Two-way street

摘要

Suppose that I gave you a highway-traffic monitor that reports total traffic density on a section of roadway but does not provide separate figures for eastbound and westbound traffic. Obviously, this sensor reveals only part of the total traffic picture. Transmission lines, like streets, support traffic in two directions. A voltage probe connected to a transmission line acts much like a traffic-density sensor. It shows an aggregate-voltage waveform but doesn't say which way the waveform is moving.