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[petec]

I have just stumbled on a fix for a long standing radio reception problem on the mega and I don't recall seeing the cause being highlighted as a particular weak point before (apologies if this is all thoroughly documented and I have missed it)
Ive had my CDX estate  for about 7 years now, the radio was ok but not particularly sensitive. When I changed head unit for an upmarket Alpine unit 6 years ago the sound was great but radio reception was not at all brilliant (standard roof mounted whip aerial). I put up with it for ages as most of time listened to ipod or CD and anticipated that the problem probably lay in the aerial amp or a dodgy connection behind the head unit or in the wheel well.

Anyway.....  just out of interest I unscrewed the whip aerial from the base and temporarily replaced it with a similar one from "she who must be obeyed"'s  clit. Must admit I thought this was probably pointless as a metal rod is a metal rod right? but reception was instantly transformed.  I just got a fleabay whip of the same size and type for £4.99 and whereas before I could only tune in 8 stations on fm band now I get 29..sorted.

It seems over a period the rubber section at the bottom of the whip continually flexed and the twig lost continuity.  So if your estate has a standard whip toward the back of the roof and your radio has trouble finding stations its worth sticking a bit of coat hanger or welding rod into the aerial base to see if it improves before delving any deeper  
Of course this does not apply to saloons with aerial in rear screen heater and not sure whether motors with satnav are different.

[terry paget]

Thanks. That's very interesting. Reception on my 2000 CDX estate is inferior to that of my saloons. I had the impression that the aerial on Omega estates was amplified and that was as good as possible. You say you bought an e-bay whip aerial for £4.99 that improved things. I imagined that I had to buy an aerial compatible with the amplifier. What aerial did you buy?

[petec]

Hi, I know I said I got a £4.99 aerial, forgot I actually went for a seemingly identical one from another seller for £6.99 because it specifically mentioned Omega although I am pretty sure they are identical.
. Not sure about linking to other sites so if you go onto ebay.uk its item number 280758567777, price now down to £5.59 with free delivery.

My radio transmission theory is a bit rusty now as I have not used it for many years but you make a fair point about matching to amplifiers. I was under the impression that the estates have an aerial amp but not sure of its location if so. I have read on other posts that its in the tailgate but that does not make much sense for a roof mounted aerial. The most likely location is in the aerial base but have not removed it on my estate so cannot be sure. If the amp is a booster in the coax from the aerial then any matching would be with regard to the coax impedance and the expected signal level in from the antenna. If its part of the aerial assembly I am guessing (happy to be corrected) that any matching would just relate to the antenna design i.e. 1/4 wave, 1/2 wave etc which relate to the physical size of the antenna.



I decided to do an autopsy on the old twig this afternoon to find out why it died. The aerial is 40cm long, and consists of a 29cm fibreglass rod crimped to the end of a 10cm stainless steel spring with the thread attached at the bottom. The aerial itself is a very thin, flat copper wire (maybe .3mm across and only a couple of thou thick) wound helically around the fibreglass and soldered where it crimped to the spring. This is to produce an ideal length aerial wire in a short package. there is another insulated wire wound round this assembly but not physically connected The whole assembly has been plastic coated then the rubber section at the bottom added.
On mine it looks as though the plastic coating had failed about half way up and the copper track had oxidised and broken near to the crimp at the bottom rendering the antenna pretty useless although there were no visible signed externally

The one I bought on Fleabay is identical to the original. It looks the same, is the same length and diameter  and uses the same helical windings so gain and impedance should match the original closely...and it works fine for me. There seem to be several sellers offering an identical looking unit at similar prices.

I did try sticking a length of wire in the base connector to see if you could prove whether you have a dodgy twig. A standard escort aftermarket aerial (straightened coathanger) works better than my old aerial, seem to remember 95cm is good length for FM

[zirk]

1/4 Wave Length centred at 98 Mhz, as you say wrapped around a fibreglass stick.   

Only improvement without looking stupid would be a 1/4 Wave straight whip (approx +3db over the Helicoil) which can be vertical or horizontal mounted as most UK Band II Commercial Transmissions are Circular Polarized.

[petec]

think I've got an old 16 foot ex-wd tank aerial somewhere in the shed, just the job although you would need to be a bit careful under low bridges

[terry paget]

Thanks petec. I've ordered one. Will report back.

[ffcgary1]

intresting thread, was hoping it was gonna help my problem but no deal, my reception is fantastic on both AM and FM, BUT  AM is wiped out as soon as the wheels move, AND I MEAN AS SOON AS THEY MOVE,i am thinking abs sensor is causing it.
Good fix all the same.

[petec]

[quote   AM is wiped out as soon as the wheels move, AND I MEAN AS SOON AS THEY MOVE,i am thinking abs sensor is causing it.
Good fix all the same.
[/quote]
That's an interesting problem, presume am band is drowned out with interference as soon as you are rolling. Often that type of interference is caused by bad earthing/ dodgy screening.  I seem to recall that there is a "speed" pulse supplied to the radio connector which allows  head unit to increase volume as road noise increases, that might be the nearest candidate for motion related interference. I wonder if you have a loose chassis earth to head unit or interrupted screen to aerial coax. The aerial screen will be grounded at the head unit and I believe there is a connector in one of the foot wells. Might be worth checking all plugs are firmly in place before looking further.

[petec]

I just found this old post from 2004 on another site which relates to MW interference when moving

I have a 2003 Vectra, which suffered from severe radio interference on
 medium wave, but only when the car was moving. There are several
 newsgroup articles describing this problem which also affects Astras,
 but with no definite solution. I took it to the dealer several times,
 and they first reseated the cables, then replaced the radio (under
 warranty), then reseated the cables again, and each time it was OK for
 a day or two, then the problem returned.

 Finally, I persuaded them to talk to Vauxhall technical support, and
 they were advised to fit a suppressor to one of the cables (sorry,
 they couldn't tell me which one), and this has cured the problem.

It sounds very similar, no idea where a suppressor may have been fitted but it sounds as though this may be a known problem and maybe there is a technical bulletin covering it if anyone can find it.

[terry paget]

I have bought and installed the aerial recommended. Sadly, it has not cured my fault.

My fault is not old fashioned ignition crackly interference, varying with engine speed. I last heard that when I fitted a set of aftermarket ignition leads to a 2.5 Omega saloon. My radio reception is fine with the car stationary, engine running, but a hissing when in motion comes and goes in waves about a second long  - this on FM. I suspect a poor comnnection somewhere affected by the motion of the car. I had hoped it might be in the aerial; now I wonder if it is between the aerial lead and the head unit. Certainly when I remove the head unit the aerial lead has already disconnected itself. I wonder how to tighten that connection. Any suggestions?

[petec]

I did read in a post somewhere on the site that the aerial lead to head unit connection is a potential weak point.  Apparently there is also a connection in the wheel well, not sure which side which needs checking. You could unplug there and check continuity between there and head unit connector if you have a multimeter. Not sure about tightening the connection, its been a few years since I saw it. Normally either the pin or the socket will have split/sprung sides which holds plug in place, if so a bit of judicious tweaking with a fine screwdriver to spring the sides out a bit might help.

It might be worth temporarily sticking an extra earth cable from the head unit chassis to a suitable nearby earthed point to prove the earth in the loom is ok if the above doesn't improve things. Looking at a Haynes wiring diagram which may be accurate   the aerial lead screen is grounded at the head unit and the amplifier looks as though it is integral with the aerial base on the roof. The screen would only normally be grounded at one end to prevent an eth loop which itself could generate noise but its possible the base unit fixing screw could be providing an eth to screen the electronics so next place is probably to check the mounting is clean and tight. Hopefully you would be able to drop the back edge of the headlining enough to check this out without major strip out although not had to go there myself (yet). I would be interested to know how you get on.