For guided signal transmission, a transmission line, a material medium forms a path. The construction of a transmission line determines the frequency range of the signal that can be passed through it. Fig. 32.1 shows some typical transmission lines.
The simplest form of transmission line is a pair of parallel conductors separated by air or any dielectric medium. These are used in telephony. However, such lines tend to radiate, if the separation between the conductors is nearly half of the frequency corresponding to the operating frequency. This may lead to noise susceptibility, particularly at high frequencies, and limit their utility.
To overcome this problem, twisted pair of wires is used. These are used in computer networking.
At high signal frequencies (≤ 3GHz) radiation losses are minimized by using coaxial cables, where one conductor is hollow and the second conductor is placed inside it at its centre throughout the length of the cable. These conductors are separated by dielectric spacer layers of polythylene and the electric field is confined in the annular space in between the conductors. These cables are used for carrying cable TV signals.
Ideally the dielectrics should have infinite resistance. But in practice, their resistance is finite and that too decreases with frequency. As a result, even coaxial cables are useful in a limited range (up to a maximum of 40 GHz when special dielectric materials are used).
Beyond 40 GHz, wave guides are used. However, for frequencies greater than 300 GHz, their dimensions become too small (4 mm) and it presents practical problems. Above this frequency, optical fibres are used for guided wave transmission.