ANTENNA, APERTURE

See Aperture antennas.

ANTENNA ARRAYS

Consisting of two to thousands of antenna elements, an ar­ray antenna presents the ultimate in flexible antenna pat­tern control. This capability includes electronic scanning, planar or conformable apertures, the ability to transmit or receive multiple shaped patterns, and adaptive control for jammers, clutter, and multipath suppression and for the reduction of cosite interference. Arrays benefit from tech­nological advances, and they are the key technology drivers for solid state T/R (transmit/receive) modules, advanced signal processing, and photonic technology.

Antenna arrays are often simply called “phased” arrays, a term that refers to the progressive phase shift intro­duced to scan the beam. We will use this term through­out this section, although the beam can be scanned by ei­ther time delay devices or phase shifters, depending on the required system’s bandwidth. The distinction will be ad­dressed later.

The basic principle behind the operation of the phased array, as illustrated in Fig. 1, is that the RF power be di­vided among a number of elements by a power divider, with each element signal shifted appropriately in phase or in time. Relative time delay is portrayed in the sketch by cir­cular phase fronts emanating from each element of the ar­ray, with signals either radiated at the same time [Fig. 1(a)] or delayed in time by an increasing amount from left to right so that the rightmost signal radiates last. The figure shows how radiation from each element adds in space so as to create an outgoing wave with an appropriate scan angle. Although not shown in the figure, the array power divider usually provides equal line lengths from the source to each element because this leads to optimum system bandwidth.

The power divider can also be used to control the signal amplitude at each element. This unequal power division is called amplitude “tapering,” and it provides for sidelobe control. At broadside, with element spacing chosen prop­erly, the array directivity is that of the broadside aperture,

ANTENNA, APERTURE

where A is the aperture area and ea is the aperture ef­ficiency, which depends on the array’s taper design. The scanned beam of Fig. 1(b) will have reduced directivity and (usually) additional losses that further reduce the array gain.

Updated: 20.02.2014 — 06:32