The Smith Chart and its application.pdf.pdf

THE SMITH CHART AND ITS APPLICATIONS TRANSMISSION LINES 1 1. TRANSMISSION LINES To understand the Smith Chart, the theory of electric waves along a uniform transmission line is first considered. A uniform transmission line can be defined as a line which has identical dimensions and electrical properties in all planes transverse to the direction of propagation . Unlike the waveguide, the transmission line consists of two conductors separated by a dielectric. If the two conductors are identical and placed alo ng side each other then a twin line is formed. These lines are usually used at the lower frequenc ies. Examples of twin lines are the familiar TV twin-lead and the open two-wire line. If one conductor is placed inside the hollowed tube of the other, then a coaxial line is formed. Coax ial lines are used for frequencies up to 30 GHz. At frequencies from 3 to 300 GHz, hollow waveguides are used. Transmission lines provide a method of transmitting electrical energy between two points in space, similar to antennas. Their behavior is normally described in terms of the current and voltage waves which propagate along it. The performance of a transmission line is normally described in terms of the secondary coefficients, as discussed below. DISTRIBUTED CONSTANTS AND TRAVELLING WAVES In a coaxial line the centre conductor may be held in place by dielectric spacers or by a continuous solid dielectric fill which supports the inner conductor keeping it central to the outer conductor. A coaxial cable is self-shielded and has no external field except possibly near terminations. For this reason it is widely used throughout the radio frequency range and well within the microwave region; which is the name given to the radio spectrum at wavelengths below half a metre. Twin lines, used as open wire lines, are normally balanced with respect to ground, but as a coaxial cable is unsymmetrical it cannot be balanced