Glendora Emergency Response Communications

What is Antenna SWR?

Jun 22, 2004

April 13, 2004

SWR (Standing Wave Ratio) is a measure of antenna efficiency. The Standing Wave Ratio is equal to the ratio of radio and antenna impedances. Speaking as a math teacher ratios are really just fractions so this means that for a 50 Ohm radio, an antenna impedance of 75 Ohms will give an SWR reading which can be expressed as 1 ?½ over 1 when you divide 75 ohms by 50 ohms. More commonly we simply say the SWR is 1.5 to 1.

What is a good SWR reading ?

The best reading possible is 1:1, but usually a reading below 1.5:1 is acceptable. To ensure an accurate reading, it is best to connect the meter at the base of the antenna.

Why bother?

There are several reasons why SWR is important. It is well known that a high SWR means that the antenna is not transmitting all the power, and so a good SWR will ensure that you get the most signal out that you can. More serious problems can be caused by the power that is not radiated. This power is reflected back towards the transmitter, where it causes extra heat in the output stage. This can lead to serious damage within the transmitter. Another problem that a high SWR can cause is EMC trouble.

Interference problems

Interference problems which may occur include interference to TV, Radio, telephone and other electronic devices. With a high SWR, the coaxial cable starts to radiate signal as if it is part of the antenna. As the cable is likely to run nearer to the ground and closer to other electronic equipment and cables, the chances of causing interference are greatly increased by this problem.

As the cable is acting as part of the antenna, the position of the cable will also affect the directivity of the antenna, and so reduce your signal to most areas.

Another problem with a high SWR, is that great "coax length" debate...

Coax length

Most radio enthusiasts have heard about the coax length issue. So let's clear this one up now...
COAX LENGTH DOES NOT MATTER. If changing your coax length seems to change your SWR, then you have a problem with the antenna.
The answer to why lies in the text above, so let's summarize it...

High SWR = coax radiates signal.

coax radiating = coax acts as part of antenna.

coax part of antenna = coax affect SWR reading.

So if you "tune" the system by trimming the coax, the actual antenna is still not efficient, and the reflected signal is being radiated by the coax. OK, so the reflected signal may not get back to the transmitter, but all the other problems still exist.

So even if your antenna is 60ft in the air, much of your signal is being radiated much closer to ground.

73 DE KF6DSA