Shaped-Beam Circularly-Polarized Practical Antenna Array for Land Imaging SAR Systems

A planar array of M x N circularly polarized microstrip patch antennas is proposed for high resolution land-imaging applications using an airborne side-looking Synthetic Aperture Radar (SAR) systems. The microstrip patch is designed to produce either right-hand or left-hand circular polarization. The planar array is designed to produce a circularly polarized three-dimensional beam of cosecant-squared shape in the range direction and uniform shape in the azimuth direction. The mutual coupling between the adjacent patches in the proposed array is studied for the assessment of the final array design and is shown to have no bad effects on the resulting beam shape or the array performance. A computationally efficient Particle Swarm Optimization (PSO) algorithm is developed and applied to find the amplitude and phase distributions of the feeding voltages over the planar array elements required to produce the desired radiation patterns. This technique is achieved by developing a fast algorithm to extend the amplitudes and phases calculated for M-element and N-element linear arrays to be applied to the elements of a two-dimensional M x N planar array without the need to apply the PSO to calculate the amplitudes and phases for the entire elements of the planar array. A perfect three-dimensional pattern with the desired shapes in both azimuth and range directions is achieved. The input impedance, the axial ratio and the shapes of the radiation patterns produced by the proposed antenna array are investigated and shown to be satisfactory over the frequency band of 3.91 - 4.11 GHz.