3.6 Types of Antenna

3.6 Types of Antenna

An antenna is a transducer to transform from a high frequency electric current to radio waves and vice versa. An antenna is used to transmit and receive radio waves. There are many kinds of antenna ranging from very small size (such as a monopoly antenna in a cordless telephone) to very large antenna reflectors of 100 meters in diameter for radio wave astronomy. In this section, antennas used for microwave remote sensing are introduced.

Typical antennas in microwave remote sensing are those of the passive type of microwave radiometer, active types of microwave altimeter, scatterometer and imaging radar.

There are three major types of antenna; horn antenna, reflector mirror antenna and array antenna.

The horn antenna such as the conical horn or rectangular horn is used for power supply to the reflector antenna, calibration of low temperatures for the microwave radiometer in the form of a sky horn looking upward, and calibration for active radar as shown in Figure 3.6.1.

Reflector antenna such as parabolic antenna and Cassegrainian antenna are composed of primary radiator and a reflective mirror as shown in Figure 3.6.2. The reflector antenna is used for microwave radiometers, altimeters and scatterometers. In case of wide angle scanning, all antenna will be controlled, while in the case of narrow beam scanning only the radiometer or reflective mirror will be controlled.

An array antenna is composed of multiple element arrays for example, linear array, area array or nonformal array. The element antennas are half-wavelength dipoles, microstrip patches and wave guide slot. The advantages of array antenna are to enable beam scanning without changing the looking angle of each array antenna and to generate an appropriate beam shaping by selective excitation of current distribution of each element.

The array antenna is used for synthetic aperture radar (SAR) and real aperture radar. Figure 3.6.3 shows a wave guide slot array antenna designed for real aperture radar.