Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!watmath!clyde!burl!ulysses!mhuxj!houxm!vax135!cornell!uw-beaver!tektronix!hplabs!sri-unix!jhs@Mitre-Bedford 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