Transmission Line Theory

Abstract This article presents an overview of one of the important topics in RF and microwave engineering, the transmission line theory. In several ways, transmission line theory bridges the gap between electromagnetic theory and basic circuit theory. At high frequencies, when the physical lengths of transmission lines are comparable with wavelength, time‐delay effects become predominant, leading to the distributed nature of the transmission line. This article focuses on studying the phenomenon of wave propagation on transmission lines by using the cascaded equivalent lumped‐element representation and application of basic circuit theory principles. First, a quick description of several commonly used transmission line structures and a brief description about the propagating modes are presented. Transmission line equations, relating the voltage and current on a general transmission line are derived in time‐ and frequency‐domains. Next, wave propagation on infinitely long and finite length transmission lines is described. Propagation parameters, such as characteristic impedance and propagation constant, are derived in terms of transmission line parameters. Special cases of lossless, and distortionless lines are cosidered and their propagation characteristics are studied. Terminations such as open‐circuit, short‐circuit, and matched‐load are described, and the variation of input impedance, voltage, and current on the line is discussed. Next, the concept of standing wave ratio and reflection coefficient and their relationship to the input impedance is presented. Smith chart, a graphical representation to solve transmission line problems, is described along with few numerical examples. The article finally presents various network parameters of a general transmission line for application to microwave network analysis.