The formation condition of standing wave is that the terminal of transmission line is broken, short-circuited or impedance mismatch, resulting in reflection. The frequency and propagation speed of the two waves are exactly the same, but the directions are opposite.
Standing wave is the definition that two waves are superimposed and the standing wave propagates in opposite directions. In the superposition area, the vibration strengthening points are always strengthened to form antinodes, and the superposition of vibration weakening points is always weakened, that is, nodes. In order to achieve such a stable superposition, the amplitudes and frequencies of the two waves must be the same.