Basic properties

The principle of reciprocity is one of the most important properties of an antenna. It means that the properties of an antenna when acting as a transmitter are identical to the properties of the same antenna when acting as a receiver. For this to apply, the medium in between the two antennas must be linear, passive and isotropic, which is always the case for communication systems.

The directional selectivity of an antenna is represented by the radiation pattern. It is a plot- of the relative strength of the radiated field as a function of the angle. A pattern taken along the principal direction of the electric field is called an E- plane cut, the orthogonal plane is called an H-plane cut. The most common plot is the rectangular decibel plot, Figure 17.3 which can have scales of relative power and angle chosen to suit the antenna being charac­terised. Other types of plots such as polar plots (used for small antennas and two dimensional), contour plots (or three dimen­sional), and isometric plots are also used. A radiation pattern is characterised by the main beam and sidelobes. The quality is specified by the beamwidth between the -3dB points on the main beam and the sidelobe level.

Communication antennas radiate in either linear polarisation or circular polarisation. In modern communications cross- polarisation is important. This is the difference between the two principal plane patterns and is specified relative to a reference polarisation, called the co-polar pattern. There are three definitions of the cross-polari­sation. The one in normal use with reflector antennas and feed systems is Ludwig's third definition (Ludwig, 1973) which assumes that the reference polarisation is that due to a Huygens source. It most closely corresponds to what is measured with a conventional antenna test range.

The power gain in a specified direction is defined by the ratio of the power radiated per unit solid angle in direction Θ, φ to the total power accepted from the source, as in Equation 17.1.

This is an inherent property of an antenna and includes dissipative losses in the antenna. The dissipative losses cannot easily be pre­dicted so a related parameter, the directivity, is used in calculations. The definition of the directivity is similar to that of the gain except that the denominator is replaced by the total power radiated. The terms gain and directivity are often used interchangeably in the literature. Normally only the peak gain along the boresight direction is specified. If the direction of the gain is not specified, the peak value is assumed. The value is normally quoted in dB's. The defini­tions given above are in effect specifying the gain relative to a loss-less isotropic source. This is sometimes stated explicitly by using the symbol dBi.

The efficiency of an aperture antenna is given by the ratio of the effective area of an aperture divided by the physical area. Normal aperture antennas have efficiencies in the range 50-80%.

As far as circuit designers are concerned the antenna is an imped­ance. Maximum power transfer will occur when the antenna is matched to the transmission line. The impedance consists of the self impedance and the mutual impedance. The mutual impedance ac­counts for the influence of nearby objects and of mutual coupling to other antennas. The self impedance consists of the radiation resist­ance, the loss resistance and the self reactance. Loss resistance is the ohmic losses in the antenna structure. Radiation resistance measures the power absorbed by the antenna from the incoming plane waves. It is one of the most significant parameters for small antennas where the problem is often to match very dissimilar impedances.

A receiving antenna is both a spatially selective filter (measured by the radiation pattern) and a frequency selective filter. The band­width measures the frequency range over which the antenna oper­ates. The upper and lower frequencies can be specified in terms of a number of possible parameters: gain, polarisation, beamwidth and impedance.

A communication link consists of a transmitting antenna and a receiving antenna. If the transmitter radiates P₍ watts, then the received power, P_f at a distance r is given by Equation 17.2 where G₍ and G^ are the transmitter and receiver antenna gains respec­tively.

This formula is known as the Friis transmission equation. It assumes that the antennas are impedance and polarisation matched. If this is not the case then extra factors must be multiplied to the equation to account for the mismatches.