SWR: The Matcher
Article
written and gratefully
re-produced courtesy of
Jack, ACBRO 469
Note - The
following article refers to 27Mhz CB only. Matchers are not available for UHF
CB.
So now you know about SWR you might be looking at adverts for CB matchers and
thinking, “that’s the thing for me, no adjustments necessary”. So lets
have a look at them and see if you need one. Matchers are not used to much for
CB, but they are used a hell of a lot by radio hams on the amateur bands, where
they use their more correct title of Antenna Tuning Units, or ATUs.
What does a matcher do? Well it uses a network of capacitors and inductors to
make the transmitter think that the antennas SWR is low, when in actual fact, it
might be quite high. So why don’t we use one of these things instead of
messing about with the hacksaw and Allen keys at the antenna? Easy, a poorly
tuned antenna will not radiate all the power that is sent to it, or put another
way, it won’t work as well as it should. A matcher will only fool the CB into
thinking the SWR is good and hence the rig will not be damaged, but it doesn’t
actually make the SWR good. It is far better to tune the antenna properly than
to go to the expense of buying a matcher, then using it to match in an antenna
which could easily be working better than it is. I can’t stress too much how
better off you would be by just doing the job right in the first place and
adjusting the antenna.
But people do use matchers, why? Two reasons I can think of. If you need to use
the CB for a short one off job, but for some reason it isn’t easy to set the
SWR, a matcher could sort your problem in the short term. It isn’t a long-term
answer, but it will stop your rig being damaged by a high SWR. Second reason is
what all those radio amateurs have one for. If you need to use the same antenna
over a greater range of frequencies than the antenna is designed for, a matcher
can lower the SWR and allow it to be used. For example, if an antenna is set so
at the bottom end of the 3.5Mhz band (say 3.510Mhz) it has an acceptable SWR but
when tuning to the top end at 3.795Mhz the SWR rises to 4 to 1. A matcher could
reduce this so the other end of the band could be used. Now this shouldn’t be
a huge problem with CB as most antennas have a wide enough bandwidth to cope,
but you might have something special in mind, and if you do. I don’t want to
know about it J
Matchers, or ATUs for amateur band use are usually very high quality and cover a
huge range of frequencies and have a high power rating. They also cost a lot of
money. For CB use we don’t need the high power rating or big frequency range
so its better to buy a cheaper CB dedicated unit. There are two types of matcher
available for CB use, the stand alone matcher and the SWR meter/matcher. The
stand alone type is a small box with two knobs on the front and two SO259
sockets on the back. To use one you connect a patch lead from the back of your
CB rig to an SWR meter, then take another patch lead from the SWR meter to the
matcher, then plug in the antenna to the matcher. Now I should go on to tell you
how to work it, but I wont. Instead I’ll offer this piece of advice. Don’t
buy one, they are cheap because they are rubbish. They use some sort of pivoting
screw in airspaced variable capacitor which is a disaster, the quality of these
components varies from very bad to absolutely unbelievably crap. I have
never seen one which didn’t deserve being thrown in the bin straight away
before even trying to use it.
Now that I’ve got that off my chest we’ll move onto the SWR meter/matcher.
The quality of these things can vary from below average to pretty good. They
usually use rotary vane type capacitors which are much better. The average
meter/matcher has a meter, a calibration knob and forward/reflect switch just
like a regular SWR meter. They often have another switch which can allow the
meter to measure output power, sometimes switchable to 2 ranges, often 10 or 100
watts. These meters are usually fairly inaccurate, often reading a bit to high.
But can be useful as an indication that the rig is working and is actually
putting power out. Then there will be the two matching knobs and a switch to
switch them in or out.
Connect the meter/matcher in line with a patch lead as you would any other SWR
meter. Switch the matcher off, or out (same thing) and switch the meter to
SWR rather than power. Then check your SWR as described in part one of this
article. Lets say that it gives a reading of 3. Now 3 is to high, you want to
bring it down a bit. Adjust both the tuning knobs so they are the central
position and switch the matcher on (or in). The meter will now probably read
higher than 3, so quickly (so as to avoid damage to the rig) rotate the first
tuning knob until it reads lower, it will read high, then peak at a low spot
then go high again. You have to find the low spot. Then do the same with the
second tuning knob. This will hopefully peak it even lower, then repeat with the
first and then the second knob. Each time you will hopefully peak the reading a
little lower, and hopefully by this time it’ll be down below about 1.5. If you
cant get it adjusted to well, and it takes a little longer than the few seconds
that is preferable. Stop transmitting and let the rig cool down. Transmitting
for to long with a high SWR reading can damage the rig, but if you do it in
short bursts with a cooling off period between, it’ll be OK. So when you’ve
got the reading down, switch the forward/reflect switch back to forward and
reset the meter on the set position, then switch back to reflect. This is the
true reading. You may find that you need to retune the tuning knobs a little to
get a better result after rechecking the meter. It sounds a bit of a fiddly procedure,
but with practice you’ll be doing it in seconds without even thinking
about it.
Now what I’ve just told you may differ from some meter/matcher instruction
booklets. They often tell you to set the SWR calibration knob to the center and
not to bother setting it on the forward position. This will allow you to peak
the meter to a low reading, but to get a true reading of “matched SWR” you
will need to set the SWR meter on forward. When you change channel you may need
to peak the tuning down a bit again.
So that’s how to work a matcher. Again it isn't to complicated. However I have
to stress again that a matcher is a temporary fix, you would be better
correcting the problem which is causing the high SWR, ie adjust the antenna or
get one with a wider bandwidth. But until then a matcher will save your rig.