On Sun, 29 Nov 2009 13:43:11 -0500, "Michael A. Terrell"
wrote:

Klaatu wrote:

You forgot to mention that with age, the antenna coaxial cable would get
water in it, and reception would suffer. Adjusting the trimmer would
make little or no difference in this situation. Replacement was the
only fix.

There would be little or no reception by that point, and replacing
the antenna was a common repair for me in the '60s & '70s. We kept a
used car radio antenna in the service department to test a radio in the
car. Unplug the car's antenna and plug in the test antenna. if you
picked up some stations, the antenna was bad & had to be replaced. At
that time we could get most OEM antennas delivered in two days or under.

That was also in the days when the coax cable was attached to the
antenna base with a weird connector. I found far more coax cables to
be defective than antennas. The cable used was some kind of very low
capacitance coax, with a very tiny center conductor floating inside a
plastic tube. The center wire would often break from the vehicle
vibration. I couldn't find any details on the coax type.

These daze, the antenna is built into the windshield or side window of
the vehicle.

Looking at the data sheet of a modern AM/FM front end chip, the AM
section appears to be Hi-Z input:
http://www.atmel.com/dyn/resources/prod_documents/doc4913.pdf
See Page 5.
ATR4251 provides an AM buffer amplifier with low input
capacitance (less than 2.5 pF) and low output impedance (5ohms).
The low input capacitance of the amplifier reduces the capacitive
load at the antenna, and the low impedance output driver is able
to drive the capacitive load of the cable. The voltage gain of the
amplifier is close to 1 (0 dB), but the insertion gain that is
achieved when the buffer amplifier is inserted between antenna
output and cable may be much higher (35 dB). The actual value
depends, of course, on antenna and cable impedance.

No mention of any AM trimmer capacitor tuning. My guess(tm) is that
older AM front ends were looking at a tuned circuit, instead of a
broadband amplifier. The tuned circuit was looking at some specific
capacitance in order to be on frequency. The trimmer compensated for
the variations in cable and antenna capacitance in order to resonate
this tuned circuit.



--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558