Clutter suppression for thinned array with phase only nulling

An active array antenna, comprising:

• a first plurality N of elemental antennas;
• a plurality of T/R modules, each including at least an input port, an output port, and a phase control port, each of said T/R modules having its output port coupled to an associated one of said elemental antennas, for, in a transmitting mode of operation, receiving signals at said input port, and for producing amplified signals at said output port at a phase controlled by a phase control signal applied to said phase control port, the signals produced at said output ports of said T/R modules being of equal amplitude;
• antenna element support means, for physically supporting said plurality of elemental antennas in a circular aperture, each of said elemental antennas being located in said aperture on one of a plurality M of concentric rings of said elemental antennas, each of said elemental antennas on each of said rings being equidistant from adjacent ones of said elemental antennas on the corresponding ring, the number N.sub.n.sup..SIGMA. of said elemental antennas in each of said M rings being accordance with a solution of ##EQU26## where i=1, . . . , M-1;
• r.sub.n+1 is the radius of each ring other than the innermost ring, normalized to the radius of the aperture;
• J.sub.0 is a Bessel function of the first kind and the zero order;
• and ##EQU27## where a.sub.M is the radial distance of the outermost ring from the center of said circular aperture;
• phase control means coupled to said control ports of said T/R modules, for controlling the phase of each of said T/R modules in such a manner as to generate a phase distribution across said aperture which results in a directivity pattern including a main beam having an angular extent, and a plurality of sidelobes outside said angular extent of said main beam, said sidelobes having a predetermined sidelobe level;
• and
• phase perturbation means coupled to said phase control means, for perturbing said phase of said T/R modules in accordance with
• .PHI.=E' (E E').sup.-1 B
• where:
• .phi. is a matrix of the individual phase perturbations of the T/R modules;
• E and B are matrices derived from a solution of
• IC .PHI.=RC.sub.0
• RC .PHI.=-IC.sub.0
• and
• E' is a matrix transpose of matrix E, for thereby generating a null in said directivity pattern at a location .theta., .phi. outside said angular extent.