In wireless communication, if the impedance between the antenna and the feeder does not match or the impedance between the antenna and the transmitter does not match, high-frequency energy will generate reflected waves in the antenna, and the reflected and incident waves will converge in the antenna feeder system to generate standing waves.

In order to characterize and measure the standing wave characteristics in the antenna feed system, that is, the situation of forward and reflected waves in the antenna, the concept of "standing wave ratio" was established. The calculation formula for the standing wave ratio is SWR=R/r=(1+K)/(1-K), where the reflection coefficient K=(R-r)/(R+r). When K is negative, it indicates opposite phase, and R and r are output impedance and input impedance, respectively.
When two impedance values are the same, a perfect match is achieved, with a reflection coefficient K equal to 0 and a standing wave ratio of 1. This is an ideal situation, indicating that the impedance of the feed line and antenna are completely matched. At this point, all high-frequency energy is radiated by the antenna without any reflection loss of energy; When the standing wave ratio is infinite, it indicates total reflection and no energy is emitted at all.
In fact, there is always reflection, so the standing wave ratio is always greater than 1. The standing wave ratio is the basis for testing the transmission efficiency of feeder lines. The voltage standing wave ratio should be less than 1.5, and the voltage standing wave ratio at the operating frequency point should be less than 1.2. If the voltage standing wave ratio is too large, it will shorten the communication distance, and the reflected power will return to the amplifier part of the transmitter, which can easily burn out the amplifier tube and affect the normal operation of the communication system.
Voltage standing wave ratio (VSWR) is a commonly used parameter in radio frequency technology, used to measure whether the matching between components is good.