Propagation and antennas

Antennas

The fundamental antenna is a piece of wire which is one half wavelength (λ/2) long, corresponding to the frequency at which radiation is desired. The voltage and current vary over the length of this antenna, as shown in Fig 7.7 (top).

Fig 7.7. Standing waves on resonant antenna showing voltage and current variation. Upper antenna is λ/2 long (fundamental antenna); lower is 1λ (second harmonic) antenna.

If the piece of wire is made a whole wavelength (1λ) long, the current and voltage variations are as in Fig 7.7 (bottom). This is known as a 'full-wave' or 'second-harmonic' antenna. Larger multiples of the basic λ/2 antenna show similar voltage/current variations. These variations are known as 'standing waves', and this type of antenna is known as a 'resonant' antenna.

It is seen that the ratio of voltage and current varies over the length of the antenna, and may be resistive, inductive or capacitive. This ratio is referred to in general terms as the antenna 'impedance'.

The 'radiation resistance' of an antenna is a fictitious resistance which would dissipate the power radiated by it.

Antenna length

The length of a half-wavelength (λ/2) in space is

metres

The actual length of a λ/2 antenna is somewhat less than this, owing to:

• (a) the velocity of propagation in the wire being different from that in space;
• (b) the presence of insulators at the end of the wire and of nearby objects (trees or buildings);
• (c) the diameter of the wire or element.

Table 7.1. Approximate lengths of λ/2 dipoles

The actual length is normally taken to be 5% less than (or 0.95 of) the electrical length. This constant, 0.95, is sometimes known as the 'correction factor', hence the actual length is

metres

Radiation patterns

If a λ/2 antenna is assumed to be parallel to and at least a wavelength above perfect ground, and also remote from all other objects, the radiation is concentrated at right-angles to its length, as shown in Fig 7.8(a). This is the radiation pattern of a λ/2 antenna and, as the antenna radiates in directions all round the wire, the radiation pattern in space is the shape formed by imagining the pattern of Fig 7.8(a) to be rotated round the antenna as an axis.

Fig 7.8. Theoretical radiation patterns of resonant antennas

Radiation patterns of antennas working on higher harmonics are shown in Figs 7.8(b), (c) and (d), where it is seen that the effect is to produce more lobes; the four lobes of the full-wave (lλ) case tend to swing towards the ends of the antenna, and subsidiary lobes appear. Thus in the case of an extremely long antenna the radiation tends to be concentrated towards the ends.

If one end of the antenna is tilted, the lobes tend to move together and hence radiation tends to become concentrated off the lower end. Consideration of the λ/2 pattern shows that radiation is all around (omni directional) when the antenna is vertical.

Angle of radiation

This is the angle with respect to a tangent at the surface of the earth at which the maximum radiation occurs. Its value depends upon interaction between the direct ray from the antenna and the ray reflected in particular from the ground. Hence it is determined by both the antenna height and the characteristics of the ground. A single lobe exists in the vertical plane at an antenna height of approximately λ/2, and above this height the one lobe splits into two, one at a higher angle and the other at a lower angle. These two lobes could be felt to be of more use than a single lobe from the point of view of total coverage (see Fig 7.2). As the height increases the higher lobe increases in angle and magnitude. It is clear that the angle of radiation produced by a multi-band antenna at the fairly average height of 7-8m varies considerably between 3.5MHz and 28MHz.

Directional antennas

It is possible to modify the radiation pattern of an antenna in order to concentrate the radiation in a particular direction. Thus a 'directional' or a 'beam' antenna is created.

Fig 7.9. (a) Arrangement of Yagi directional (beam) antenna. (b) Radiation pattern of directional antenna

This is generally achieved by the addition of parasitic elements known as a 'director' and a 'reflector' parallel to the antenna, as shown in Fig 7.9(a), which also shows the approximate spacing in terms of the operating wavelength. This arrangement is known as the 'Yagi array'. The radiation pattern is shown in Fig 7.9(b). The addition of more directors produces a narrower beam, but more than one director is usually only possible at VHF, where up to 20 or more directors may be used.

Means must be provided for rotating a beam antenna so that it can be turned to the required direction.

An additional feature of the directional antenna is the fact that when used for reception, signals to the back of the beam are attenuated, ie interfering signals from an unwanted direction may be significantly reduced in strength. The characteristics of a beam antenna are the 'forward gain' (compared with a dipole) and 'front-to-back ratio', these terms being self-explanatory; they are expressed in decibels.

Photo 7.1. A typical 3 element HF 'Yagi array' atop of a large mast. These are normal arranged so they can be rotated, to direct the beam in the required direction.