CB RADIO GUIDE
WHAT IS SWR?
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 (£17.99 for a basic one), click here to view our range. (note: to connect the meter you will also need a patch lead).
1. Connect the SWR meter between the antenna and the radio, as per the diagram above.
2. Now you need to switch the meter to the ‘Forward’ or ‘Set’ mode. Sometimes this switch can also say ‘Cal’ or ‘Dir’ as all SWR meters are different.
3. Now you need to make sure the radio is connected to power and turned on. Then you need to transmit on the radio using the microphone.
4. Whilst transmitting you will see the needle move on the meter. You now need to use the ‘Set’ or ‘Cal’ knob to move the needle to the top of the scale where it normally says ‘Set’.
5. Now let go of the transmit button, and switch the SWR meter to ‘Reflected’ mode. Sometimes this switch can say ‘SWR’.
6. Now transmit again, using the microphone, whilst checking the meter reading. This will be the SWR reading of your antenna on that channel.
WHAT DOES THE READING MEAN?
Check the table below to find how well your antenna is tuned in.
|Below 1.5||Excellent, your antenna is perfectly tuned.|
|1.5 -> 2.0||Your antenna is acceptably tuned and should work fine|
|2.0 -> 3.0||Your SWR reading is slightly high, it shouldn’t damage your radio but you should try and tune your antenna to bring down your SWR reading|
|Above 3.0||Your antenna isn’t tuned acceptably and could damage your radio|
The best SWR reading would be 1:1 but as stated earlier this could only be achieved with a perfect antenna and no antenna is perfect. So basically the lower the SWR reading, the better.
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…
This means that your antenna is too short. To adjust it please see the instructions below.
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".
CB RADIO GUIDE
CB RADIO HISTORY
THUNDERPOLE ON THE BBC
CB RADIO USER MAP