# Electronics/Expanded Edition Aerials

This is about what aerials and antennae look like to a circuit, effective impedance, coupling of current with radio waves.

I. Introduction to Electromagnetics / Transmission Line Theory

Previous sections have presented an overview of DC and AC circuit principles. The circuits we have looked at until now all have one thing in common: they are electrically small compared to the wavelength of the voltages and currents induced on them. In other words, the physical size of the circuit is much smaller than the length of the voltage or current wave. Many other circuits, however, have physical dimensions the same as or larger than the wavelengths of their voltages and currents. In order to understand their operation, it is necessary to introduce the principles of electromagnetic theory.

All alternating voltages and currents move through space the form of waves. This is a fundamental principle of physics discovered in the 19th century and elaborated by the Scottish physicist James Clerk Maxwell, among others. Like a wave in water, an electromagnetic wave contains "crests" and "troughs". Wavelength is the distance from one crest to the next. In the electrical case, a crest is a point in space where the alternating voltage or current value is at maximum, and a trough is a point where the value is at minimum.

In the case of a DC current or voltage, the value is the same everywhere, so the wavelength is infinite. Alternating voltages and currents have finite wavelengths. The pattern the values take in space is that of a sine wave. If voltages and currents are measured at locations along the path of an electromagnetic wave, they will fit the curve of a sine wave.

Frequency in electromagnetics is the number of crests passing a particular point in space per unit of time, as the wave moves. It is the inverse (one over) wavelength.

What has been discussed so far is a subset of electromagnetic theory.