bpnjensen wrote:
On Jun 13, 7:22 am, wrote:
On 13/06/2010 4:30 AM, DEFCON 88 wrote:

OK, this deviates somewhat from the topic, but I've always wondered
about window antennas. My Mitsubishi Galant has an antenna embedded in
the rear window. It's not one of those dipoles that you used to (or
still?) see embedded in the front windshield of GM cars. Rather, it
has a peculiar folded design that's hard to describe. It works very
well. I get exceptional reception, especially on AM. In fact, one of
the reasons I chose the Galant is because it had the best AM reception
of all the cars I test drove. What's the theory on these window
antennas? Do they have a pre-amp ahead of the radio?

The theory is they are simple and vandal proof! They also lessen the
effect of an external aerial on the aesthetics and the aerodynamics of
the car. No wind whistles on these window aerials!

Krypsis

I don't think the windshield-internal dipoles work as well on FM as a
good external vertical whip of the proper dimension. They are not
omnidirectional, nor are they vertically polarized, both of which are
helpful for FM in a moving car. The folded ones may have different
characteristics that overcome these drawbacks, I don't know for sure.

Moreover, they are not likely the AM antennas at all - almost every
car AM radio since dirt has used a ferrite- or air-core coil antenna;
far more effective than a whip or wire ever could be. Especially the
ferrite, it gets you what a whip cannot - ~ full wave resonance and
high relative gain. Match that with a quality AM radio and you'll
have a nice MW DX machine in your car. I love listening to stations
on the Great Plains when driving overnight to Yellowstone :-)

Bruce Jensen

Since dirt until very recently AM car antennas have been active whips.
Note the very tiny center conductor in a car antenna co-ax; because the
impedance is so high at 1 MHz. The 31" whip, plus the lead, is your AM
antenna. Same as an MFJ or Stoner Dymek active. No difference. Short
whip into an FET. The old radios had slug-tuned PTOs. Maybe that's
what you're thinking of.