Conceptual Analysis of Antenna Standing Wave Ratio

Standing wave ratio is called voltage standing wave ratio, also known as VSWR and SWR, which is short for English Voltage Standing Wave Ratio.

Where the incident wave and the reflected wave have the same phase, the voltage amplitude adds up to the maximum voltage amplitude Vmax to form an antinode; where the incident wave and the reflected wave have opposite phases, the voltage amplitude subtracts to the minimum voltage amplitude Vmin to form a node. The amplitude values of other points are between the antinode and the node. This composite wave is called a traveling standing wave. The standing wave ratio is the ratio of the sound pressure amplitude Vmax at the antinode of the standing wave to the sound pressure Vmin at the node. In the standing wave tube method, by measuring the standing wave ratio, the sound reflection coefficient and sound absorption coefficient of the sound absorbing material can be calculated.

In radio communication, if the impedance of the antenna and the feeder do not match or the impedance of the antenna and the transmitter do not match, high-frequency energy will be reflected and folded back, and will merge with the forward part of the interference to generate a standing wave. In order to characterize and measure the standing wave characteristics in the antenna system, that is, the forward wave and reflected wave in the antenna, people have established the concept of "standing wave ratio".

　　SWR=R/r=(1+K)/(1-K)

　　reflection coefficient K=(R-r)/(R+r)

　　(When K is negative, the phase is opposite)

Where R and r are output impedance and input impedance, respectively. When the two impedance values are the same, a complete match is achieved, the reflection coefficient K is equal to 0, and the standing wave ratio is 1. This is an ideal situation, in fact there is always reflection, so the standing wave ratio is always greater than 1.

Of course, there are other calculation methods for standing wave ratio, but the calculation results are the same. The higher the standing wave ratio is, the more mismatched the impedance is. Generally speaking, the standing wave ratio is less than 1.5.

Only when the impedance is completely matched can the maximum power transmission be achieved. This is more important at high frequencies! When there is a mismatch, a part of the radio waves emitted by the transmitter will be reflected back, and a reflected wave will be generated in the feeder. The reflected wave will reach the transmitter, which will eventually be consumed as heat. When receiving, the received signal will also be bad due to mismatch.

Complete matching will not produce reflected waves, so that the voltage amplitude at each point in the feeder is constant.

The quality of the antenna cannot be determined solely by the standing wave ratio. The reason why everyone is so superstitious about the standing wave ratio is simply because the standing wave meter is so cheap and easy to buy. Don't think everything is OK just because the antenna standing wave ratio is very low. It is really fun to study other characteristics of the antenna (such as directivity).