From: ornitz@kodak.kodak.com (Barry Ornitz)
Subject: Re: Car antenna design help wanted
Followup-To: sci.electronics
Summary: Why?
Keywords: antenna, amplifier, impedance matching, trunstile, Alford
Organization: Eastman Chemical Company
Date: Fri, 9 Oct 92 23:18:33 GMT

In article <7'#@byu.edu> richard@ee.byu.edu (Richard Christensen) writes:
>I want to design a better car stereo antenna system. The 
>system would include impedance matching elements, baluns, 
>and an amplifier.

This seems a little silly to me since the typical auto antenna is an
unbalanced antenna feeding an unbalanced load.  What is the balun for?
Why do you need an amplifier?  With most radios, you have more than
adequate sensitivity.  Adding an amplifier will only make the radio
perform poorer because most receivers are not designed to perform well
in a high-signal environment.  Intermodulation (RF) and "front-end
overload" will prevail.

>  My question is, does anyone have a good 
>book on how the antenna works on a car.  Almost all books 
>on antenna theory, do not include anything about the car 
>antenna system.  For instance, I understand that the antenna 
>is capacitively loaded. Do the receivers anticipate and 
>match the antenna correctly, or do they just live with 
>the mismatch?  Does the antenna impedance match the 
>impedance of the feed cable or the receiver?  Any help 
>would be greatly appreciated.

Automobile antennas work just like any antenna.  The car acts an a ground
plane and the antenna is a monopole.  Unfortunately most auto antenna
systems are designed on the basis of "automobile styling" rather than
radio performance.

Typical auto antennas are a little short of a quarter wavelength for the FM
band, thus they are capacitively reactive and relatively low impedance.
Impedance matching is rarely a problem in receiving applications, however.
In fact, an optimum low-noise amplifier will be deliberately mis-matched for
lowest noise figure.  Impedance matching for maximum power transfer is fine
for transmitters - but an entirely different situation for receivers, especially
when weak-signal reception is desired.

If you really want an optimum antenna for your car radio, you will site the
antenna in the middle of your roof with the maximum ground-plane surrounding
the antenna.  If you want to see how antenna placement affects antenna
patterns on automobiles, you should check into the ARRL (American Radio
Relay League) Antenna Handbook, a book for amateur radio operators but you
should find it interesting.

The problems with good FM reception from an automobile are really due to the
fact that the auto is mobile (no pun intended).  Multipath reception will
cause fading and distortion independent of the antenna.  You might try a
spatial diversity antenna system, but you will need two receivers and the
necessary circuitry to select the audio source with the best signal.  Some
autos are too small for a spatial diversity antenna system to work well.

Alternately, you might consider a horizontal turnstile antenna on the roof
of your car to get approximately omnidirectional horizontal polarization. The
roof of an auto might be a little too small for an Alford array!  ;-)

I suggest you read the ARRL Antenna Book.  It is a good practical reference
(and I think it still costs less than $20).  I doubt, however, if you will
see spectacular improvements in your reception.  This sounds like a school
project, so I encourage you to experiment and prove this.  Good luck.

						Barry
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|  ___  ________  |       Dr. Barry L. Ornitz          WA4VZQ
| |  / /        | |       Eastman Kodak Company
| | / /         | |       Eastman Chemical Company Research Laboratories
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Newsgroups: sci.electronics