Design of Quasi-periodic and Aperiodic Array Lattices to Improve Array Antenna Performance

Abstract

The thesis addresses one of the possible solutions to the grating lobe occurrence in the beam steerable periodic arrays for large angular beam scans. The controlled aperiodicity has been introduced to the periodic array to achieve the same and the object is set to design the beam steerable array antenna offering improvement in the peak SLL performance with beam scan and reducing the number of array elements. The designs of such aperiodic and quasi-periodic array antennas have been carried out using the innovative strip projection (SP) based method. The strip projection method uses the area of rotated higher dimensional lattice and projects it to lower dimensions to generate an aperiodic array. The designs of aperiodic linear and planar arrays have been carried out to achieve �30� conical beam scan range with peak SLL <-10dB over -90� to 90o angular range. The novelty of the proposed SP method is that the number of optimization variables is fixed and independent of the size of the aperiodic array. The reported techniques to generate the aperiodic arrays lack in this aspect. The proposed method facilitates a significant reduction of the design efforts, especially in the case of the larger beam-steerable arrays. The proposed method is relatively straightforward to implement compared to the reported algorithms.The performance of the aperiodic linear array antenna has been compared with the aperiodic arrays designed using evolutionary optimization algorithms, namely, genetic algorithm (GA), particle swarm optimization (PSO) and Jaya algorithms and it is found that the proposed design method is comparatively more efficient and faster. The aperiodic array lattice is also populated with X-band electromagnetically coupled patch antenna integrated with a phase shifter and simulated. The aperiodic patch array antenna has been fabricated and characterized in the anechoic chamber. The comparison of the measured and simulated results is presented. In measurement, a significant improvement of 5.72dB in peak SLL is achieved at �30� beam scan angle.The design of an aperiodic planar array antenna has been carried out for a 15 x 15? aperture size. The optimized array has 21.9% less elements than the conventional periodic rectangular lattice. The Pinwheel based aperiodic array lattice has also been designed for the same beam scan requirement and presented for comparison. It is observed that the peak SLL performance is maintained at <-11.63dB and <-12.70dB over the 0�-30� beam scan range by the proposed aperiodic array and Pinwheel based array, respectively. Moreover, both types of lattice have been populated with S-band cubic-shaped dielectric resonator (DR) antenna element and their simulations have been carried out using a 3D electromagnetic solver. For the quantification of the aperiodicity in the structure, position standard deviation (O) is also defined and computed.The projection concept is generalized and implemented to design a quasi-periodic beam steering array antenna by projecting the vertices of co-centric polyhedrons on the 2D aperture plane. The modelling and design of quasi-periodic array lattice are carried out by projecting the vertices of co-centric polyhedrons, namely dodecahedrons and icosahedrons, on the aperture plane. The angular orientation of the polyhedrons is optimized to achieve a 4.2dB peak SLL improvement for a �30� beam scan. The optimized array lattice is populated with cubic shaped DR based elements and integrated with a Voronoi based metallic fence and decoupling network (DCN) for mutual coupling improvement between the elements. The polyhedron projection based concept has been extended to design interferometric arrays for radio astronomy. The stereographic projection has been used for the projection of vertices of the rotated polyhedron and forms the aperiodic array, whose performance is subsequently evaluated for radio interferometric imaging. The necessary test framework for imaging of 1" sample image using the designed array lattice has been developed in Matlab and the array lattice has been optimized to achieve the maximum fidelity index (FI). The various cases of the aperiodic array with various combinations of polyhedrons have been evaluated and compared with the Giant Metrewave Radio Telescope (GMRT) array. The aperiodic array generated by the three co-centric polyhedrons, i.c., dodecahedron, octahedron and tetrahedron, is proved to have a better fidelity index (FI) over the various declinations (8). In addition, the projection based aperiodic array antenna has also been evaluated for minimum variance distortionless response (MVDR) type of beamformer, which is a widely used technique in various fields like communication, radar, acoustics, and sonar. Matlab codes are developed to implement DoA estimation using the MVDR technique and applied to the conventional periodic and proposed aperiodic linear arrays. It is shown that aperiodicity in the element position has eliminated the unwanted lobes in the detection range.