11 - Antennas: How to Make Circuits That Radiate

Abstract

This chapter demonstrates how to make circuits that radiate efficiently. In practice, circuits usually have a return current that flows in the opposite direction of the signal current. For example, in a two-wire transmission line, the two wires are parallel with signal current flowing in one direction and return current flowing in the opposite direction. There is a field between the wires and in the immediate vicinity of the wires, but at far distances, the fields from the two wires tend to cancel one another. The result is that very little radiation occurs. There are two basic ways to create an antenna from a transmission line. One method results in an electric dipole and the other method results in a magnetic dipole. The loop antenna can actually save space in low-frequency radio applications, where wavelength is tremendous. For instance, the wavelength at the center of the AM radio band is about 300m.