An Antenna is a device which allows us to collect or radiate electromagnetic fields. In this section we'll consider the general properties of antennas. Later sections will consider specific types of antenna and how they can be used to transmit patterns of electromagnetic energy from place to place.

### 6.1 Antenna power pattern.

In figure 6.1 the output from a coherent source (e.g. an oscillator) is directed out into free space using an antenna. The signal source is linked to the antenna by some kind of waveguide (microwave guide, light fibre, or whatever). The antenna acts as a sort of transformer. It takes the electromagnetic field pattern, moving along the guide and transforms it into some other pattern, which is radiated out into free space.

Using this simple picture we can establish two basic properties of any antenna:-

Firstly, the antenna doesn't itself generate any power. So, unless the antenna is imperfect and dissipates some power, the total powers carried by the guide and free space fields must be the same. (In reality, all practical antennas tend to be slightly resistive so some power is normally lost, but for now we can assume any loss is small enough to ignore.)

Secondly, the antenna is a reciprocal device — i.e it behaves in the same way irrespective of which way we pass signal power through it. Imagine making a video showing how the field flows from the source into the system and is transformed/radiated into space. If we were to replay the video backwards the field would appear to enter from space, move through the antenna, and be transformed into a guide field heading towards the ‘source’. In both cases the process would satisfy Maxwell's Equations. For an antenna dealing with a coherent field we therefore can't tell, by looking at what happens, which way we're playing the video unless we independently know something extra — e.g. that the device connected to the waveguide is a source, not a detector.

This reciprocal behaviour is a useful feature of a coherent antenna. It means that, in principle, the only real difference between a ‘transmitting’ and a ‘receiving’ antenna is the direction we've chosen to pass signals through it. A practical consequence is that we can often used identical antennas for transmitting and receiving signals through space. This reciprocal property also turns out to make analysing and understanding antennas a bit easier!

Theoretically one of the simplest types of antenna is a Hertzian Dipole of the kind illustrated in figure 6.2. Although this sort of antenna isn't used much at high frequencies it is a good place to start because its properties are easy to analyse.

The dipole consists of a straight piece of wire of length L with a small break at its centre. A pair of wires are then used to connect the break to a generator which can produce a current

For the sake of simplicity we can assume that

- i.e. the dipole is very short compared with the wavelength of the radiation we're attempting to radiate. This means that we can neglect the time it takes for any current to flow along the dipole. We can therefore assume that the current equals I{t} everywhere along the dipole. In reality, of course, the current would have to “come from and go somewhere” at the wire ends since charge can't appear and vanish into/out of nothing. We can imagine two spheres or discs placed at the ends of the wires to act as charge reservoirs. (In a real antenna the currents on these would affect the antenna's behaviour, but we'll ignore that fact here to avoid complications!)

Content and pages maintained by: Jim Lesurf (jcgl@st-and.ac.uk)
using HTMLEdit3 on a StrongARM powered RISCOS machine.
University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.