Antenna SWR.

SWR
SWR (Standing wave Ratio) is a measure of how well the antenna is matched to the transmitter. A poorly matched antenna will not perform as well as if it were correctly tuned.
The Standing Wave Ratio is equal to the ratio of radio and antenna impedances. This means that for a 50 Ohm radio, an impedance of 75 Ohms will give a SWR reading of 1.5:1.

The SWR meter The typical CB SWR meter does not actually measure standing waves. The standing wave ratio is calculated by measuring the amplitude of the forward and reflected signal.

What is a good SWR reading ?
The best reading possible is 1:1, but a reading below 2:1 is often acceptable. It must also be noted that many CB antennas are not exactly 50 Ohms and, as noted above, this will mean that the antenna will not give a 1:1 reading even if tuned correctly.
Examples include the standard dipole, which has an impedance of 75 Ohms - giving a 1.5:1 reading when correctly tuned.

Why bother?
There are several reasons why SWR is important. It is well known that a high SWR means that not all the power is being transmitted, and so a good SWR will ensure that you get the most signal out that you can.
There are also the dangers of damage to your transmitter and interference to nearby equipment. This is discussed in more detail on the other pages in this section.

Coax length
Most radio enthusiasts have heard about the coax length issue. So let's clear this one up now... As long as your antenna is correctly tuned, COAX LENGTH DOES NOT MATTER.
If changing your coax length appears to change your SWR, then you have a problem with the antenna. Rather than worrying about coax length, sort out the problem with the antenna.
For those wanting to understand why coax length may appear to change the readings, this topic is covered in more depth on the other pages in this section and the feeder section.

SWR and efficiency.

Chart. SWR Radiated % Loss 1 100 ------- 1.3 98.3 .08dB 1.5 96 .18dB 1.8 91.8 .36dB 2 88.9 .51dB 2.5 81.6 .86dB 3 75 1.25dB 3.5 69.1 1.61dB 4 64 1.94dB 5 55.6 2.55dB

When using this chart, you should keep it in mind that reflected signal is only one factor involved in antenna efficiency.
Antenna design, damage and aging can also make a large difference to efficiency.
Some of these may not show up on an SWR meter, as some do not change impedance while others may be hidden by detuning the antenna to compensate for the problems which you do not know exist.
This may sound strange but, as the meter is really only showing impedance ratio, detuning the antenna changes it's impedance which may then cancel out the change caused by the damage.
The SWR may appear OK, but the antenna will still not be working at it's best. It can be important to also visually check the antenna for damage or water.