TABLE OF CONTENTS When the term antenna is mentioned, many people think of a car antenna, either a long metal rod or a piece of wire imbedded in the windshield that allows them to hear their favorite radio station. That, of course, is one application of antennas, but there is much more to an antenna than a piece of metal sticking up in the air. Antennas are a vital part of many RF, microwave, and wireless applications, both commercial and military.
The simplest, and probably the most understandable, way to explain antennas is to start with a transmission line with its output end left open. You will recall from the discussion of open-circuited transmission lines that the voltage at this point is a maximum value, and there will be radiation of the energy from the open-ended line. That is something that will happen even though you are not planning on making your transmission line an antenna. Any open-ended transmission lines, including microstrip transmission lines, will radiate and act as antennas. That is the reason these lines are kept a distance from other transmission lines. Figure 4.47 shows an open-ended transmission line. Although a certain amount of electromagnetic energy radiates from all open-circuit transmission lines, the distance for which the radiation is of any consequence is minimal. That certainly would not be a very good intentional antenna. What we need is some method to get the line to radiate power over a much longer range. We could, of course, put a...
