CB radios, as with all radios, rely on their antenna to transmit and receive signals. To function at their best, antennas must be tuned to specific frequencies. Even an antenna designed for a CB radio will operate more effectively when tuned, and you'll enjoy a boosted antenna signal that will improve overall radio performance.
Disconnect the antenna cable from your radio. Plug the cable into the "input" jack of the SWR meter. Connect a short coaxial cable between the CB radio and the "output" jack of the SWR meter.
Turn on the radio and the meter. Set the radio to Channel 1 and key the microphone. While holding down the microphone key, check the reading on the meter. Set the radio to Channel 40 and key the microphone, noting the reading on the meter.
If the reading for Channel 40 is higher than for Channel 1, the antenna must be shortened. Loosen the nut at the bottom of the antenna mast and push the whip down about one-quarter inch. If the reading for Channel 40 is lower, extend the antenna slightly. Retest and adjust as necessary to match the readings. When the readings are closely matched, the antenna will operate more effectively across the frequency spectrum on which you're operating.