CB Radio Guide - What is SWR?
SWR is something that even very experienced CB'ers have trouble understanding, however this doesn't mean that you can ignore it and carry on regardless. SWR is very important to every CB set-up and should be checked every time a new antenna is purchased or an antenna is moved. Click here to view our SWR Meter range.
'A CB antenna with a high SWR reading will not work to it's full potential and in the worst case could even damage the CB radio.'
What is SWR?
SWR stands for 'Standing Wave Ratio' and basically speaking it is "the ratio of power that is reflected back down the antenna lead from the antenna."
Therefore SWR is concerned with matching the antenna to the signal being transmitted through it. A perfect antenna would have an SWR reading of 1:1 - meaning that all the power transmitted up the antenna lead would be radiated by the antenna. However no antenna is perfect and there is always a small amount of power reflected back down the antenna lead.
In conclusion, the higher the ratio (SWR reading), the less power being radiated by the antenna, and the more power coming back down the antenna lead.
Why is SWR Important?
SWR is very important because if you have a high SWR reading then your antenna will not be performing as well as it was designed to, and will not be transmitting all the power you have put into it, therefore you will not achieve the best transmit range.
Simply put, the better tuned your antenna is, the better range you will get.
If your antenna is not tuned and the SWR is very high (over 3:1 or 'in the red') then there will be a large amount of reflected power coming back down the antenna lead which will go back into the radio and possibly destroy the output transistors!
How can I check my SWR?
To check the SWR reading you must use an SWR meter. We sell a range of SWR meters and they are relatively inexpensive (£13.99 for a basic one), click here to view our range. (note: to connect the meter you will also need a patch lead).
What does the reading mean?
Check the table below to find how well your antenna is tuned in.
Which is the best channel to check the SWR on?
You should aim to tune the antenna for low SWR across the whole frequency band you are using. We suggest checking the SWR on the top, middle and bottom channels on the band you will be using, i.e. channel 1, 20 and 40 on the UK CB band.
What if my SWR is too high?
Your SWR is over 2 on channel 40… This means that your antenna is too long. To adjust it please see the instructions below.
Your SWR is over 2 on channel 1…
Your SWR is in the red (higher than 3) on all channels…This normally means that there is a problem, either with the cable, the mount or the groundplane on the antenna.
Most mobile antennas have a grub screw, which you can unscrew slightly to alter the length of the antenna whip. Simply undo this screw and adjust the length, one small bit at a time then re-check your SWR. Remember, don’t adjust it too much, a small adjustment of only 5mm can make a big difference.
If it doesn’t have a grub screw, then it may be a top loaded antenna with a screw on the top that you can adjust to bring the SWR down. Some antennas have rings around the bottom to adjust the SWR, again though, a small adjustment can make a big difference so don’t get carried away.
If it is a homebase antenna, then there is probably a way of altering the length somehow, maybe with jubilee clips. If you are unsure on how to adjust your antenna please refer to the instructions that came with the antenna.
Sometimes the antenna is not to blame for a high SWR. If you have a short circuit in your cable or mount for instance then you will have a high SWR, this is easily spotted though because the SWR will read infinity on the meter (needle full over). Another cause of high SWR is a bad 'Ground Plane'. To find out more about ground planes click here. If you adjust the length of your antenna and have a good ground plane and still cannot get the SWR reading down then you may need to reposition your antenna on your vehicle. If you have an acceptable SWR reading you are now ready to start using your CB.
There was a time when every new CB antenna had to be fine tuned when it was put on a vehicle, luckily there now are antennas that are Pre-tuned. Most of the THUNDERPOLE antennas are now pre-tuned making the hours of fine-tuning a thing of the past (thankfully!).
Most notably the THUNDERPOLE Orbitor HD and Trucker HD antennas. These antennas have been tuned at the factory for the lowest possible SWR reading on a test rig (on the rare occasion they may need a slight bit of fine tuning).
Please Note:"When you purchase a new antenna or move your existing one you should always check the SWR reading(even if the antenna is pre-swr'ed) to make sure you have no other problems with your mount/groundplane etc. and that your antenna is tuned for optimal performance".