Most of the radars have to cover an area in searching and/ or tracking mode. This means that antenna should cover maximum area speedily. A single antenna fails to track a large number of targets simultaneously and accurately. Such problem can be overcome if a moving beam can be produced by a stationary antenna. This is done with the help of array comprising of large numbers of individual radiators placed in a particular fashion. Beam steering can be achieved by introduction of variable phase differences in the individual antenna feeders.

All radar beams are generated by careful control over the phase of energy across the antenna's aperture and thus are called as phased arrays. Fig shows linear phased array, phase shifting devices are introduced into each connection between the combiner and each antenna element.

The basic principle of phased array is based on interference among electromagnetic waves in free space i.e, when electromagnetic energies from different radiators cover the same space at same time, they combine either constructively and sometime destructively. The combination is constructively means the patterns are aiding each other and if the combination is destructive means the patterns are opposing each other. The radiation patterns is selected by changing the phase delay introduced by each phase shifter.

Broadly there are two different types of phased array possible.

In the first type, a single relatively high-power output device supplies power to transmit. The feeding is done through high level power dividers and high-power phase shifters. The phase shifters are usually ferrites or PIN diodes. The total transmit power to each antenna and its phase are determined by a suitable combination of adjustable attenuators and time delays. The amount of loss is attenuators and phase shift in each element is controlled.

The second type of phase array system uses approximately as many low power variable output devices as radiating. elements and the phase relationship among the output signal is controlled by phase shifters.