From: jbs@rti.rti.org
Newsgroups: rec.radio.cb
Subject: Re: SWR Meter
Date: 24 Jul 92 14:17:39 GMT
Distribution: rec.radio.cb
Organization: Research Triangle Institute, RTP, NC

In article <5524@umriscc.isc.umr.edu> sdrumm@mcs213e.cs.umr.edu (Scott Drumm) writes:
>
>My physics teacher was kind enough to loan me an SWR meter to calibrate my
>CB, but I'm not quite sure how to use it (i.e. No instructions were provided).

Regardless of the type of meter, you'll need a chunk of coax commonly
referred to as a "jumper", with male connectors on both ends. This will go
between your CB and the meter (there should be a notation of "TX" or
some-such near one of the meter's connectors; the jumper to the radio goes
there, and the antenna cable goes into the socket marked "ANT"). You can
buy a jumper at Radio Shack, or make one yourself out of some extra coax
and a couple of male connectors. Many people insist that the jumper
should me a multiple of quarter-wavelengths long, and if they're correct
then the shortest jumper you should use for CB would be about 27" long.

>There's an indicator showing the percentage of power reflected into the system
>as well as a field strength meter.  There's also two connectors (both female)
>that will mate with a CB antenna lead.

Can't tell from this descrip whether this is a cross-needle meter or not. I
suspect not; in this case, there will be a calibration knob and a switch
that has two positions either marked "CAL/SWR" or "FWD/REF". Put your CB on
channel 20 (assuming it's got 40; else put it on 11), and set the switch to
"CAL" or "FWD". Now key the mike (don't whistle or make any other noise
during this process) and turn the calibration knob until the needle is on
the calibration mark on the meter scale (probably near the far right of
the scale). Next, put the switch in the "SWR" or "REF" position, and read
your SWR off on the scale. You want as close to no needle movement as
possible (1.0:1 SWR).

If you're not happy with your SWR, determine whether the antenna is too
long or too short by repeating the whole process on channel 1 and on
channel 40. If your SWR is better on channel 40 than on channel 1, then
the antenna is too short. Vice-versa and the antenna is too long. If
you're trimming an antenna to get the SWR down, trim only a very little at
a time, as once you've trimmed too much you either have to live with a
high SWR or go find another antenna element to replace the one you've
chopped.

If you're tuning a base station antenna, it's better to check the SWR at
the antenna if possible (i.e., if the antenna itself has a female coax
socket, get a friend to help (you'll need somebody to key the mike),
take your meter outside, and plug your jumper into the ANT socket in
the meter and into the base of the antenna; plug the long cable from
your radio into the TX socket in the meter).

I mentioned a "cross-needle" meter before. If your meter has two needles
that cross each other, you (probably) won't have a calibration knob, and
checking the SWR is much easier because one needle shows forward power
and the other reflected power, so you just look at the markings on the
meter face at the point where the two needles meet to read the SWR.

Good luck!

 -joe