Electromagnetism for Engineers by Andrew J. Flewitt

Author:Andrew J. Flewitt [Flewitt, Andrew J.]
Language: eng
Format: epub
ISBN: 9781119406204
Published: 2023-01-30T00:00:00+00:00

6.6 Loads on Transmission Lines

The purpose of defining a characteristic impedance becomes clearer when we want to know how a transmission line will respond when other devices are connected to either end of the line or when transmission lines are connected to each other. In this section, we will consider specifically what happens when a load is connected to the end of a transmission line. In practice, this will almost always be the case. For example, we will probably want to connect a load to the end of a power transmission line, such as a motor, heating element or lighting, to transduce the electrical energy into some other form. Alternatively, we may have some analogue signal processing circuit at the coaxial cable input to a television from an aerial, or in a digital logic circuit the input to a logic gate will probably sit at the end of a microstrip line. In all of these cases, the load will have some input impedance Z L .

To understand quantitatively how the voltage and current waves on the transmission line respond to the presence of such a load, let us consider the general situation shown in Figure 6.12 where we have a load impedance connected to the end of a transmission line at a point x = 0 . We will imagine that the line extends infinitely into the negative x ‐direction so that we do not have to consider any effect of an impedance at the other end of the line. For simplicity, we will use the ideal transmission line to understand how the system behaves, but the same result applies to lossy lines as well.

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