Antenna

Radio antenna (or aerials) are the means by which:

  • radio-frequency transmitter energy is radiated into space as electromagnetic energy and
  • electromagnetic energy is received from space for transfer as electrical signals to a receiver.

 Antenna were first used in 1888 by Heinrich Hertz to demonstrate the existence of electromagnetic waves. Originally just referred to as 'aerial wires' (wires in the air), Guglielmo Marconi's tests in 1895 included supporting poles similar to tent poles which in Italian are called l'antenna centrale. Marconi adopted the term l'antenna for his aerial pole, from which we get the word 'antenna'.

Antenna are designed to be as efficient as possible for a particular task. Because there are many different tasks and a vast range of possible radio frequencies, there are many different antenna designs. We discuss a few different design aspects such as resonant or non-resonant, polarisation, directivity, reciprocity, power levels, and frequency.

Resonant or non-Resonant

Early experiments used long antenna wires. These antenna were ultimately found to work best when the wire was raised up a pole at one end, with the other end sloping towards the ground in the direction of the other station. The antenna worked even better when the end of the wire was terminated in a resistor. The length of these wires were not related to the frequency (or wavelength) of the signal and are known as non-resonant antenna. Such longwire antenna are commonly used for receiving medium-wave broadcast signals (around the 1MHz frequency range).

The vast majority of antenna are constructed so that their 'elements' or conducting wires are made a half wavelength long or multiples of a half wavelength. These antenna work very efficiently because of a resonant effect which builds up energy in the wires. Consequently, these antenna are referred to as being of a 'resonant' type.

The shortest and most efficient resonant antenna (in terms of cost and effectiveness) is the halfwave type where energy is transferred at its centre with equal length wires on each side. This basic antenna is called a 'dipole' and is the standard around which most antenna are designed.

Polarisation

Early low frequency, long wavelength antenna were erected on top of very tall masts, or were strung between tall masts. This orientation of an antenna parallel with the ground is referred to as horizontal polarisation. However, as radio frequencies became higher and wavelengths shortened, it became possible to erect antennas vertically - i.e. with vertical polarisation. This was very useful for shipping because aerials could then be connected to the top of a single mast rather than between a series of masts or chimneys.

With a horizontally polarised antenna it is easy to connect equipment to its centre. It is less easy to connect to the centre of a vertical antenna without upsetting its operation. However, designers found that with a good earth or 'ground plane' under a vertical antenna, the earth would act like a radio 'mirror'. Radio signals would work with half an aerial above ground and with the mirror image that seemed to be below ground. This gave rise to the use of the 'monopole' antenna as used on the roof of cars and trucks, where a metal roof acted as the reflecting earth or ground plane. A monopole could be connected at its base, yet it would act as though it were still a dipole!

Changing an antenna from horizontal to vertical also changes the nature of the radio signal from horizontal to vertical - a horizontal antenna is least effective when receiving a vertical signal and vice versa. This is useful when trying to minimise interference from nearby frequencies and is why some rooftop TV antenna are mounted horizontally and some are mounted vertically.

Directivity

A resonant half-wave dipole radiates the majority of its electromagnetic energy from the sides and very little of it from the ends. The strength of energy radiated is like the shape of a donut where the antenna is positioned through the central hole.

An antenna's directional characteristics can be enhanced by placing additional wires near the main element. A typical example is the TV 'yagi' antenna which has a 'reflector' behind a dipole and a series of 'directors' in front of the dipole. Improving an antenna's directivity allows a designer to reduce transmitter power to save cost, to achieve longer communication distances (such as to satellites), or to minimise interference from other services.

Reciprocity and Power Levels

One of the features of antenna is that changes made to an antenna gain design affect equally both transmit and receive operation. Therefore an antenna designed to say, give 10 times more transmit gain, if used at the receive end of a circuit will give 10 times more receiver gain. This effect is known as 'reciprocity' because the antenna design results are reciprocal.

However, not all antenna design features are intended to provide more gain. A transmitter antenna may handle many kilowatts of energy and must be constructed of relatively robust components that can withstand high voltages and currents. By contrast a receiving aerial handles micro-volts (millionths of a volt)  and can therefore be made of finer material.

A good example of this difference is to consider an antenna's feeder wires - the connection between an antenna and its transmitter or receiver. The feeder line for a high power transmitter could be an open wire line where two wires are spaced say 350mm apart, or could be a high voltage coaxial cable capable of withstanding 20,000 volts. At a receive antenna the very low voltages being passed allow the use of a simple coaxial cable and lightweight transformers. However, at both transmit and receive sites there is always a need to allow for high voltage atmospheric effects such as lightning strikes by providing appropriate high voltage 'arrestors'.

Frequency

The major impact on resonant aerial design is the frequency of use for as this gets higher, the size of its antenna becomes smaller. Examples are:

  • The long wire antenna used for reception of broadcast stations (0.5 - 1.5 MHz)
  • New Zealand's original hilltop analog television stations with comparatively small TV aerials on our roofs (45 - 230 MHz)
  • The landmobile radiotelephone service use of short whip antenna as seen on car and truck roofs (75 to 400 MHz)
  • New Zealand terrestrial (hilltop) digital TV channels (500 - 700 MHz)
  • Mobile cellphones with antenna so small they are hidden inside the cellphone handset (800 MHz+)
  • New Zealand satellite digital TV services use of rooftop satellite dishes (12,500 MHz)

If the digital TV satellite signal is way higher than the frequencies our cellphones use, how come we end up needing a big dish on our roof? The answer is that the dish is not the receiving antenna, it is a reflector to focus the very weak satellite signal onto the antenna. The satellite antenna is very tiny and is hidden inside the knob on the end of the stick which pokes out in front of the reflector.