In reality, this situation is never quite true. In basic circuit theory, you assume that the wires that make up the transmission line are ideal and hence that the voltage at all points on the wires is exactly the same. The figure shows the transmission line as a pair of parallel conductors. In a simple transmission line, a source provides a signal that must reach a load ( Figure 1a). For pc boards, the common transmission lines are strip-line and microstrip. Some examples of uniform transmission lines are coaxial cables, twisted-wire pairs, and parallel-wire pairs. That is, the conductor shape, size, and spacing are constant, and the electrical characteristics of the conductors and the material between them are uniform. However, implicit in most discussions of transmission-line theory is the assumption that the lines are uniform.Ī uniform transmission line is one whose geometry and materials are uniform. In general, every connection in an electric circuit is a transmission line. Why are transmission-line effects usually noticeable only at high frequencies? What happens at low frequencies? What are the definitions of “high” and “low”? In practice, you can more easily and completely grasp transmission-line theory just by understanding the basic physics.Īs the name implies, a transmission line is a set of conductors used for transmitting electrical signals. In addition, you’ve probably heard that transmission-line effects become apparent at higher frequencies, but rarely does anyone explain why. If so, you learned transmission-line theory with wave equations and a lot of difficult math. Unfortunately, you probably learned transmission-line theory during a few lectures in an electromagnetic-fields class. Moreover, this approach requires little background in electromagnetic-field theory. With the aid of graphical representations of analog and digital signals, you can gain a solid intuitive understanding of transmission lines. As wireless designs become more prevalent and as digital designs reach higher and higher frequencies, a thorough understanding of transmission-line theory is becoming increasingly important.
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