Infinite and Finite Lines

Author: Edmund A. Laport

Any line of continuous and uniform parameters exhibits an important circuital property known as the "characteristic impedance." This is usually designated as Z₀. This is the impedance of the line as seen at its input terminals if the line were of infinite length. Such a line would continuously absorb energy from a generator and propagate it outward forever. A line of finite length is simply a portion of an infinite line. The missing remainder, going on to infinity and having the same characteristic impedance Z₀, has been cut away. To simulate this missing portion, any load with an impedance equal to Z₀ can be used to terminate or match the line and absorb all the energy propagated from the generator. When a line is terminated in an impedance equal to its characteristic impedance, the propagation of currents between the generator and the termination is the same as in an infinite line and energy travels only away from the generator. At radio frequencies the characteristic impedance of low-loss lines is resistive. When open-circuited, close-circuited, or terminated in any arbitrary impedance other than Z₀, there is reflection of energy from the end back toward the generator. The presence of waves traveling in both directions on the conductors gives rise to various current and potential distributions and causes the input impedance to vary widely in magnitude and phase angle. Because of this, transmission lines have uses as impedance transformers and energy-storage circuits as well as for the guidance of energy from a generator to some load circuit.