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Re: Lousy reception with aftermarket stereo in Audi

> From quattro-owner@swiss.ans.net Fri Oct 28 09:50:59 1994
> Date:     Fri, 28 Oct 94 9:18:18 EDT
> From: Chris Idleman (FSAC-AAD) <cidleman@PICA.ARMY.MIL>
> I've already taken care of this one (or so I thought)!  The aftermarket 
> antenna I installed is mounted in the original antenna hole (halfway up
> the left "c-pillar" or "sail panel".  I made sure I put in the longest
> antenna I could find -- it's got a 39" mast so approximately 2-1/4 feet
> extend above the roof of the car.  I originally thought that this would
> solve my poor reception problem but it only marginally improved the FM
> and didn't seem to help AM at all.  Without the in-line signal booster
> I added, the signal is too weak to lock on to.  I also suspected that
> there was a problem with the antenna lead, so I replaced the entire 
> cable.  Since the only places my stereo system "touches" my Audi's 
> factory wiring is at the battery + terminal (hard wired) and miscellaneous
> grounding points, I have to suspect that the problem is in the supply
> voltage -- ie: something in the Audi electronics is causing my reception
> problems.

Not so fast: there's a big difference between interference and nothing
at all.  If your reception is so weak that you cannot lock onto
stations, the problem is probably in your pull-out box, since that is a
variable that hasn't been addressed and the one most likely to kill all
FM reception.  A shorted or open antenna connector will do the trick,
so I would suggest connecting an ohmmeter across the antenna connectors
on the box while the radio is in the box.  The reading should not be
less than an ohm nor more than a few hundred (the stated 75-ohm
impedance of the input/cable has nothing to do with dc resistance).

Also, having an antenna longer than 33.6 inches will decrease FM
reception as a mast antenna's optimal length is lambda/4, where
lamda=wavelength of the signal.  This is determined from lambda=c/f,
where c is the speed of light (3x10^8 m/sec) and f=frequency of the
signal.  For the low end of the FM band (88 Mhz), which would need the
longest antenna, this works out to 33.6 inches, and the optimum length
decreases as frequency increases (to about 28" for the high end of the
band.) Calculation of this number is left as an exercise to the

John Greenstreet, Senior Engineer           (jgreenst@motown.ge.com)
Martin Marietta Government Electronic Systems    Moorestown NJ 08057
WPI Class of '75, Temple Class of '94

My new car history:
  1975    1978    1982       1986        1989      1992      1995
   VW ->  Audi -> Audi  -> Mercedes -> Mercedes -> Audi -> Mercedes
Scirocco Fox GTI  4000S    190E 2.3    190E 2.6    100CS     S320

POSSLQ's* new car history:
         1978       1981       1985      1988        1990     1993
       Triumph ->  Toyota ->  Toyota  ->  VW    ->   Audi  -> Audi
       Spitfire    Tercel     Corolla   Jetta GL      80      90S

*POSSLQ = Person of Opposite Sex Sharing Living Quarters
Note: All Audis and Mercedes above were sold to friends or family.