Heated rear screen and interference with radio reception

This week my 89 RR has decided to spring yet another sinister surprise on me. When I turn my heated rear screen on I get a dreadful noise on
the radio. The filter or choke on the screen must have given up the ghost. Any idea where it is so as I can replace it
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On 22 Feb 2005 15:15:20 -0800, Hugh Hogan wrote:

What sort of "dreadful noise"?

Why would a bit of wire bonded to the glass require a filter or choke?
Depending on the description of the noise you give I suspect that there is some problem with the alternator that only really shows up under hefty load.
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On or around Wed, 23 Feb 2005 00:27:07 +0000 (GMT), "Dave Liquorice"

I wouldn't bet. how is the heated screen supplied, doe sit have any sort of controller?
some of the mobile phone chargers I've had play merry hell with the radio reception.
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Austin Shackles. www.ddol-las.fsnet.co.uk my opinions are just that
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On Wed, 23 Feb 2005 07:33:14 +0000, Austin Shackles wrote:

Thats almost certainly due to them being made down to a price and containing a DC - DC convertor with minimal, if any, attention being paid to interference supression.
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">

Because the heater element is used as a radio aerial and the choke prevents the radio signal form being sent into the wiring rather than the coax to the radio.
Jeff
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On Wed, 23 Feb 2005 08:35:32 -0000, Jeff wrote:

Fairy Nuff. But a bit of wire bonded to glass is not going to generate noise, the noise is coming from else where in the vehicle. Any filter will be to keep that out of the radios RF input.
Due to the currents involved with a heated screen I doubt that the filter is in the power DC wiring but in the RF side somewhere, probably where the coax connects to the screen elements, which might be at both ends.
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The choke is to be high impedance to RF so any signal received goes down the coax and can't simply treat the power connections as a flat short to the chassis. I would expect there to be one in each leg (power and ground) or a common one in both legs.
The DC power goes through the choke like it isn't there.
nigelH
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Usually the wiring would be long enough to prevent shorting the RF to ground. Just a capacitor to prevent the radio from 12V on its antenna jack, and there you go.
regards - Ralph
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No, without a choke signals would be severely compromised, especially at VHF. The length of the wire is almost irrelevant. If you think about it any load, eg a bulb would shunt the signal to ground, and that is neglecting the capacitive effects of other wiring and the body. Also any interference in the loom would be fed directly to the radio without a choke.
Regards Jeff
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Then start calculating how long the wire from the rear window to the fuse box is - its length is absolutely no shortcut for frequencies around 100MHz. Been there, done that :-)

The capacity weakens the signal a little bit, but in most cases not very much.

regards - Ralph
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Utter cobblers!!! particularly for MW & LW, but neglecting that, say the wiring is about 4m long, this is about 1 wavelength at the low end of VHF, so what you have done is put an indeterminate element on to the antenna of about 1 wavelength long shunted to other wiring and the body by indeterminate capacitances. Now for such a connection to appear open circuit to the antenna it would have to look like one quarter of a wavelength (or multiples there of). The configuration that you are suggesting will look nothing like that, it will be a complex, and probably variable, impedance which will act as a load on the antenna. The other problem is that the vhf band is very wide and the effects will vary across the band.
At MW & LW the effect will be very marked, the wiring is very short compared to a wavelength and will be a very good shunt for the signal.
Also don't neglect the reverse path, i.e injecting signals into the radio from the rest of the wiring, without a choke there is a direct path straight into the radio with no attenuation.
Jeff
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Ralph A. Schmid, DK5RAS wrote:

But it's not high loss so the signal will reflect from every discontinuity and the impedance on a Smith chart will be all over the shop and annoy the radio front end no end. The dimensions being comparable to the wavelengths involved is very bad news for a nice flat response over the whole FM band.
Jeff is right about a choke being important and at around 100MHz it is a trivially simple/cheap component.
nigelH
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The choke is to be high impedance to RF so any signal received goes down the coax and can't simply treat the power connections as a flat short to the chassis. I would expect there to be one in each leg (power and ground) or a common one in both legs.
The DC power goes through the choke like it isn't there.
nigelH
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Thanks for the replies to date. However I can tell you that there must be a filter or choke on the system somewhere as the problem is only new and there is no problem with my alternator. Nor is there any problem with any other electrical unit in the RR.
My question was where is the choke / filter and I guess no one really knows. My guess is it is included in the amplifier under the headlining but I dont want to take that down unless I know for real.
Thanks anyhow
Hugh
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Hugh Hogan wrote:

To the best of my knowledge the choke _is_ in the amplifier unit in the headlining. But, before you dig that out check for breaks in the HRW element. My radio developed similar symptoms to those described and I went chasing all over the place trying to find a branch of Maplin that had old stock of their kit that enabled the HRW to act as an aerial (without success) until someone suggested checking for breaks in the HRW element. Breaks found and repaired _very carefully_ with Halfrauds HRW paint and all was well.
HTH
Richard
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