Role of the Antenna

The basic role of the radar antenna is to provide a transducer between the free-space propagation and the guided-wave propagation of electromagnetic waves. The specific function of the antenna during transmission is to concentrate the radiated energy into a shaped directive beam which illuminates the targets in a desired direction. During reception the antenna collects the energy contained in the reflected target echo signals and delivers it to the receiver. Thus the radar antenna is used to fulfill reciprocal but related roles during its transmit and receive modes. In both of these modes or roles, its primary purpose is to accurately determine the angular direction of the target. For this purpose, a highly directive (narrow) beamwidth is needed, not only to achieve angular accuracy but also to resolve targets close to one another. This important feature of a radar antenna is expressed quantitatively in terms not only of the beamwidth but also of transmit gain and effective receiving aperture. These latter two parameters are proportional to one another and are directly related to the detection range and angular accuracy. Many radars are designed to operate at microwave frequencies, where narrow beamwidths can be achieved with antennas of moderate physical size.

The above functional description of radar antennas implies that a single antenna is used for both transmitting and receiving. Although this holds true for most radar systems,...