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 brought or an antenna is moved.
'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'. Sounds complicated doesn't it! Dont worry, all it really means is "the ratio of power that is reflected back down the antenna lead from the antenna."
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.
Basically speaking the higher the SWR the less power being radiated by the antenna, and the more power coming back down the antenna lead. Therefore you should aim to get the lowest SWR reading possible
Why is SWR Important?
|SWR is very important because if you have a high SWR reading then you will not achieve the best transmit range, and if the SWR reading is very high (over 3:1) then the reflected power will come back down the antenna lead into the radio and destroy the output transistors in the radio! |
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 quite cheap (£12.99 for a basic one), click here to view our range. To connect the meter you will also need a patch lead, click here. |
The meter simply connects between the antenna and the radio. There are two connections on the meter, one for the antenna to plug into, and the other to connect to the radio(using the patch lead). When you have successfully connected up your meter you are ready to check your SWR.
There should be a switch on the meter labelled 'FWD/REF' (Forward and Reflected). Make sure the switch is set to FWD. Now set your radio to a channel that is half way through the band, i.e. if you have a 40 channel radio then set your radio to channel 20, for an 80 channel radio set it to UK channel 1.
Now key up your microphone and use the control knob (usually labelled CAL) on the meter to adjust the needle so that is at the'Set' position on the meter scale, usually labelled on the far right of the meter scale. Let go of the mic and now switch the meter to 'REF'. Key up your mic again and the meter will now show the SWR reading of the antenna.
What reading should I get?
|The best SWR reading would be 1:1 but as I said earlier this could only be achieved with a perfect antenna and no antenna is perfect. So basically the lower the SWR reading the better. Over 3 is usually unacceptable and could potentially damage your radio, 2 or below will give you a good range and is quite acceptable, 1.5 or below is fantastic and will give you optimal TX and RX performance. || |
| || || || |
| || |
| || |
| || || |
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 fun 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 very rare occasion they may need a slight bit of fine tuning).
Note:"When you get a new antenna or move your existing one you should always check the SWR reading(even if the antenna is pre-swr'ed) just to make sure you have no other problems with your mount/groundplane etc. and that your antenna is tuned for optimal performance".
What if my SWR is too high?
You can usually tune your antenna quite easily by altering the length of the whip, most antennas have a grub screw in the side that you can undo to adjust it. Remember though only a very small adjustment is needed, 5mm can dramatically change the SWR!
Sometimes the antenna is not to blame for a high SWR though. 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!
| || || || || || || |