Heated rear screen and interference with radio reception

H

Hugh Hogan

Guest

• Feb 22, 2005

• #1

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

 

D

Dave Liquorice

Guest

• Feb 23, 2005

• #2

On 22 Feb 2005 15:15:20 -0800, Hugh Hogan wrote:

> When I turn my heated rear screen on I get a dreadful noise on
> the radio.

What sort of "dreadful noise"?

> The filter or choke on the screen must have given up the ghost. Any
> idea where it is so as I can replace it

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.

--
Cheers [email protected]
Dave. pam is missing e-mail

 

A

Austin Shackles

Guest

• Feb 23, 2005

• #3

On or around Wed, 23 Feb 2005 00:27:07 +0000 (GMT), "Dave Liquorice"
<[email protected]> enlightened us thusly:

>On 22 Feb 2005 15:15:20 -0800, Hugh Hogan wrote:
>
>> When I turn my heated rear screen on I get a dreadful noise on
>> the radio.
>
>What sort of "dreadful noise"?
>
>> The filter or choke on the screen must have given up the ghost. Any
>> idea where it is so as I can replace it
>
>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.

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.

--
Austin Shackles. www.ddol-las.fsnet.co.uk my opinions are just that
0123456789112345678921234567893123456789412345678951234567896123456789712345
1 weebl: What's this? | in recognition of the fun that is weebl and bob
2 bob: it a SigRuler! | check out the weebl and bob archive:
3 weebl: How Handy! | http://www.weebl.jolt.co.uk/archives.php

 

J

Jeff

Guest

• Feb 23, 2005

• #4

">
> Why would a bit of wire bonded to the glass require a filter or choke?
>

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

 

D

Dave Liquorice

Guest

• Feb 23, 2005

• #5

On Wed, 23 Feb 2005 07:33:14 +0000, Austin Shackles wrote:

> some of the mobile phone chargers I've had play merry hell with the
> radio reception.

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.

--
Cheers [email protected]
Dave. pam is missing e-mail

 

D

Dave Liquorice

Guest

• Feb 23, 2005

• #6

On Wed, 23 Feb 2005 08:35:32 -0000, Jeff wrote:

>> Why would a bit of wire bonded to the glass require a filter or
>> choke?
>
> 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.

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.

--
Cheers [email protected]
Dave. pam is missing e-mail

 

N

Nigel Hewitt

Guest

• Feb 23, 2005

• #7

Dave Liquorice <[email protected]> wrote:

> 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.

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

 

N

Nigel Hewitt

Guest

• Feb 23, 2005

• #8

Dave Liquorice <[email protected]> wrote:

> 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.

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

 

H

Hugh Hogan

Guest

• Feb 23, 2005

• #9

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

 

R

Ralph A. Schmid, DK5RAS

Guest

• Feb 24, 2005

• #10

"Nigel Hewitt" <[email protected]> wrote:

>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.

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

--

Want to get in touch? http://www.radio-link.net/whereisralph.txt

 

J

Jeff

Guest

• Feb 24, 2005

• #11

>
> 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.
>

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

 

R

Ralph A. Schmid, DK5RAS

Guest

• Feb 25, 2005

• #12

"Jeff" <[email protected]> wrote:

>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,

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

>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

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

>would be
>fed directly to the radio without a choke.
>
>Regards
>Jeff
>




regards - Ralph

--

Want to get in touch? http://www.radio-link.net/whereisralph.txt

 

J

Jeff

Guest

• Feb 25, 2005

• #13

>
> 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.
>

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

 

N

Nigel Hewitt

Guest

• Feb 25, 2005

• #14

Ralph A. Schmid, DK5RAS wrote:
> "Jeff" wrote:

>>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,

> 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

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

 

R

Richard

Guest

• Mar 7, 2005

• #15

Hugh Hogan wrote:

> 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
>
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

--
Real email address is RJSavage at BIGFOOT dot COM

The information contained in this post
may not be published in, or used by

http://www.diyprojects.info