tooth. The antenna is one of those short things stuck on the rear of the roof of the car, and combined with the tailgate washer jet. It is not one of the shark fin type, nor is it a telescopic one that fits in the boot, so replacing it will not be easy, and probably expensive.
Thanks
8 REPLIES
Community Veteran
Posts: 16,834
Thanks: 1,124
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Registered: 06-11-2007

Re: Car Radio Antenna problem

1. Fault in the antenna lead.....  as it is a roof fixed antenna... the connection where it screws into the base MAY have got water in and that will "short out" the antenna to the vehicle "ground".. (metal work).. via the outer sheathing of th coax...  If you have been through carwashes with it in place.. you may well have broken the wire connection under the fitting where the aerial screws in...
2, If you can access the back of the radio.... ( which you probably can as you fitted it).... then you could probably just run a wire  securely poked into the centre of the aerial socket along the inside of the dash in convenient places... to exit near the left or right side of the windscreen  ( you could do this with some coax if you wanted to "screen" that path,)... then run the wire up the side of the screen .... blobs of evostick glue to secure it ....  ( I did similar and it worked quite well .... at least whilst stationary, with no engine... cos I don`t listen to radio when driving )...
3. If petrol engine.... usual things to suppress.... plug leads.. (usually incorporated in the caps as standard fitting)...  Distributor lead.. (same as spark plugs).. coil, and points.  usually have a  capacitor fitted... also the Alternator...
Community Veteran
Posts: 7,913
Thanks: 594
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Registered: 02-08-2007

Re: Car Radio Antenna problem

There are a couple of things you might like to check, one is the earth lead, it might look ok but it only takes a minute to remove the bolt, sand off any rust and apply a bit of copper grease before refixing.
As for static, the question is what is causing it , several electrical devices could be at fault including worn leads or supressors, it's possible to buy supressors for various electrical items on the car but knowing which one is at fault may be the biggest headache.
If it was me other than checking the earth & aerial lead I would leave it as you will be getting rid of the car soon.
,
Community Veteran
Posts: 6,586
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Registered: 16-02-2009

Re: Car Radio Antenna problem

Fords had an issue with the roof mounted aerial getting a poor earth.
Other cars may have similar issues, to fix remove the light, then look at the wires on the aerial, if possible remove the connector and clean with a wire brush to get a better earth and reconnect. see if that works.
penneck
Grafter
Posts: 655
Thanks: 8
Registered: 03-08-2007

Re: Car Radio Antenna problem

I haven't taken the car through a carwash - it is an Sri140 so the spoiler on the tailgate means I shouldn't do that, so I don't. I just don't understand why I get VHF clearly and loudly, while the MW and LW are awful - that is what I am trying to get an answer to.
As for the suppression problem - the suggested solutions are the same as those we used to do as far back as when I bought my first car 45 years ago. In those days, cars had dynamos and points and such, unlike today when they have alternators and electronic ignition
I say they haven't changed, but that doesn't include shutter's suggestion about sticking a wire to the windscreen - nice one, shutter. I may try a temporary one with coax, to see the result, before I 'glue' it in.
I'm not so keen on taking the antenna apart to check the earth connection. It means taking the roof lining apart to get at the fixing screws, and I'm not sure I could get it
the lining back afterwards. However, if needs must....
Is the antenna coax screen earthed at both ends? That might not be such a stupid question because earthing both ends can cause earth loops which puts electrical 'noise' on the signal conductor.
Regards
Community Veteran
Posts: 6,586
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Registered: 16-02-2009

Re: Car Radio Antenna problem

It is earthed at both ends, at the aerial end on the chassis. at the radio end by the radio chassis.
Community Veteran
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Registered: 02-08-2007

Re: Car Radio Antenna problem

I guess the reason you get a good signal on VHF is due to the fact it is very high frequency but equally has a much shorter signal range than MW and LM.
Many things can effect MW and LW signals such as weather conditions whereas VHF is not as vulnerable.

PowerLee
Rising Star
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Registered: 12-03-2013

Re: Car Radio Antenna problem

Wouldn't be surprised if you find the rubber seal around the aerial base has failed & started to let in water.
The aerial base is also plugged in to the cars electrical system as there is a aerial signal amplifier used on the Vectra.
Removing the aerial base, cleaning up the area the teeth bite in to the metal skin & adding a small bead of clear silicone sealant around the hole in the roof on the outside fixes water leaks with roof mounted aerials on cars.
Community Veteran
Posts: 16,834
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Registered: 06-11-2007

Re: Car Radio Antenna problem

If you use coax. for my suggestion.... make sure that the outer shielding does not have any loose hairs/single strands of wire floating loose, at both ends... tape up with a bit of insulating/sellotape, where you use the ordinary "speaker" wire for the bit up the windscreen..
As gleneagles says... VHF can be picked up on a very short aerial... ( hence the little bit of stick on the roof)... whereas to get MW and LW.. ideally you need a good long wire... so you may notice some roof aerials have a sort of spiral spine to them... ( long bit of wire wound round to keep it short ! ! )... The length of wire up the windscreen is not critical... but if you are not getting enough signal... add some to run across the top of the screen....
If you can get some black gaffa tape, then you could run a strip of that to hide the wire... !..