An antenna transmits or receives electromagnetic waves. It is a transducer to convert electromagnetic waves into high-frequency electrical currents and vice versa. The mechanical sizes of the antenna are fractions of the used wavelength. There are antennas of more than a hundred meters in length using frequencies in long-wave range, and antennas with a length of a few millimeters for microwave ranges. This lesson deals specifically with antennae used in radar installations.

9.1. Antenna Functions

An antenna is a device that acts as a transformer to provide a good match between the feeding line as a local source of power and free space. If the antenna is not matched to free space, power will be reflected back toward the transmitter, resulting in a loss in radiated power. The antenna is one of the most critical parts of the radar system. It performs the following essential functions:

• It transfers the transmitter energy to signals in space with the required distribution and efficiency. This process is applied in an identical way upon reception.
• It ensures that the signal has the required pattern in space. Generally, this has to be sufficiently narrow in azimuth to provide the required azimuth resolution and accuracy.
• It has to provide the required frequency of target position updates. In the case of a mechanically scanned antenna, this is associated with the revolution rate. A high revolution rate can be a significant mechanical problem given that radar antennas in certain frequency bands can have a reflector with immense dimensions and can weigh several tons.
• It must measure the pointing direction with a high degree of accuracy.
• The antenna structure must maintain the operating characteristics under all environmental conditions. Radomes are generally used where relatively severe environmental conditions are experienced.

The basic performance of radar can be shown to be proportional to the product of the antenna area or aperture and the mean transmitted power. Investment in the antenna, therefore, brings direct results in the system’s performance.

Taking into account these functions and the required efficiency of the radar antenna, two arrangements are generally applied:

• the parabolic dish antenna
• the array antenna