In 1928 Phillip Smith was told to push a radio signal down two kilometres of transmission line into an antenna array. Part of it kept bouncing back. The reflected wave met the forward wave and stood still in the line — a standing wave, with peaks that can double the voltage and burn the cable open. The cause is a mismatch: the line has a characteristic impedance, the antenna has another, and at the boundary between them the wave is asked to change its terms. It refuses, and part of it returns. Smith drew a chart that turns that invisible reflection into a single point you can see and move. Every module after this one is this same picture wearing different physics.