There is a misconception among CB operators that changing the length of your coax can "tune" your antenna system. This is absolutely untrue! This short discussion will detail why.
The coax cable is a means for transferring your RF signal to the radiating portion of the radio system. The cable, in theory, is meant to be a contained, non-radiating link. Because it does not radiate and serves only to transfer RF between two components of the transmitter system, it's performance in terms of efficiency is affected by length, but only in terms of overall resistance. In other words, using a long run of coax will reduce the total amount of signal at the antenna, but only because of loss due to resistance and NOT because of standing waves.
Ideally, you want to check the SWR of your antenna at the antenna feedpoint. In a perfect world, this is the best way. However, we all know that this is ludicrous to expect in a standard base antenna installation. Unless a remote SWR meter head is incorporated, we usually use the standard SWR meter located at the radio. The drawback is that resistance and slight impedance mismatch of the coax affects the overall SWR reading.
Because radio waves are tuned wavelengths of energy, we have to take into account the coax cable length. A typical 11-meter signal has a basic wavelength of 36 feet/wave. "Tuning" the coax for the exact full wavelength tends to throw off the SWR meter by not allowing any standing waves to return to the meter. Excess RF on the coax has been given an ideal medium by which to "hide" electrically from your SWR meter. That is not to say that the excess RF is not returning to the radio, you just can't see it on your meter.
What we want to do is create an environment where any excess RF (standing waves) are rendered as visible as possible to the meter. This is effectively done by using multiples of the 1/2-wavelength of the radiated signal. One half wave for the 11-meter band is 18 feet. However, this is not the length that you will cut your coax. There is another factor that affects the length. This is Velocity Factor. The velocity factor is basically a term for how fast the signal moves through the coax. This factor affects the overall electrical performance of the coax and thus needs to be accounted for when determining the true half wave length
Here are the velocity factors of the various Belden coaxial cables:
RG-59/U (foam) .79
RG-58/U (foam) .79
RG-8/U (foam) .80 9913 .84
Here is how to figure out your true 1/2-wave:
492 x (Velocity Factor) / Frequency (MHz)
For example, I want to figure out the true half wave coax length for RG-59/U (foam) on my home channel (ch. 33 - 27.335):
492 x .79 / 27.335 = 14.219 feet
Now add 14.219 to itself to determine your 1/2 wave multiples. Remember to use every other number. See the example below:
14.219 feet 1/2-wave multiple, 28.438 feet 1-wave multiple, 42.657 feet 1/2-wave multiple, 56.876 feet 1-wave multiple, 71.095 feet 1/2-wave multiple, 85.314 feet 1-wave multiple
and so on . . . .
Use only the lengths that fall on the 1/2-wave multiples and you will be all set.
Now in order to get the true SWR of the system, you have to throw away that 3-foot jumper cable for now. The SWR meter has to fall on a 1/2-wave point on the coax run. Using the example above, you need a 14.219 foot jumper from the radio to the SWR meter, and a 1/2-wave multiple length from the SWR meter to the antenna. If my antenna is 65 feet away from my radio, I need a 14.219 foot jumper from the radio to the SWR meter, and a 71.095 foot length between the SWR meter and the antenna.
In laymans terms, the coax length fools your SWR meter into thinking that your SWR is different to what it actually is !
The ONLY way to know what your SWR actually is, is to use a correct length of coax for the operating frequency