In order to work properly, an antenna needs to be resonant on the frequency [band] you are operating.  Frequency and wavelength are directly in-proportional, meaning that as frequency increases, wavelength decreases.  This information can be used to easily and accurately tune your antenna.

For CB use, If the SWR is lower on channel 1 than it is on 40, make the antenna shorter.  Channel 40 is higher in frequency than channel one, so it's wavelength is shorter, thus requiring a shorter antenna.  Likewise, if the SWR is higher on channel 1 than it is on 40, make the antenna longer.  Only adjust in small increments, like 1/4 inch at a time.  Sometimes it is necessary to trim a bit off the whip. In this case, a bolt-cutter works well, just remember to wear safety glasses and take care, the small pieces can fly off with a lot of force. Cut from the bottom, in 1/4 inch increments.

There is much confusion concerning the proper length of coax to use with your cb / ham setup.  In a perfect world, the length of coax does not matter, and should only be as long as needed. If your antenna is a pure 50 ohms, this rule holds true. However, this is rarely the case, and the general rule of thumb is to cut coax lengths to be 1/2 wavelength, or a multiple thereof.   For a typical CB installation, 1/2 wavelength is about 18 feet in free space.  However, the signal travels through coax at a different speed than in free space, and this speed differs depending on the coax.  The difference in speed at which the signal travels through the coax is referred to as the velocity factor, and can be found in the specifications for that type of cable.  For the velocity factor of other cables, click here.

For standard RG8, the velocity factor is 66%.  To determine the length of RG8 coax you should use, take the desired length (1/2 wavelength in this case) multiplied by the velocity factor:  18 x .66 = 12.  This means the actual length of coax you should use would be 12' of RG8.