Planar array

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In telecommunications and radar, a planar array is an antenna in which all of the elements, both active and parasitic, are in one plane.

Note 1: A planar array provides a large aperture and may be used for directional beam control by varying the relative phase of each element.

Note 2: A planar array may be used with a reflecting screen behind the active plane.

Source: from Federal Standard 1037C and from MIL-STD-188

A planar array with a reflecing screen is related to a Radar absorber. Both are supposed not to reflect incoming radiation of the desired wavelength. Radar absorbers have the advantage that they can use magnetic materials to avoid reflections at the interface to air at least for some frequencies. For antennas to be broadband, the transition from air (vacuum) to the metal of the screen has to be a gradually one. Radar absorbers have the advantage that they do not have to collect the received energy. The antenna needs a 3D tree of twin-leads and chokes to connect a single cable to a large number of micro antennas. Micro dipole antennas are only resonant at a single high frequency, to be broadband the arms of adjoining antennas have to be connected.