An Introduction to Wireless Technology, Second Edition

Another property of electromagnetic radiation is that it can be polarized. The

concept of polarization is most familiar to us in the use of polarized sunglasses

to eliminate reflections off shiny surfaces such as water. Polarized sunglasses

will only allow light of one polarization to pass through them and will cut out

light reflected from the surface. This is because electromagnetic radiation

undergoes a 90

° change in polarization each time it is reflected. Radio waves

can be polarized in the same way and selection of polarization of a transmitted

signal may be achieved by the position of the transmitting elements in a

horizontal or vertical attitude. This property can be used to reject unwanted or

spurious signals which may arrive at the receiving antenna with a different

polarization to that of the wanted signal.

2.1.2 Antennae

Radio frequency signals are transmitted using an antenna which is designed to

provide the most efficient method of radiating the signal. Its design will be

dependent on the frequency of the signal, the spread of the signal required, and

the environment in which it is to be used. In general, the same design of

antenna can be used for both transmitting and receiving.

The basic form of antenna is known as the half-wave dipole. It consists of a

single element with the feed from the transmitter or receiver at its center. Its

length is exactly equal to one half of the wavelength of the signal.

28 An Introduction to Wireless Technology

Figure 7. Half Wave Dipole Polar Diagram

You can see from the diagram that it radiates (or receives) equally well from any

direction (omni-directional), assuming that it is mounted vertically. In a vertical

plane its radiation pattern is a figure-eight producing an overall

three-dimensional pattern in the shape of a doughnut or torus. The half-wave

dipole may be used as the standard antenna on which comparisons of other

antenna designs are made. In this case it is considered to have unity gain

(0 dB).

Note: An antenna is a passive device, and cannot amplify a signal. However, a

uni-directional antenna will have most of its transmitting/receiving capability in

one direction, and this is represented in terms of antenna gain.

The antennae commonly seen mounted on vehicles for broadcast reception are

a special form of dipole. The top half of the dipole is the element mounted on

the vehicle. The bottom half of the dipole is formed by a reflection from the

metal of the vehicle body. The vehicle body is known as the groundplane.

Chapter 2. Wireless Methodologies 29

Figure 8. Vehicle Mounted Dipole

Another common type of antenna is the Yagi array. The familiar externally

mounted TV reception antenna is called a Log Periodic antenna and is similar to

a Yagi array.

30 An Introduction to Wireless Technology

Figure 9. A Six-Element Yagi Array

A Yagi array consists of one element called the driven element and at least two

other elements parallel to it. The driven element has the feed to the transceiver

connected to it. The element is a reflector, normally slightly longer than the

driven element and mounted just behind it, (away from the direction of

transmission or reception). The second element is called a director. It is

mounted in front of the driven element and is slightly shorter. There may be

several director elements each shorter than the first and each other. The more

director elements, the more directional the Yagi array antenna. The size and

position of each element is carefully calculated to ensure the best performance.

In general, the more directive an antenna, the higher its gain. Antennae are

reciprocal devices; that is, they provide directive gain in both transmitting and

receiving modes. This means that a correctly designed system will benefit from

enhanced radiated power from the transmitter and improved receiver sensitivity.

The overall system performance will be improved by an amount that is

equivalent to twice the antenna gain. A good antenna design has more effect on

performance than any other single part of a radio communications system.

The performance of an antenna is quoted by its gain (dB) and its capture angle

or beamwidth in degrees. The gain is the ratio of power transmitted or received

for a standard half-wave dipole compared to that of the antenna being described.

The beamwidth is the angle between the two edges of the main lobe of the

antenna polar diagram for a specific transmitted field strength or received

sensitivity.

A different principal of antenna design is used for satellite TV reception and

other satellite communications.

Chapter 2. Wireless Methodologies 31