Re: Radio static/Defroster

[dan_masi@MENTORG.COM (Dan Masi)]

On Dec 3,  1:40am, STEADIRIC@aol.com wrote:
> Subject: Re: Radio static/Defroster
> >Another data point: it is signal strength dependent.  I'll be on Route 4 in
> >Durham NH listening to Maine Public Radio or WGBH in Boston, and turning on
> >the defroster will trash the reception.  If I then switch to WUNH, whose
> >antenna I am then right under, the static almost disappears.  It's not being
> >masked by volume - even if they are playing something quiet this is apparent
> >.  So, dear listers, if you have any knowledge of RF arcanery, what's up? 
> >Seems to me that putting the defroster into an electrical circuit is causing
> >it to interfere in the antenna's ability to pick up a clean signal.  Is this
> >how radio works?  can the defroster "out-antenna" the regular antenna, or
> >sheild it?  Is it acting as a diffraction grating/prism?  Could there be
> >some sort of filtering that would help, i.e. a small inductor in series with
> >the defroster, and/or a high quality capacitor to ground right at its
> >connections?
> 
> First off when your directly under (or within 2 miles) of a broadcast 
> "Stick" (Tower for those not in the biz...) your actually getting a 
> greatly REDUCED signal (On the order of 70db...) Because of antenna 
> polarization patterns. 

Has nothing to do with polarization, it's simply the transmission
pattern (lobes) of the radiator.

> I think that you have some problem with the antenna and ground 
> plane because what your describing is a Faraday shield 

Nope.  Faraday shielding isn't affected by potential.  

> ..... The other thing that you might look at is your alt, it's voltage 
> reg and it's diodes.... If it is passing ANY A/C it would be reradaited 
> by the defroster grid and it might just be powerful enough to mask you 
> antenna... (Think Jammer here Joe...)

More likely ignition noise.  I suppose that if there's a large
amount of ignition noise being passed back to the defroster, the
receiver's front end (antenna amp) could be overloaded and thus
gained way back.  Another possibility is just a DC problem.
It's been mentioned that some Audis have an antenna amplifier that
is powered by the same lead as the defroster.  A defroster will
draw a fairly high current.  If there is a connection somewhere
in that circuit that has degraded and turned too resistive, that
current draw will spell voltage drop.

The first thing I'd do is to get a multimeter, and measure the
power for the antenna amp, RIGHT AT the antenna amp's input.
Measure it with the defroster off, then on.  If it drops by
more than, say, a volt, something needs to be fixed.

If the supply voltage is OK, then I'd try putting an inductive
filter on the supply.

Dan Masi
'96 A4Q