What is Standing Wave in microwave transmission line

A standing wave is a stationary wave pattern formed when two waves of the same frequency and amplitude travel in opposite directions and interfere with each other. Standing waves are important in RF transmission lines, antennas, acoustics, microwave systems and electromagnetic wave propagation analysis.

A standing wave is a wave that appears to stay in one place, not traveling through the medium.

In a transmission line that isn’t perfectly matched to its load (i.e., a mismatched terminated line), the incident signal (the signal traveling from the source) and the reflected signal (the signal bouncing back from the load) interfere with each other. This interference creates a standing wave pattern along the line.

An important characteristic is that the current and voltage standing waves are 90 degrees out of phase with each other along the transmission line. This means that where the voltage is at a maximum, the current is at a minimum, and vice versa.

The magnitude of these standing waves is measured using a metric called the Standing Wave Ratio (SWR). SWR is defined as:

$SWR = \frac{\text{max. voltage or current}}{\text{min. voltage or current}}$

In simpler terms, the reflected wave interferes with the forward wave. Due to phase changes along the line, these waves add together at some points, creating areas of high voltage and current. At other points, they cancel each other out, resulting in areas of low voltage and current. This creates a stationary interference pattern that we call a standing wave.