SWR: The Meter
Article
written and gratefully
re-produced courtesy of
Jack, ACBRO 469
So you have
connected up your new CB and installed the antenna. Now you have to tune, or SWR
it. You may have been told that this is the next move, but most likely will not
have been told why or how.
What is SWR? Its full title is really VSWR, and it stands for Voltage Standing
Wave Ratio. Now I wont go into huge detail about the physics involved in
voltage, current and different types of RF fields involved, cos if you get
interested in that sort of stuff, you can get a book about it written by someone
more knowledgeable than I. While you are driving about in your car yakking to
other CBers, all you want to know is that it’s working. Put basically, the SWR
is a reading of how well your antenna is tuned to the frequency you are using.
When you transmit with your CB, The transmitted power goes out of the plug on
the back and up the co-ax cable to the antenna. It is then radiated out into the
atmosphere to be received by other CBers and arrives at the speaker of their
rig. Oversimplified maybe, but as complicated as we need to get. If your antenna
is properly tuned it will radiate all the power sent up the cable. If it is
poorly tuned it will be unable to radiate all the 4 watts, and some will be
reflected back down the co-ax cable to the rig. If the antenna is very badly
tuned, or if the antenna or cable has a fault and is not connecting properly, a
lot of the transmitted power will be reflected back to the radio, and if this
happens the radio is in danger of being damaged. This is no fairy tale; I’ve
done it myself. I had a cable fault causing a very high SWR, and within a
minutes fiddling to see why the thing wasn’t working I had blown the output
transistor. It only cost a few bucks to repair, but was a major pain in the bum.
So how do we adjust this “SWR” thingy then? Well first we must check to see
what the SWR reading is at the moment. Dealing with 27Mhz CB first, you should
start by getting yourself an SWR meter. There are loads available, some
complicated with several meters and different things to measure, but a basic
simple meter is all that is required. The basic meter has a meter (Doh!), a
calibration knob and a switch. The switch is usually marked with the words
Forward and Reflect, or FWD and REF. You’ll also need a patch lead which is a
little bit of cable with PL259 co-axial plugs (that’s the same kind as your
antenna has) at either end.
Connect the patch lead into the plug on the back of the radio, and connect the
other end to the plug marked “transmitter” on the meter. Try looking on the
front, its not always marked next to the socket. Then plug the antenna into the
“antenna” socket on the meter. Next switch the radio on and switch to AM, as
you need to transmit a carrier to check the SWR, which SSB doesn’t. Next
select channel 20 and assuming there’s nobody talking on channel 20, press the
button and transmit. Select “forward” on the little switch. Now the meter
will be reading something, so adjust the knob so the needle is over at the right
side of the meter on the “set” mark. Now stop transmitting and slide the
switch over to “reflect”. Transmit again, and whatever the meter reads now
is your SWR reading.
Now this reading is a ratio, so the lowest reading possible is 1 (as in 1 to 1).
You will never get a reading of 1, its very difficult to achieve and not worth
the hassle. If it’s below 1.5, be happy, that’s great. If it’s under 2,
fine no worries. If it’s under 3, you’ll need to adjust that to get it below
2. If it’s above 3. STOP TRANSMITTING, you could damage your radio.
Ninety percent of antennas adjust their SWR by changing their length. You’ll
probably see a little allan grub screw at the base. Alter the length by maybe a
quarter of an inch, and then check it again. Some antennas are helically coiled
(i.e., coiled all the way up) and their length is pre-set at the factory. You
should still check them just to make sure. To determine whether the antenna
should be made longer or shorter, check the SWR on channel 1 and 40. Remember to
recalibrate the meter each time you change to another channel. If you get a
higher reading on 40 than 1, the antenna is to long, so shorten it. If you get a
higher reading on 1 than 40, the antenna is to short, so lengthen it.
After adjustment, check the reading on channels 1, 20 and 40. If all are below
2, and channel 20 is lower than 1 and 40, you’ll not improve it much. It’ll
work fine and wont cause damage to your radio so just get on and enjoy your CB.
There is much effort wasted trying to achieve the fabled “one to one SWR” in
the misguided opinion that the antenna isn’t working properly unless the SWR
meter doesn’t move off the stop. If you are a lonely and bored individual with
plenty spare time, then go for it, but if you have a life, do something else.
Once you get all the channels SWR below 2, don’t worry about it; the amount of
power that isn’t being radiated by the antenna is so small it wont make any
noticeable difference.
But what about UHF? Most UHF antennas are either a set length and are not
adjustable, or have been set during manufacture and will not require changing.
This doesn’t mean you should assume they are properly tuned. It doesn’t do
any harm to check. SWR meters for 477Mhz are much more expensive so you might
prefer to try and borrow one rather than buy one. But the checking procedure is
the same as 27Mhz.
But what if you’ve adjusted the length until your pulling your hair out and it
won’t tune in at all. If the SWR is in the red (above 3) on channel 40 but
maybe 2.5 on 1, and you have reached the end of the adjustment, it maybe needs a
bit cut off it. Some antennas are supplied a little bit to long on the
assumption that different people want to use the antenna for different purposes,
and it’s easy to cut a bit off, but not so easy to add a bit on. If this is
the case, get a hacksaw and trim a quarter of an inch off it. If this improves
the situation, trim a little more. Hopefully this will get the SWR down to
acceptable readings. Remember to trim a little at a time though. You can’t
stick it back on once you’ve chopped it off.
If the SWR is way into the red on all channels. Likelihood is you have a fault
somewhere. Check the cable with an ohmmeter to make sure there is no shorts or
faults and that the plugs are properly fitted. Check there is no rainwater in
the plugs or cable, as this will screw everything up big time. Check the antenna
for the same, but be warned some antennas will give a “short to earth”
reading even when they are healthy as there is an inductor across the loading
coil. Make sure the antenna has a good ground plane, i.e. it’s solidly secured
to a large metal object, like your car. Or in the case of a magnetic mount, is
stuck to a large metal object. Fiberglass panels are commonly used, especially
in truck cab roofs, and they don’t help efficient tuning of antennas one bit.
If you can’t see anything wrong try someone else’s antenna and try to find
what’s faulty by elimination. If you don’t know how to work an ohmmeter, its
time to call someone who does. There are some things that are easier to learn by seeing them.
People will tell you the co-ax length is the problem. They will tell you it has
to be a certain length and that by trimming it you’ll get a better SWR.
CRAP!! The only thing you need to remember about co-ax cable is to
avoid hugely long runs, as you’ll start to loose power, particularly on UHF.
It does not affect SWR. If you have a very long run in the base station, make
sure you buy good quality co-ax and not the stuff that the guy at “Honest
Johns mobile phone and CB supply cupboard” tells you is good. Consult an
amateur radio shop in this case and be prepared to spend a bit more money. The
issue of co-ax length is a keenly debated subject by people a hell of a lot
cleverer than you or I, and they can’t decide what’s best. So for all
practical purposes, the shortest length of co-ax you need the better.
I hope you’ve managed to adjust your SWR to acceptable levels, or confirmed
that’s its OK. Remember to check it once in a while or whenever you whack that
antenna off a low tree. Keep the mounting clean from rust, as that’ll make the
SWR go wonky. If you have space, leave the SWR meter in line all the time so you
can check it whenever you want. It won’t do any harm or cut down your output
power, as some people seem to think.
You see, it’s not really that complicated after all.