TABLE OF CONTENTS In this chapter, you will explore the physical features that compose a transmission line. You will examine how the geometries of chunks of metal influence the flow of electrical signals, terminology used to describe this flow, and the mathematical models that explain it. When these chunks of metal become large compared to signal wavelengths, interesting things happen. Of course, these things happen at lower frequencies, too, but often they can be ignored. When signals were running in the low hundreds of megahertz, wavelengths were meters long, and circuit board sizes were small compared to wavelengths. Now with signals in the mid-gigahertz range, board sizes are large compared to wavelengths. Even individual traces are often long compared to wavelengths. In some instances, even board thicknesses are large compared to wavelengths. Effects that were too small to be of concern when frequencies were lower now can become dominant factors.
The full, exact evaluation of a complicated arrangement of metals and dielectrics at microwave frequencies is often not possible. Good approximations are often possible, but they take excessive amounts of time. Running a model of a few inches of transmission line in SPICE takes a millisecond or so. Running a full-wave analysis of a square inch of multi-layer circuit board takes hours to days. The engineering question is, how good is good enough?
To make practical use of microwave frequencies on production-level boards, design engineers need useful approximations and simplifications that allow them to quickly identify critical issues while ignoring...
