ABSTRACT
The study of transmission lines is the investigation of the properties of the system of conductors used to carry electromagnetic waves from one point to another. Here, however, our attention will be limited to high-frequency applications, i.e. when the length of the transmission line is of at least the same order of magnitude as the wavelength of the signal. In this chapter an idealized model of the line will be used to represent the many different forms found in practice, ranging from twisted pairs to coaxial cables. The theory of transmission lines, which was developed in the early years of the study of electromagnetic propagation, is strictly applicable only to systems of conductors that have a ‘go’ and ‘return’ path, or that, in electromagnetic field terms, can support a TEM wave. Hollow-tube waveguides do not fall into this category, although, as we discuss in the chapter on microwaves, many of the concepts of transmission line theory can be applied to them. Transmission line theory is important to communications engineers because it gives them the means for making the most efficient use of the power and equipment at their disposal. By applying their knowledge correctly they can ensure that a transmitting system is designed to transfer as much power as possible from the feeder line into the antenna, or they can take steps to ensure that a receiving antenna is correctly matched to the line that connects it to the receiver itself, so that no power is wasted. The range of systems over which transmission lines are used is as extensive as the subject of communications engineering itself, and the general theory we develop in this chapter may be used to solve a very wide variety of problems.
