Synthesis of Conformal Antenna Arrays on Polygonal Cross-Sectional Cylindrical Conductors for 360 Degree Azimuth Coverage Applications

Abstract

Antenna arrays are one of the most important part of any RF communications system nowadays. Ranging from military to communication applications, the versatility they provide in selectively enhancing and rejecting signals from various directions and sources by electronically steering the radiation pattern beam, is a key factor of their widespread use. Such electronically steerable arrays form a super-set of the well-known phased arrays, where phase-shifters or devices have be used to provide phase-difference across array elements. In recent times, conformal arrays which can adapt or conform to the shape of the mounting body (aircraft wing, tank surface, etc), have been of particular interest. Such arrays have found widespread applications in the present day radar, MIMO and 5G technology as well. Pattern synthesis for conformal phased arrays has been widely researched, with commonly used numerical methods such as moment method, fi nite element method, etc. The numerical computing methods are limited by array size, and computation speed is very slow. Furthermore, another motivation for the present work is to nd an excitation similar to the Chebyshev exci- tation for similar arrays. Since its introduction, the Chebyshev excitation has been thoroughly explored in various aspects relating to practical utility such as scanning limits, HPBW and SLL behaviour vs number of elements, etc alongwith work along the lines of reducing the disadvan- tages of the same, leading to the Taylor's distribution. Hence, new faster synthesis methods must be developed that can handle the requirements of conformal antennas and provide an excitation similar to the Chebyshev excitation for conformal arrays. In order to address and resolve issues highlighted above, we have analyzed and synthesized arrays of directive antenna elements on wedge-shaped conformal conductors and their extension to form polygonal cross-sectional cylinders for 360 scanning applications, which is at the core of present work. In the 1st part of this work, important issues with scanning arrays of directive elements have been dealt with in detail. The broadside case for scanning array of directive elements has been explored thoroughly in literature, therefore, the present work contains the analysis and synthesis of equal-sidelobe excitation for the scanning array. The excitation is optimum in the same sense as is the Chebyshev excitation for isotropic elements with respect to the FNBW and SLL and is different in terms of excitation values. A novel procedure based on certain aspects of the array factor expression has been proposed in order to take care of the above aspects and issues, namely beam direction skew and sidelobe envelope distortion. This work forms an important basis for the primary problem as several key observations and evaluations are at the foundation for certain extrapolations in the following work. Next, we deal with the analysis and synthesis of arrays of such directive elements, in partic- ular, microstrip patch antenna elements, on wedge-shaped conductors. The synthesis methods, for beam direction pointing to both apex and away from apex scan angles, has been discussed in detail. Furthermore, the variation of the radiation characteristics of array pattern with respect to bend angle, scan angle, number of elements, SLL, element pattern, etc have been discussed in detail. Certain observations, in particular the unchanged HPBW of an array on a wedge-shaped conductor, have been highlighted. In further sections, utilizing the foregoing fact about HPBW, a comparison of various geometries of a polygonal cross-section cylinder have been given with respect to radiation characteristics such as SLL, HPBW, etc. Lastly, a size reduction scheme has also been proposed for the speci fic case of fixed square cross-sectional geometry in order to obtain the same overall HPBW, within certain limits, characteristics for complete 360 azimuth scanning applications.