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Using a SWR meter
What is a SWR meter used for?
Simply put, a SWR meter compares the power that can be delivered by the CB to what is actually leaving the aerial.
By adjusting the length of the aerial you "tune it" to your CB rig.
Get it right, and the SWR will be 1:1 thus all the power goes out into space and the aerial is "tuned" to pick out incoming transmissions.
Get it wrong, if something is not quite right i.e. the connectors, coax, or the aerial connector is faulty, the aerial is not correctly tuned or damaged, some of the power will be “reflected†back into the CB.
Too much “reflected†power can destroy the transmitter let alone affect your reception. A badly tuned aerial is rather like a badly tuned guitar string. Grating on the ears, makes loads of noise, and in radio terms causes loads of interference that can affect your neighbours TV & stereo. Not a good thing.
So what's good and bad levels of SWR?
1.5:1 to 1:1 is brilliant. 96-100% of your power hits the airways.
2.0:1 to 1.75:1 isn't great. 89-93% of your power goes out
10:1 to 3.0 is disasterous. 33 to 75% is all that gets out.
Above 10, you've got VERY serious problems. Either a lead or aerial is broken or you've got an electrical short.
Tuning Your Aerial To Your Rig
Install everything into it's correct place. Once you have finished your installation, don't move things round. SWR may change if you do.
Read the little paper that came with the SWR meter to familiarise yourself with the switches (Forward / Reverse or FWD /REV), the scale, the control knob (CAL) to control the needle, and the connections on the back.
About costs. A cheapy little CB only SWR meter will set you back £9 - £15.
You'll need a litle patch lead which plugs into the back of the CB sets aerial socket and into the "IN" plug of the SWR meter. That costs £4 - £8.
The aerial lead then goes into the "OUT" plug of the SWR meter.
Switch on the set and select LOW POWER until you have SWR'd the aerial.
Put the switch to “FWD†and transmit. Turn the CAL knob so that the needle swings over to the far right of the scale and align the needle with the set (or cal) mark.
Still transmitting, switch over to “REV†(Reflected) and the needle should drop down. Note that reading.
CAUTION! If the needle hardly drops when you switch over, slams into the right hand side and stays there, or does not come out of the red portion of the scale -
STOP TRANSMITTING. Something is VERY WRONG.
Check your coax and aerial for shorts, damage, or breaks.
Assuming that it did drop, you can happily carry on.
For UK Only Sets
Take readings on Ch. 1 and 40 and make a note of them.
On each test re-tune the needle to the set mark on FWD each time you change channel. DON'T FORGET, EVERY TIME!!!
If the SWR is higher on Ch.1 than Ch. 40 Lengthen the aerial
If the SWR is Lower on Ch.1 than Ch. 40 Shorten the aerial
Tuning for 80 channel UK and CEPT (EU) sets.
Note:- The whole frequency range is often more than a little aerial can handle. If you want to transmit on every channel, it's generally better to fit a LONGER aerial to start with.
You'll be using:-
Ch. 1 on the CEPT (EU) band and UK Ch. 40
Once again, Take readings on Ch. 1 and 40 and make a note of them.
On each test re-tune the needle to the set or cal mark on FWD each time you change channel. DON'T FORGET, EVERY TIME!
If the SWR lower on UK band Ch. 40 than CEPT (EU) Ch. 1
Lengthen the aerial.
If the SWR is higher on UK band Ch. 40 than CEPT (EU) Ch. 1
Shorten the aerial.
You're aiming for a SWR of 1.5:1 or less.
Sometimes no matter what you do the SWR won't drop down to 1.5 or less.
There are a few quickie things to check that can cause this.
Loose connectors, badly soldered wires, damaged cables, poor grounding of an aerial mount (not applicable on a mag mount), extending cables with taped joints. Coax runs not in multiples of 9 feet (3 metres).
Once you have tuned the aerial in on low power, recheck it on HIGH power. It shouldn't change a lot but needs to be checked.
Remember. IF you change, add, or move things round, always Re-SWR.
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