Re: Antenna research

[Sandy Duffy <sandy@pcnet.com>]

At 10:28 PM 3/6/96 CDT, you wrote:
>Using them one at a time with the replacement Sony stereo installed, 
>the front antenna can't pick up SQUAT on AM.  Not even local 
>stations.  The rear antenna can pick up moderately well on AM, but 
>gets hideous interference with my favorite AM station - and it's 
>being generated by a local FM station!  I have NEVER had this problem 
>before - but the Audi had it with the stock Bose system, and it's 
>even worse with a more sensitive radio, which the Sony is.

Al and all else interested:

You failed to mention this, but, is the FM station fairly nearby?  If it is,
the FM signal is getting into the AM antenna amplifier and modulating it
with its AM noise component.  This component is generally 50 db or so
beneath the FM component but it is there.  It will show up if you are
reasonably close to the transmitter. If you are far away from the
transmitter, then you probably have a bad amplifier for the rear antenna or
a bad connection between the rear antenna and the amplifier.   My guess on
these Audis with all these problems regarding AM and lack of sensitivity is
a bad amplifier on the AM/FM rear antenna.  You see the rear AM/FM antenna
is nowhere near adequate for AM unless it is electrically doctored and has a
non-standard impedance to perform for the AM band.  When the amplifier
fails, you get no AM at all.  When the connection between the antenna and
the amp fails, you still get no AM but the amp is now running wide open
looking for any signal at all and amplifying whatever it finds.  In your
case it is amplifying the AM noise component of an FM station.  In effect
the antenna becomes super-sensitive to ignition noise, alternator whine,
other stations, CB radio, Ham radio, television...anything in the electrical
spectrum! 

>I even tried using them without connecting the outer shield of the 
>antenna coax - which worked great for SteadiRic, but it just 
>eliminates all reception for me.

Steadiric has a shorted antenna problem.  Your antenna is not shorted and so
you're removing the "ground" side of the antenna from your radio when you do
this.  Every antenna consists of two parts; the "ground plain" part of the
antenna and the "stick" part of the antenna.  Both parts are equally
important.  If one part is bad the reception will suffer.  Most people
ignore the ground plain of the antenna because it is not really the visible
part.  It is just as important as the stick.  Most radio reception problems
can be traced to a poor ground plain not a bad antenna (except, I think,
Steadiric, who is describing a classic case of a shorted stick).  When the
antenna is replaced a new ground is created because the new antenna contacts
fresh metal and gets a new good "ground".  Hence, a new "stick" part has
cured the problem when in reality the ground has been reestablished.

>THEN, I tried something which worked.  I keep a good ol' $15 Radio 
>Shack cheapie fender mount antenna in the garage in case one of out 
>others gets broken, etc.  I plugged this guy in and went for a drive.
>Good AM reception, with NO interference.  None.  Not even in the 
>worst spots for interference.  As long as I had any of the 
>traditional antenna above the body metal (even inside the car) it 
>worked.  FM seems as good as it was before.

The reason this worked is because the Sony is electrically matched to any 50
ohm antenna which is what that "good-ol' $15 Radio Shack
cheapie-fender-mount-antenna-in-the-garage" is.  If you have a Japanese
radio, 50 ohms is the standard impedance and it will perform best with that
antenna. (Use the japanese Harada antenna) Unfortunately Audi is a European
automobile and the Euro standard is 75 ohms.  If you have a stock radio, the
replacement antenna should be 75 ohms.  (Don't use Harada, use the german,
and much more expensive, Hirschmann.)  Of course, both companies make both
impedance antennas but you need to CAREFULLY read the data sheets to make
sure you're getting the correct impedance for your radio.  To make matters
worse, some radios put in OEM in Audis require 50 ohm antennas because they
are made in Japan for Audi and others even though they are made in Japan
were made to the 75 ohm spec.  It helps to consult someone.  The system
you're referring to it your set up is most likely neither 50 or 75 ohm but a
weird impedance to allow the diversity system to work,  to have AM
reception, and to lock you into the "Bose" radio and Audi dealer network.
<grin> (see above note) This is why other radios plugged into the existing
antenna system will not operate well.

Can you understand why I've been lurking watching this thread go on?  There
is a lot to say that DOESN'T meet the eye here.  Most car stereo places such
as Crutchfield etc. don't even know about the different impedances for Euro
vs. Japanese radios.  I been through electrical engineering at college and
have been in the stereo business since 1969 about two-thirds of that time as
a manufacturer's rep.  I am very familiar with diversity receivers and 12
volt applications.  It is amazing to me to see the misinformation given out
by many of the people in the 12 volt biz today.  I got fed up with it about
three years ago and resigned my 12 volt catalog to concentrate on home and
home theater systems.  

Sorry for all the bandwidth I'm taking up on this. I am posting this to the
list to help answer some questions that seem to be persisting.  I hope it
helps.  I couldn't keep quiet any longer.  

Sandy

PS.  I can also tell you why most people's rear speakers aren't functioning
after a radio replacement (equal amount of bandwidth).  Let me know if you
all still need this.