Planar Electromagnetic Bandgap Structures and Applications

Abstract

With the emergence of novel wireless technologies, the demands for larger bandwidth, high efficiency and high capacity RF and microwave devices have been growing tremendously. To satisfy the demand novel materials, new processing techniques and refinement of existing devices have been evolving. Using electromagnetic bandgap (EBG) is a new technology to improve the performances of existing RF active and passive devices. EBG structures (EBGSs) are periodic structures, which exhibit distinct passband and stopband characteristics. The passband can be used as phase shifters and slow wave structures. The stopband characteristics can be used to suppress surface waves in dielectric media. Due to these unique properties, EBGSs find potential applications in many active and passive RF and microwave devices including antennas, filters, amplifiers and oscillators. Recently, the dispersion properties in passband of EBGSs are used in the phased array antennas for wide angle beam steering. Among various EBG configurations, planar EBGSs become most popular due to their low profile, ease of fabrication and integration with monolithic microwave integrated circuits (MMICs). The thesis concerns the planar EBG structures in the forms of conventional circular and rectangular photonic bandgap structures (PBGSs) and defected ground structures (DGSs). Novel PBGSs in the form of non-uniform Binomial and Chebyshev distributions of unit PBG cells have been proposed. This novel PBGSs yield ripple free passband and wide stopband properties, which are very useful to suppress higher order harmonics in the bandpass filters. Next the chirped PBGSs are investigated that yield better passband return loss, low ripples and wider stopband than those for conventional PBGSs as well as non-uniform PBGSs mentioned earlier. The parametric study of dumbbell shaped DGSs has been conducted with the gap width and length and the size of dumbbell slots. Three novel designs are proposed: non-uniform dumbbell shaped DGSs with Chebyshev distribution, hybrid DGS and PBG and modified hybrid DGS and PBG configurations. The novel designs outperform the conventional DGSs reported in the literature. The DGSs yield perfect LPF responses with negligible passband ripples and extremely wide stopband. The modified hybrid DGS and PBG configuration yields dual stopbands that can be used as a dual stopband filter. The novel design are used in asymmetric coupled line bandpass filters, aperture coupled single band and dual-band patch antennas and finally in 4-element conventional and reconfigurable phased array antennas. For filters, PBGSs suppress second and third harmonics by about 40 dB, A comprehensive investigation of the number, filling factor and position of the PBGS under the filter has been conducted. The frequency response of the filter has been observed. This investigation is novel and illustrates the significance of PBGSs in suppression of higher order harmonics of the bandpass filters. It has been found that non-uniform distribution of PBGSs is more effective to suppress higher order harmonics than the conventional uniform PBGSs. DGSs are very effective for simultaneous suppression of both 2nd and 3rd harmonics due to their very wide stopband performances. This phenomenon of DGSs has been demonstrated in various designs of asymmetric coupled line bandpass filters. Finally, the passband phase properties of the PBG and DGS have been investigated. The comprehensive investigations of relative phase delays of PBGS and DGS assisted microstrip transmission lines have been performed. Such investigation is not found in the open literature. It is observed that DGSs yield much larger phase delay compared to a similar size PBG structure. Therefore, DGS assisted feed network can scan the beam over a wide angle. After thorough and satisfactory investigation of phase delays for PBGSs and DGSs, phased array theory is studied. The required phase delays for a 4-element phased array are calculated based on the element spacing and number of elements. A wide beam scanning up to approximately 60° is achieved with 0-8-16-24 DGS assisted corporate feed network for the 4-element patch antenna array. So far such wide angle scanning using DGS assisted beamforming network has not been reported in the open literature.