ABSTRACT
The power density, in , at range away from the radar (assuming a lossless propagation medium) is
where is the peak transmitted power and is the surface area of a sphere of radius . Radar systems utilize directional antennas in order to increase the power density in a certain direction. Directional antennas are usually characterized by the antenna gain and the antenna effective aperture . They are related by
where is the radar operating wavelength. The relationship between the antenna’s effective aperture and the physical aperture is
where is referred to as the aperture efficiency, and good antennas require . In this book, unless otherwise noted, and are used interchangeably to refer to the antenna’s aperture, and will assume that antennas have the same gain in the transmitting and receiving modes. In practice, is widely accepted.
