ABSTRACT
At the end of this chapter you should be able to: • appreciate the purpose of a transmission line • define the transmission line primary constants R, L, C and G
• calculate phase delay, wavelength and velocity of propagation on a transmission line
• appreciate current and voltage relationships on a transmission line
• define the transmission line secondary line constants Z0, γ , α and β
• calculate characteristic impedance and propagation coefficient in terms of the primary line constants
• understand and calculate distortion on transmission lines
• understand wave reflection and calculate reflection coefficient
• understand standing waves and calculate standing wave ratio
44.1 Introduction
A transmission line is a system of conductors connecting one point to another and along which electromagnetic energy can be sent. Thus telephone lines and power distribution lines are typical examples of transmission lines; in electronics, however, the term usually implies a line used for the transmission of radio-frequency (r.f.) energy such as that from a radio transmitter to the antenna. An important feature of a transmission line is that it
should guide energy from a source at the sending end to a load at the receiving end without loss by radiation. One form of construction often used consists of two similar conductors mounted close together at a constant separation. The two conductors form the two sides of a balanced circuit and any radiation from one of them is neutralized by that from the other. Such twin-wire lines are used for carrying high r.f. power, for example, at transmitters. The coaxial form of construction is commonly employed for low power use, one conductor being in the form of a cylinder which surrounds the other at its centre, and thus acts as a screen. Such cables are often used to couple f.m. and television receivers to their antennas. At frequencies greater than 1000MHz, transmission
lines are usually in the form of a waveguide which may be regarded as coaxial lines without the centre conductor, the energy being launched into the guide or abstracted from it by probes or loops projecting into the guide.
