A study of TMn1-mode circular microstrip antennas for GPS interference mitigation applications
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This thesis involves a study of TMn1 circular microstrip antennas in planar controlled reception pattern antenna (CRPA) configurations, to spatially filter interfering signals in global positioning system (GPS) receivers. The main goals of the study are CRPA performance improvement and size reduction. Conventionally, planar CRPA designs consist of multiple dominant-mode right-handed circularly-polarized (RHCP) antenna elements. Various configurations of arrays consisting of multimode and/or dual-polarized antenna elements are considered in this work, their radiation characteristics studied analytically, and their performance in the presence of interferers is evaluated by studying their steady-state adaptive response through many Monte Carlo simulations of the interference environment. In this study, the number of array ports, and thus the number of degrees of freedom (DoF) are kept fixed to keep the computational requirement of each adaptive array design fixed and thus put the emphasis on the effects of the physical characteristics of the array on its performance. Two cases of array sizes are considered, one with nine ports (eight DoF) and one with four ports (three DoF). In each case, the performances of the arrays are evaluated in the presence of both RHCP and randomly-polarized interferers. First, it is shown that for the case of the nine-port CRPA array, utilizing dual-polarized elements (could be single-mode or multimode) in general leads to improvement in the performance of the array in the presence of randomly-polarized interferers. Furthermore, it is shown that utilizing dual-mode and dual-polarized TM21 elements alongside RHCP TM11 elements in an array configuration, thus utilizing both polarization diversity and pattern diversity, leads to performance improvements for all interference scenarios. Specifically, it is shown that the performance of the proposed dual-polarized dual-mode TM11-TM21 array is as good as or better than the non-planar spherical arrays which are the state-of-the-art in terms of CRPA performance. The proposed dual-mode array therefore, is a planar alternative to the bulky spherical arrays. Next, the case of a smaller CRPA array with four ports is studied and it is demonstrated that a stand-alone dual-polarized dual-mode TM11-TM21 antenna element is suitable for a very compact CRPA design, as its nulling resolution is independent of the size of the antenna beyond a certain miniaturization point. This is in contrast with a conventional dominant-mode antenna array whose nulling resolution degrades as the inter-element spacing is decreased. In this case, the polarization and pattern diversity of the multimode TM11-TM21 antenna is exploited to design an ultra-compact CRPA antenna. The issue of mutual coupling in the multimode CRPA configurations is studied qualitatively using the concept of beam coupling factor, and it is shown that the coupling can be made very small within a multimode TMn1 element, as well as between multiple TM11-TM21 elements with about half-wavelength spacing. Finally, proof-of-concept of the proposed arrays are provided through full-wave simulation and measured fabrication results for three CRPAs: 1) The nine-port TM11-TM21 array at GPS L1 band. 2) The four-port TM11-TM21 antenna at GPS L1 band. 3) A dual-frequency version of the four-port TM11-TM21 antenna, operating at both GPS L1 and L2 bands.