From: jkearman@arrl.org (Jim Kearman)
Newsgroups: rec.radio.shortwave
Subject: Antenna Tuners
Date: 20 Jul 92 13:02:38 EDT
Organization: American Radio Relay League

          
In article <3138@peking.gdwb.oz.au> mtonkin@gdwb.oz.au (Malcolm Tonkin) writes:
>I live in south east Australia and intend setting up a long wire antenna
>for my Sangaen 803.
>
>I have a basic understanding of the functions of an antenna tuner but would be
>greatfull for any advice on the relative merits of different tuners that would
>be suitable to my needs.
>
>To date I have been unable to locate any suppliers of antenna tuners in
>Australia.  I would really like to hear from an Australian who can suggest a 
>local supplier or who has a tuner for sale.
>
Richard Crisp writes:

>Antenna tuners are primarily useful in transmitting applications where
>minimal SWR is key to delivering full power to your antenna and eliminating
>potentially harmful (to the radio) standing waves. For receivers they typically
>don't offer any real benefit except in the situation where you have a strong
>station that causes front end overload and therefore affects your ability
>to receive weaker stations properly.
 
>This is the conclusion I have reached after spending money on antenna tuners
>for my various receivers over the years and not realizing any perceptible
>improvement in my reception. 
 
>Others may have different experiences but those are mine.

My experiences are different from Richard's. A tuner can be a useful
tool for the listener, especially one using a random-length antenna.
The tuner transforms the unknown impedance of the antenna to the
input impedance of the receiver. A gross mismatch will result in
diminished signal strength. I use a tuner between my R8 and a
wire antenna, and it works well.

The Sangean ATS-803A has two antenna inputs. One is the whip antenna.
I think that a tuner between a random-length wire and the whip 
might not work well, because the whip input is probably other
than 50-ohms impedance. (Most tuners have a 50-ohm output impedance,
as that has become an industry 'standard.') The other antenna 
input is that phono jack on the back of the set. It's impedance
is closer to 50 ohms. I have successfully used a tuner between
a wire antenna and the antenna jacks of both a DX-440 (Radio
Shack version of '803A) and a Sony ICF-2010.

It's easier to adjust a tuner that's only used on a receiver,
as you don't have to tweak it for a perfect match. I would
suggest an "L-match" for use on a random wire. A T match has
two controls: a switch and a variable capacitor. Set the
capacitor to about midrange and rotate the switch until the
background noise peaks. Then adjust the capacitor for a peak.
That will usually do the trick.

I use a T-match, which has an added capacitor. It permits 
matching to a wider variety of antennas, but I use it
because it was on hand when I needed a tuner. Mine was 
made by MFJ for amateur-radio use, so it has lots of extra
gee-gaws and an SWR meter.

One possible advantage to using a tuner with a radio like the
803A, is that an L- or T-match tuner acts as a high-pass
filter, passing signals above the frequency to which it is
tuned, but attenuating those below. If you live near a MW*
station, hanging a big wire on a portable SW set can cause
more problems than it solves. The set's gain is adjusted to
provide good performance with a whip antenna. Adding considerable
length to that antenna may result in overloading from strong 
signals. That's why 'good' portables have RF gain controls. On
the Sangean, I'm pretty sure the RF gain control is just a
potentiometer connected across the antenna input. That way, 
the offending signals are attenuated before they can get into
even the first stage of the receiver and overload it. 
*Medium-wave; 540-1700 kHz broadcasters

Receiver overloading makes the receiver sound noisy. Distorted
signals appear in strange places, and weak signals are unreadable.
A tuner won't solve the whole problem, but it can help.

MFJ, Grove and others make simple tuners for SWL purposes. 
Dick Smith Electronics in Australia carries the MFJ line.
There's a magazine called "Amateur Radio Action" in 
Australia that has lots of ads for dealers. You should be
able to find it at a newsstand. The cartoons are pretty
outrageous, too.

Good listening!

Jim

****************************************************************

JIM KEARMAN, KR1S                    Voice: 203-666-1541 ext 279
ARRL Assistant Technical Editor	     FAX:   203-665-7531
Author of `The DXCC Companion'	     ARRL Headquarters
Editor of `Now You're Talking!,'     225 Main Street
ARRL License Manuals,		     Newington, CT 06111
ARRL Instructor Guides		     e-mail: jkearman@arrl.org

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Newsgroups: rec.radio.shortwave