Antenna directivity and gain

Antennas (aerials) do not radiate equally in all directions. It is found that all realisable radio antennas radiate more in some directions than others. The actual pattern is dependent upon the type of antenna, its size, the environment and a variety of other factors.This directional pattern can be used to ensure that the power radiated is radiated in the desired directions.

It is normal to refer to the directional patterns and gain in terms of the transmitted signal. It is often easier to visualise the antenna is terms of its radiated power, however the antenna performs in an exactly equivalent manner for reception, having identical figures and specifications.

In order to visualise the way in which an antenna radiates a diagram known as a polar diagram is used. This is normally a two dimensional plot around an antenna showing the intensity of the radiation at each point for a particular plane. Normally the scale that is used is logarithmic so that the differences can be conveniently seen on the plot. Although the radiation pattern of the antenna varies in three dimensions, it is normal to make a plot in a particular plane, normally either horizontal or vertical as these are the two that are most used, and it simplifies the measurements and presentation. An example for a simple dipole antenna is shown below.

Polar diagram of a half wave dipole in free space

Antennas are often categorised by the type of polar diagram they exhibit. For example an omni-directional antenna is one which radiates equally (or approximately equally) in all directions in the plane of interest. An antenna that radiates equally in all directions in all planes is called an isotropic antenna. As already mentioned it is not possible to produce one of these in reality, but it is useful as a theoretical reference for some measurements. Other antennas exhibit highly directional patterns and these may be utilised in a number of applications. The Yagi antenna is an example of a directive antenna and possibly it is most widely used for television reception.

Polar diagram for a yagi antenna

There are a number of key features that can be seen from this polar diagram. The first is that there is a main beam or lobe and a number of minor lobes. It is often useful to define the beam-width of an antenna. This is taken to be angle between the two points where the power falls to half its maximum level, and as a result it is sometimes called the half power beam-width.

Antenna gain
An antenna radiates a given amount of power. This is the power dissipated in the radiation resistance of the antenna. An isotropic radiator will distribute this equally in all directions. For an antenna with a directional pattern, less power will be radiated in some directions and more in others. The fact that more power is radiated in given directions implies that it can be considered to have a gain.

The gain can be defined as a ratio of the signal transmitted in the "maximum" direction to that of a standard or reference antenna. This may sometimes be called the "forward gain". The figure that is obtained is then normally expressed in decibels (dB). In theory the standard antenna could be almost anything but two types are generally used. The most common type is a simple dipole as it is easily available and it is the basis of many other types of antenna. In this case the gain is often expressed as dBd i.e. gain expressed in decibels over a dipole. However a dipole does not radiated equally in all directions in all planes and so an isotropic source is sometimes used. In this case the gain may be specified in dBi i.e. gain in decibels over an isotropic source. The main drawback with using an isotropic source as a reference is that it is not possible to realise them in practice and so that figures using it can only be theoretical. However it is possible to relate the two gains as a dipole has a gain of 2.1 dB over an isotropic source i.e. 2.1 dBi. In other words, figures expressed as gain over an isotropic source will be 2.1 dB higher than those relative to a dipole. When choosing an antenna and looking at the gain specifications, be sure to check whether the gain is relative to a dipole or an isotropic source.

Apart from the forward gain of an antenna another parameter which is important is the front to back ratio. This is expressed in decibels and as the name implies it is the ratio of the maximum signal in the forward direction to the signal in the opposite direction. This figure is normally expressed in decibels. It is found that the design of an antenna can be adjusted to give either maximum forward gain of the optimum front to back ratio as the two do not normally coincide exactly. For most VHF and UHF operation the design is normally optimised for the optimum forward gain as this gives the maximum radiated signal in the required direction.

Gain / beamwidth balance
It may appear that maximising the gain of an antenna will optimise its performance in a system. This may not always be the case. By the very nature of gain and beamwidth, increasing the gain will result in a reduction in the beamwidth. This will make setting the direction of the antenna more critical. This may be quite acceptable in many applications, but not in others. This balance should be considered when designing and setting up a radio link.


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