9.2.1. Antenna Gain

Due to the special design of the antenna, the radiation density can be concentrated in certain spatial directions. A measure of the directivity of a lossless antenna is the antenna gain. It is closely associated with the directivity of the antenna. In contrast to the directivity, which only describes the directional properties of the antenna, the antenna gain also takes into account the efficiency of the antenna. It, therefore, indicates the actual radiated power. This is usually smaller than the power provided by the transmitter. However, since this power is easier to measure than the directivity, the antenna gain is used more often than the directivity. Under the assumption that a lossless antenna is considered, the directivity can be set equal to the antenna gain.

A reference antenna is used to define the antenna gain. In most cases, the reference antenna is a lossless hypothetical omnidirectional radiator (isotropic radiator) that radiates uniformly in all directions or a simple dipole antenna, which can also be a reference, at least in the plane being considered.

For the antenna to be measured, the radiation density (power per unit area) at a point at a certain distance is determined and compared with the value obtained using the reference antenna. The antenna gain is then the ratio of the two radiation densities.

If, for example, a directional antenna generates 200 times the radiation density of an isotropic antenna in a certain spatial direction, the antenna gain G has the value 200 or 23 dB.

9.2.2. Antenna Pattern

The antenna pattern is a graphical representation of the spatial distribution of the radiated energy of an antenna. Depending on the application, an antenna should only receive from a certain direction but should not pick up signals from other directions (e.g., TV antenna, radar antenna); on the other hand, the car antenna should be able to receive transmitters from all possible directions.

The desired directivity is achieved by the targeted mechanical and electrical construction of an antenna. A directivity indicates how well an antenna receives or transmits in a certain direction. It is indicated in a graphical representation (antenna pattern) as a function of the azimuth angle (horizontal diagram) and the elevation angle (vertical diagram).

Either a Cartesian or a polar coordinate system is used. The measured values in the graphical representation can have linear or logarithmic values.

9.2.3. Polarization

The radiation field of an antenna is composed of electric and magnetic lines of force. These lines of force are always at right angles to each other. The electric field determines the direction of polarization of the wave. When a single-wire antenna is used to extract energy from a passing radio wave, maximum pickup will result when the antenna is oriented in the same direction as the electric field. The oscillations of the electric field may be oriented in a single direction (linear polarization), or the oscillation direction of the electric field may rotate as the wave travels (circular or elliptical polarization).

Electric field (blue) and magnetic field (red) of a vertical mounted (polarized)

9.2.4. Linear Polarization

Vertically and horizontally mounted receiving antennas are designed to receive vertically and horizontally polarized waves, respectively. Therefore, changes in polarization cause changes in the received signal level due to the inability of the antenna to receive polarization changes. Two planes of polarization are used mainly: 

• In a vertically polarized wave, the electric lines of force lie in a vertical direction. 
• In a horizontally polarized wave, the electric lines of force lie in a horizontal direction. 

The linear polarization can obviously take all planes but besides the horizontal plane and vertical plane only the positions. When a single-wire antenna is used to extract energy from a passing radio wave, maximum pickup will result when the antenna is oriented in the same direction as the electric field. Thus a vertical antenna is used for the efficient reception of vertically polarized waves, and a horizontal antenna is used for the reception of horizontally polarized waves.

9.2.5. Circular Polarization

Circular polarization has the electric lines of force rotating through 360 degrees with every cycle of RF energy. Circular polarization arises by two 90-degree phase shift income signals and also by plane-polarized antennae moving 90 degrees simultaneously. The electric field was chosen as the reference field since the intensity of the wave is usually measured in terms of the electric field intensity (volts, millivolts, or microvolts per meter). In some cases the orientation of the electric field does not remain constant. Instead, the field rotates as the wave travels through space. Under these conditions both horizontal and vertical components of the field exist and the wave is said to have an elliptical polarization. 

Circular polarization can be right-handed or left-handed. A circularly polarized wave is reflected by a spherical raindrop in the opposite sense of the transmission. On reception, the antenna rejects waves of the opposite sense of circular polarization thereby minimizing the detection of rain. The reflection from the target will have significant components in the original polarization sense because unlike rain, aircraft are not spherical. The strength of the target signal is therefore enhanced relative to rain.

Rising of circular polarization

For maximum absorption of energy from the electromagnetic fields, the receiving antenna must be located in the same plane of polarization. If a wrongly polarized antenna is used, then considerable losses arise, in practice between 20 and 30 dB. At the appearance of strong weather clutter the air traffic controllers prefer to switch on the circular polarization. In this case the hiding effect of the targets by the weather clutter will be decreased.