An ordinal antenna is used for transmitting radio waves in a specific direction or for receiving radio waves from a specific direction. Therefore it can be said that the antenna is a spatial filter for radio wave.

Relative power, given as a function of the beam angle is called the radiation pattern or beam pattern. Usually the beam pattern is given in an orthogonal coordinate system or polar coordinate system, as shown in Figure 3.7.1.

The characteristics of the beam pattern can be determined by making a Fourier transformation of the aperture distribution. If the size of antenna aperture is infinite, the beam pattern should be an impulse pattern. But as actual antenna are limited in size, the beam pattern has several lobes with respect to beam angles, as shown in Figure 3.7.2.

The point with zero power is called the null and the pattern between two nulls is called the lobe. The central biggest lobe is called the main lobe, while the other robes are called sidelobes.

The beam width of the antenna is defined as the beam width at the power level of 3dB downward from the peak of the main lobe (equivalent to the half power beam width). The difference between the peaks of the main lobe and the biggest sidelobe is called the sidelobe level. Antenna gain is given as the ratio of power density of an antenna to the reference antenna with a given constant power at a specific angle. The antenna gain that is obtained by an isotropic antenna as the standard antenna is called standard gain. The ratio of the power density at a specific angle to the average power density determined from all radiative power is called directivity and is given as follows.

where E (, ) : field strength at the direction of and (horizontal and vertical angles)

Usually characteristics of transmitting antenna and receiving antenna are identical to each other.

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