Standing wave

A standing wave is the fixed pattern of crests (antinodes) and stationary zero points (nodes) that appears when a wave reflects back on itself and the forward and reverse waves superpose. Unlike a travelling wave, the pattern doesn't move down the line — it just pulses up and down in place, like the vibrating string of a guitar where certain points never move while others swing wide. On a transmission line this happens whenever a reflection sends a wave back toward the source.

The spacing between successive voltage nulls is exactly half a wavelength, which makes a standing wave a handy ruler: by sliding a probe along a slotted line and finding the minima, early microwave engineers measured both wavelength and the unknown load impedance. A pure standing wave (total reflection) carries no net power down the line — all the energy sloshes back and forth — which is precisely the failure mode you want to avoid when feeding an antenna.