Functions of an Antenna
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 a 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 on 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 equates to the revolution rate. A high revolution rate can be a significant mechanical problem given that a radar antenna 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 a 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 terms of system performance.
Taking into account these functions and the required efficiency of a radar antenna, two arrangements are generally applied: