What is Standing Wave Ratio (SWR) in Radio Communication?

The Standing Wave Ratio (SWR), also known as VSWR, which is short for Voltage Standing Wave Ratio. The SWR (VSWR) is a numeric value used to indicate whether the Antenna and the RF transmitter matches well.

In radio communication, if the impedance of the antenna and the feeder does not match, or if the impedance of the antenna and the transmitter do not match, the high frequency energy will produce a reflected wave, and the reflected wave and the incoming wave will converge to produce a standing wave in the antenna-feeder system. 

In order to characterize and measure the standing wave characteristics in the antenna feed system, namely the situation of forward and reflected waves in the antenna, the concept of "Standing Wave Ratio" is established, which refers to the ratio of standing wave belly voltage to valley voltage amplitude, also known as standing wave coefficient.

Its formula is SWR=R/r=(1+K)/(1-K), where the reflection coefficient K=(R-r)/(R+r), and negative K indicates the opposite phase, "R" and "r" are the output impedance and input impedance respectively. When the two impedance values are the same, the perfect match is reached, the reflection coefficient K is 0, and the Standing Wave Ratio is 1.

When the Standing Wave Ratio is equal to 1, it means that all the waves transmitted to the antenna are emitted without any reflection, which is ideal situation, but there is always reflection, so the SWR is always greater than 1. The more mismatched the antenna, the more power it reflects, and the larger the SWR.

The SWR is the basis for checking the efficiency of feeder transmission. Generally, it should be less than 1.5, and it is better to be less than 1.2 at the working frequency. If the SWR is too large, the communication distance will be shortened, and the reflected power will return to the power amplifier part of the transmitter, which will easily burn the power amplifier tube and affect the normal operation of the communication system.

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