Rocket's Guide to Antenna SWR for CB & Amateur Radio

Posted by Mark Russell on

1) Q : What is SWR ?

A : SWR stands for Standing Wave Ratio

2) Q : Why is SWR important for my transceiver and antenna setup ?

A : Your SWR reading gives you an idea of how well matched (or tuned) your antenna is to your transceiver.

3) Q : What if my setup has a high SWR, or I don't bother to check it ?

A : You may not get the best range, and at worst you could cause damage to your transceiver. *Please note that no manufacturer will honour the transceiver or linear amplifier warranty if damage is caused by excessively high SWR*

4) Q : How will a high SWR cause damage to my transceiver ?

A : The higher the SWR reading, the less RF power is radiated from your antenna, and the more power that is returned back into the transceiver. To give you an idea, a SWR of 1:1.5 sends approx 4% of your RF power back into the transceiver, SWR of 1:2.0 it's 12%, and SWR of 1:3.0 it's a whopping 25% ! Now you can see why a high SWR is not good, a 100W Ham rig transmitting through an antenna with an SWR of 1:3.0 will send 25 Watts back into the output stages of the radio, this causes excessive heat, and it's the heat that does the damage !

5) Q : OK, what SWR should I aim for ?

A : As a rule of thumb, lower than 1:2.0 is OK for CB radios and lower power transmitters. For linear amplifers and higher power transmitters, it should be below 1:1.5. Just get it as low as possible without spending too much time trying to achieve that elusive 1:1.0 on every single channel.

6) I have connected my SWR meter in line with a patch lead between the radio and antenna, how do I get an SWR reading ?

A : Pick a mid channel (say channel 20) on a band. Set the SWR meter switch to FWD and then press the transmit button on the microphone, then turn the Calibrate knob on the SWR meter so that the needle lines up with the SET mark on the scale. Then flick the switch on the meter to REF, and the reading you get from the meter needle is the SWR for that particular channel. Carry out this procedure on channels 1 and 40 and check the SWR is OK over the whole frequency range.

7) I've heard that something called a 'Groundplane' can affect SWR, what is it ?

A : Lets say you have an antenna which is mounted on a steel bodied vehicle, then the steel body will hopefully act as a good groundplane for your antenna. Poor groundplanes include those vehicles bodies which are made from aluminium and fibreglass. Solutions for those vehicles include groundplane kits and electronic groundplanes as well as using sheets of steel and copper inside the vehicle (even aluminium foil tape can sometimes work, as well as running earthing wires to the chassis of the vehicle.)

8) Q: Oh no ! I think my groundplane is fine, my connections on the coaxial cable are ok, but I have a good SWR of 1:1.5 on channel 1, but a bad SWR of 1:3.0 on channel 40, what can I do ?

A : This just means that over that range of 40 channels, the antenna is a bit too long. Just undo the grub screw at the bottom of the antenna whip and slide the whip down by a few mm, tighten the grub screw, then re check the SWR. Keep fine tuning like this until you have a good compromise on the SWR across the frequency range of your transceiver. (As well as the example quoted above, if the SWR is higher on channel 1 than it is on channel 40, then the antenna is too short)

9) Q : I have an antenna which is described as 'pre-tuned'. So I don't need to check the SWR then ?

A : Yes you do. The antenna is 'pre tuned' in the factory in ideal conditions. You need to check SWR even on this antenna to take variables into account such as your coaxial cable and connections, the antenna mounting, the groundplane, as well as that the frequency range of your radio my be different or wider than the bandwidth of the antenna.

That's it. I do hope you find this guide helpful.

73's and good DX, Mark.M0ROC


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