Overview

One of the first things that you learn in radio communications and broadcasting is that antenna impedance must be matched to the transmission line impedance and that the transmission line impedance must be matched to the output impedance of the transmitter. The reason for this requirement is that maximum power transfer between a source and a load always occurs when the system impedances are matched. In other words, more power is transmitted from the system when the load impedance (the antenna), the transmission line impedance, and the transmitter output impedance are all matched to each other.

Of course, the trivial case is where all three sections of the system have the same impedance. For example, an antenna could have a simple 75- ? resistive feedpoint impedance (typical of a half-wave dipole in free space) and a transmitter with an output impedance that will match 75 ?. In that case, you only need to connect a 75- ? standard-impedance length of coaxial cable between the transmitter and the antenna. Job done! Or so it seems .

But, in other cases, the job is not so simple. In the case of the standard antenna, for example, the feedpoint impedance is rarely what the books say it should be. That ubiquitous dipole, for example, is nominally rated at 75 ? but even the simplest antenna books show that value is an approximation of the theoretical free-space impedance. At locations closer to the earth s surface, the impedance could...