Measuring SWR and adjusting an antenna
Measuring SWR with the meter above is described below:
- You set the sliders of the meter to positions FWD (Forward) and SWR. First slider does not affect the SWR measurement.
- The level adjust slider must be on the left.
- You'll push the PTT button of your microphone and use the level adjust slider to set the needle on the meter so it sits at the SET mark.
- When transmitter is still on, you will turn the slider on the right side to REF (Reflected) position and read the indicated SWR value on the meter. If SWR is over 2, your antenna needs adjusting or you have a fault in your coax cable.
- When adjusting the antenna you will need to repeat points 3 and 4 one by one until you get the SWR to minimum (usually 1.2 … 1.5) and so it's about the same on the lowest and highest channel.
In some more modern SWR meters it's not necessary to calibrate the meter (points 3 and 4), these meters show the SWR value directly without separate calibration.
The antenna adjustment starts by checking the SWR reading on channels 1 and 40 (Modern 40 channel CB radios) or on channels 1 and 22 (Older models), if the reading is higher on the highest channel you must shorten the antenna and if the reading is higher on the lowest channel (1) you need to lengthen the antenna.
In practice the basic SWR measurement is recommended to be done at the middle of the channel area of the CB radio (on modern radios on channel 20 and on older 22 or 23 channel radios on channel 11). When you have the radio on the middle channel, you can adjust the antenna to the lowest SWR you can get on that channel. After that you can start the fine adjustment by checking the SWR at the lowest and highest channels. It's not such a big deal if the SWR rises a bit from the middle channel value, especially on radios with 40 channels.
Longer wavelength than on channel 40
|If the SWR is higher on channel 1, you need to lengthen the antenna|
Shorter wavelength than on channel 1
|If the SWR is higher on channel 40, you need to shorten the antenna|
In some cases the SWR might not be optimum at the middle channel and in that case you might need to seek a little bit, and that seeking is easier by checking the SWR on the top and bottom channels. Especially mobile antennas can often have a low SWR in a surprisingly narrow range of channels, and then you have to compromise a bit. Base antennas on the other hand often work with a low SWR on a broad range of channels.
Below is some info about power loss with different SWR values:
When measuring SWR, you should take into consideration that when possible the SWR should be measured at the antenna. When the antenna is not tuned to the best SWR and you're measuring SWR from different spots along the transmission line, you will get different readings because the impedance changes along the transmission line. If your transmission line's length would be electrical ½-wavelength long you would see the real SWR of the antenna at the end of the cable. Otherwise the length of the cable does not matter, and the length of the cable does not impact the tuning of the antenna. When the antenna is tuned correctly to the best possible SWR (near 1:1) the SWR is the same along the whole transmission line.
For more info on SWR
Firestik has more extensive docs about measuring SWR and troubleshooting antennas at their Tech Docs Library: Firestik Library
Useful data at Procomlab on Procom A/S website (SWR nomographs, SWR including feeder loss and lots of other useful info): Useful data at Procomlab