RF and Microwave Engineering: Fundamentals of Wireless Communications

4.1 Overview

Transmission lines may be used in various ways and applications. First, transmission lines can carry signals from one circuit component (e.g. the output of an amplifier) to another circuit component (e.g. an antenna). Ideally, in this area of application the transmission line is loss-less and without reflection at both ends. In order to make practical lines low-loss, good conductors (e.g. copper) and low-loss dielectrics are used to construct the lines. Reflections at both ends of a line are reduced by matching the load and internal source resistance to the characteristic impedance of the line.

Another area of application for transmission lines is the design of passive circuits. Transmission line segments can be arranged to—for instance—build filters, power dividers, couplers and matching circuits. In the case of transmission line circuits, reflections are useful for obtaining the desired transfer behaviour.

Figure 4.1 shows some technically important types of transmission lines. Coaxial lines, two-wire lines and optical waveguides are used to bridge larger distances. Planar transmission lines (microstrip, striplines) are suitable for the design of passive circuits. Rectangular waveguides are used especially in high power applications (e.g. radar).

Figure 4.1 Transmission line types: (a) Coaxial line, (b) rectangular waveguide, (c) optical waveguide, (d) two-wire line, (e) twisted two-wire line, (f) star-quad line, (g) microstrip line, (h) stripline and (i) coplanar ...