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From: jhs%Mitre-Bedford@sri-unix.UUCP
Newsgroups: net.ham-radio
Subject: Re: some questions about UNIDEN CR2021
Message-ID: <12144@sri-arpa.UUCP>
Date: Tue, 18-Sep-84 11:54:00 EDT
Article-I.D.: sri-arpa.12144
Posted: Tue Sep 18 11:54:00 1984
Date-Received: Wed, 26-Sep-84 00:53:26 EDT
Lines: 31

Not knowing anything about the radio you are asking about, I will speculate
that the "antenna adjust" knob is like those on other radios since time
immoral:  a tuning capacitor provided to resonate the input tank circuit
for maximum signal transfer to the first amplifier, mixer, or whatever.
Traditionally, these "antenna trimmers" are used to allow manual correction
for any detuning effect of an antenna whose feed-point impedance differs from
the ideal value contemplated in the design of the front-end resonant circuit,
i.e., is highly reactive.

Some receivers in the old days provided a low-impedance input arrangement
for "real" antennas plus a tap on the top of the coil for a "short" antenna.
An especially popular and useful arrangement was to have a center-tapped
primary coil giving your choice of 75 ohms unbalanced or 300 ohms balanced
input, with a convenient grounding strap to make the selection.  (Remember
that impedance varies as the square of turns, so that a center tap gives a 4:1
choice of ratios, not 2:1.)  Often the short antenna was provided as a
built-in whip, as in your radio.  It might well be left connected at all times
on the theory that if you collapse it and use a "real" antenna, the whip's
small residual capacitance can be corrected for by twiddling the antenna
trimmer.

Still sticking to my position that full-blown antenna tuners are a waste of
time on most receivers, I will allow as how the combination of several
impedance taps and the trimmer gives a "mini" antenna tuning capability which
is useful.  In fact I suppose if one of these newfangled receivers didn't even
give you a trimmer capacitor to adjust, a tuner would be a reasonable way to
correct the designer's oversight.

					73,
					John H. Sangster, W3IKG
					jhs at mitre-bedford