What Is SWR?

What is SWR? “Standing Wave Ratio” is the measurement of quality of how well the antenna of a CB radio is tuned.  Technically, the ratio of the amplitude of a partial standing wave at an antinode (maximum) to the amplitude at an adjacent node (minimum), in an electrical transmission line. In laymen terms, it is the ratio of how much output power reaches its destination versus how much is reflected back.

Any problems with the antenna, antenna cable, or antenna mount will show up during an SWR check, poor SWR is never a result of the radio itself. When the output power is impeded, the waves can be called “standing,” meaning they are not radiating the way they are supposed to. A low SWR means that your antenna is transmitting a satisfactory amount of RF (radio frequency) power instead of sending it back into your transmitter. A high SWR means there is low output from your antenna. Valuable RF power is sent back through the feed line and into the radio. This means that your outgoing signals will be weak. An increased SWR can also result in damaging your unit, all that power being sent back to your radio is in the form of heat which can destroy the transistor. (This damage is not covered under normal warranties).

The SWR reading should be below 3. A reading of above 3 means that too much power is being transmitted back to the radio via the feed line which prevents optimal signal output.  It is not mandatory to check the SWR, but in order to receive peak performance and minimize the possibility of damage your antenna should be checked to ensure that it is functioning properly by checking the SWR. By law, a CB radio is only entitled to four watts of RF power, so small adjustments to the SWR can make a significant difference in radio performance.

When comparing the readings from channel 1 vs 40:

  • If the SWR is lower on channel 1, then the antenna is too long.  Make the antenna shorter.
  • If the SWR is lower on channel 40, then the antenna is too short.  Make the antenna longer.
  • Continue to adjust the antenna until the SWR is approximately the same on channels 1 & 40 AND is less than 3.  The antenna is now properly tuned for the CB band.