Transmission line theory

A transmission line is a two conductor system, usually with uniform cross-section, used to guide electromagnetic energy between two places. At low frequencies (and to a good approximation for many microwave guiding structures) the energy is transferred via the voltages developed between the conductors, and the currents flowing through the conductors. A coaxial line and a two-wire line are examples of ideal transmission lines. When the transmission line is shorter in length than the wavelength of the signals travelling along it, the line voltage and the current will be constant at a given instant in time. However as the line length becomes comparable with or smaller than the operating wavelength, then at any instant the voltage and current will be functions of position along the line. To allow the line to be analysed using Kirchoff's Laws, it is divided into very short segments over which the voltage and current may be considered to be constant. The line performance is then evaluated by summing or integrating the results from the line segment over the line length.