Transmission Line

A transmission line is a distributed parameter network and is described by parameters distributed throughout its length. The electric lines, which are used to transmit the electrical waves along them, are known as transmission lines. Transmissions lines are used for connecting the transmitter arid its antenna and arc usually considered as a part of the antenna system. Transmission tine elements are integral parts of the antenna. Transmission lines are also known as feeders. Feeder is actually a special case of a transmission line. Transmission line is not only used to transmit energy from one place to another but it is also used as a circuit element like inductor, capacitor, resonant circuit, filter, transformer and insulator at high frequencies. It means to convey electrical signals between two points separated in distance. A problem in transmission line differs from the simple four terminal network of circuit theory.  The input voltage K is applied at the input terminals and a finite time is taken in reaching the load end terminals. A finite time is needed for the voltage and current to travel the length of the transmission line just as the electromagnetic wave has a finite velocity in space.

A transmission line has inductance (L), capacitance (C) and resistance (R). The electrical parameters are uniformly and continuously distributed along the line. Each conductor of the line has certain length and diameter; it must have resistance and inductance. Since the two wires or conductors are close to each other there is capacitance between them. The wires are separated by a medium known as dielectric that cannot be perfect in its insulation and hence current leaks through it. This leakage of current through the dielectric is represented by a shunt conductance (G). Assume that the unit length is a very small fraction of the total length so that there are a number of identical lumped unit sections. One unit section of line is represented up to x = 1 per unit length. When R, L, G, Care uniformly distributed, it is said to be a uniform transmission line. R, L, G, C should not be assumed to be lumped at any one point as they are uniformly and continuously distributed along the entire length of the line.

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• What is transmission line?