Electromagnetic band gap resonator antennas: from narrowband to extremely wideband

This dissertation introduces several novel electromagnetic band gap (EBG) resonator antennas (ERAs) that are designed to have wide directivity bandwidths as well as significantly reduced footprints. ERAs are well known for their simple configuration and high directivity, as compared to other complex and bulky alternatives. Antennas of this type find numerous applications in modern communication systems including point-to-point links, sensor systems, and electronic warfare. Although these highly directive antennas are very attractive for their simplicity, their applications are increasingly limited due to their inherent narrowband behaviour and large footprint. This dissertation investigates several ERA designs, both numerically and experimentally, to achieve large bandwidths alongside high directivity and small footprint areas. Half power directivity bandwidths exceeding 50% with broadside directivity ranging between 15-20dBi were successfully achieved, thereby representing an improvement of nearly two orders of magnitude over the 1-3% directivity bandwidth of classical ERAs. Initially, a novel method is proposed to design single-feed high-gain wideband ERAs. Axial permittivity gradient (APG) was introduced to multi-layer 1-DEBG superstructures composed of unprinted dielectric slabs, and the thicknesses of each of these slabs were optimised to achieve a wideband defect mode using the Defect Cavity Model (DCM). By further investigating the 1-D scattering response of this superstructure, it was revealed that simultaneous use of defect cavity model (DCM) and superstructure reflection model (SRM) is imperative to predict wideband performance in ERAs, specially with bandwidths exceeding15%. Following this approach, a prototype ERA designed with a single feedand superstructure area as small as 1.5 × 1.5λ20was designed. It demonstratesa measured 3-dB directivity bandwidth of 22% at a peak gain of 18.2 dBi. Furtherstudying the truncation of this three-layered superstructure, an extremelycompact ERA was designed with a footprint of only 1.7λ20. To the best of theauthor’s knowledge, no other planar antenna with such a small footprint presentsa broadside gain greater than 15 dBi with a matched 3dB gain-bandwidth exceeding25%. This ERA has a measured average aperture efficiency of 90% withinthe 3dB directivity bandwidth. In contrast to the multi-layer 1-D superstructures having APGs, single dielectric layer superstrates having transverse permittivity gradient (TPG) are introduced and investigated to design wideband ERAs. A relationship between broadband directivity enhancement of TPG superstrates and their aperture-field phase uniformity is established to provide valuable insight into the underlying principle of operation. Prototype ERAs were fabricated and the measurements validated the concept. A measured 3dB directivity bandwidth of 52.9% is demonstrated with a measured directivity of 16.4 dBi for an ERA that has a very small total footprint area of 1.54λ20. This represents an increase of 90% over the previous best measured ERA directivity bandwidth of 28%. While designing these wideband ERAs, the finiteness of the ERA superstructures is found to be a vital contributor to the DBP as well as the overall antenna performance. To systematically quantify these effects, detailed case studies were conducted using a single layer as well as a two-layer dielectric superstructure with uniform permittivity in single-feed ERAs. It is found that even without any special treatment of the superstructure, the finiteness of the superstructure strongly influences the DBPs. In particular, intriguing cases are observed when the dielectric layers in the two-layer superstructure are truncated to different finite sizes.Conventionally, although smaller superstructures yield wider bandwidths with reduced directivity, the truncation of superstructure layers to different finite sizes yielded more than 65% enhancement in the DBP, experimentally, while resorting to the same physical footprint. The detailed empirical relationships between the peak directivity, bandwidth, and the finiteness of the superstructure layers provide valuable physical insight into the operation of superstructures that have non-uniform reflection and transmission profiles, similar to the TPG superstrate.