All-dielectric near-field transforming structures to dielectric-less metasurfaces for high-gain antenna systems
thesisposted on 2022-03-29, 00:00 authored by Ali Lalbakhsh
This dissertation presents several novel Electromagnetic (EM) metsurfaces that are engineered to manipulate the electric near-field of any EM sources for various purposes. The approach combines the use of aperture antennas, such as Resonant Cavity Antennas (RCAs), or horn antennas as the EM radiators and the introduced metasurfaces to improve or alter the radiation patterns of the antennas. Initially, a conventional RCA is diagnosed with sub-optimal radiation patterns as a result of poor EM near-field characteristics. So, a novel all-dielectric structure is designed to completely remove this deficiency over a large frequency band, which has never been performed, opening new doors into other cutting-edge applications of this class of antenna, such as passive beamsteering. So, an all-dielectric structure named Near-Field Correcting Structure (NFCS) is designed, which is composed of circular correcting regions, determined by a numerical method in which time-average Poynting vector in conjunction with a phase gradient analysis is utilized to suggest the initial configuration of the NFCS. It is then completed using a customized particle swarm optimization algorithm implemented in MATLAB. The NFCS was fabricated and placed in a subwavelength distance from the RCA and measured. According to the predicted and measured results, the phase and magnitude distributions of the electric near-field of the antenna have been greatly improved over a large bandwidth of 40%, resulting in a high aperture efficiency of 70%. The antenna under NFCS loading has a peak measured directivity of 21.6 dB, a 3 dB directivity bandwidth of 41% and a 10 dB return loss bandwidth of 46%. Using the same concept of the near-field transformation, all-dielectric metasurfaces composed of near-field rotatable graded-dielectric plates are designed to realize an EM-wave beamsteering antenna. According to the numerical results, the beam can be scanned within a large conical region with an apex angle of 82.2o with a significantly less profile than the mechanically scanned reflector dishes. Unlike the all-dielectric structures mentioned above, a printed metasurface is designed to exhibit a negative transverse-reflection magnitude gradient and, at the same time, a progressive reflection phase gradient over frequency. The first phenomenon has already been realized using either multiple printed dielectric surfaces or a large single FSS, while the proposed metasurface is made of a compact single dielectric. The second phenomenon was also realized using costly fabrication techniques, which is not the case with the proposed low-cost metasurface. A prototype of the metasurface was fabricated and tested with a partially shielded cavity, creating an improved RCA, showing a very high aperture efficiency of 83% with a peak directivity 16.2 dB and a 3 dB directivity bandwidth of 22%. In all aforementioned metasurfaces, dielectric substrates are an integral part of the metasurface configurations, where the realization of the metasurfaces is not impossible without the dielectric substrates. However, in some less-specialized applications, the high cost of dielectric substrates can be an impediment to the applications of metasurfaces. Furthermore, dielectric-based metasurafces cannot directly be used in high-power applications, as they are prone to dielectric breakdown. To address this issue, several dielectric-less metasurfaces are presented in this thesis for a variety of applications, including single-frequency phase correction of RCAs, wideband spatial filtering, and wideband phase correction of shortened horn antennas. Unlike the existing Phase Correcting Structures (PCSs) of RCA, a hybrid topology of fully-metallic spatial phase shifters are developed to form an All-Metal PCS (AMPCS), resulting in an extremely lower prototyping cost as that of other state-of-the-art substrate-based PCSs. The APMCS was fabricated using laser technology and tested with a RCA to verify its predicted performance. Results show that the phase uniformity of the RCA aperture has been remarkably improved, resulting in 8.4 dB improvement in the peak gain of the antenna and improved sidelobe levels (SLLs). The antenna system including APMCS has a peak gain of 19.42 dB with a 1 dB gain bandwidth of around 6%. In order to achieve wideband performance using dielectric-less metasurfaces, a design methodology is presented in which the integrity of metasurfaces are ensured by metallic inductive grids, which have a wideband bandpass frequency response. Therefore, additional metallic resonators can be integrated into the grid, without any mechanical stability concerns. Based on this design mechanism, Orthogonal Dipole Resonator (ODRs) are integrated into the metallic grids to form a wideband bandpass metasurface filter with excellent performance. The metasurface filter is composed of multiple segregated metallic layers, where the selectivity of the filter can be improved by increasing the number of layers. The metasurface filter has the capability of harmonic suppression and has a fully adjustable wide passband of 31%. Extension of this method can be used to develop wideband phase correcting metasurfaces, which can be used to significantly enhance the near-field of shortened horn antennas, leading to a small horn antennas with plane wave. The horn antenna with the metasurface has a uniform phase distribution over a large frequency band of 25%, resulting a high aperture efficiency of 66% at 11.7 GHz. The antenna system has a measured peak directivity of 20.9 dB with a large frequency band from 9.70 GHz to 12.45 GHz -- abstract.