PhD Oral Exam - Syed Muhammad Sifat, Electrical and Computer Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Wireless and satellite communication systems demand a high data rate, which necessitates operating at Millimeter-wave bands. Thus, new efficient antennas and microwave devices are developed, which play an essential role in the design of an efficient frontend future communication system. Many challenges exist related to the present material properties and conventional challenges related to fabrication. Most printed circuit board (PCB) based guiding systems suffer from high dielectric and radiation losses, support surface waves, and excite cavity modes. In addition, copper cladding surface roughness introduces losses and affects the substrate's equivalent dielectric constant. Surface roughness would introduce invisible air gaps between two conducting surfaces creating a parallel plate (loosely referred to as PEC-PEC) waveguide supporting substantial signal leakage and losses due to the surface impedance. The newly developed printed gap waveguide technology would resolve such a problem by changing one of the plates with a magnetic conductor surface, thus, suppressing the leakage. This will be loosely referred to as PEC-PMC. As the magnetic conductor physically does not exist, an engineered artificial magnetic conductor (AMC) surface is realized by a periodic structure that has to be designed for the required frequency band. The Dissertation aims to highlight some of these issues and solve some electromagnetic problems. In addition, a problem related to the limitation of the commercial ferrite materials that limit the bandwidth and possibly its application at mm-wave frequencies is tackled by building in-house new ferrite material capable of overcoming such problems. In-house disks of ferrite with high-magnetization made of nickel-zinc are presented in terms of their properties and application in the circulator design at mm-wave frequencies, which is considered a significant contribution of this Dissertation.

The dissertation comprises three parts. The first part presents a design of an integrated multilayer horn antenna providing wide bandwidth and high gain. For the first time, a solution is provided to control the leakages due to the surface roughness from imperfect copper cladding using the PEC-AMC configuration of the horn radiator that is realized by a multilayer of PEC-AMC. In each layer of the horn, the opening is surrounded by periodic cells to suppress leakage and surface waves. In addition, the upper surface surrounding the horn's opening is covered by EBG mushroom cells to act as a soft surface that suppresses the surface currents around the horn aperture and reduces edge diffraction, improving the radiation characteristics of the horn. The proposed horn antenna achieved 11 dBi gain and 20.5 % impedance bandwidth, with better than 26.5 dB co- to cross-polarization in both E- and H-planes within 90^∘.

Most existing PCB-based circulators encounter massive dielectric losses, leading to spurious radiation and surface waves, degrading the system's overall performance. In the second part, this problem is resolved using printed gap waveguide technology that provides a packaged, low-loss, planar, cost-effective solution. The printed gap waveguide circulator provides 28.5% impedance bandwidth with better than 10 dB return loss and 12 dB isolation level throughout the operating frequency from 28.5-38 GHz. The first and second parts are integrated to form a full-duplex antenna system on the same layer with the proper matching between their ports. These studies highlight the advantages of using the gap waveguide technology, which could be a step towards standardizing this technology.

The final part of the work focuses on fabricating high-magnetization ferrite powder, which was baked at a different temperature to verify the structural and magnetic properties. Later, the manufactured powder is pressed with a hydraulic machine to form a disk shape that can be used in the mm-wave spectrum. The microwave industry's highest available saturation magnetization is around 5300 (G). In this work, we have fabricated different ferrite disks with Nickel-zinc ferrite (NZF) composition utilizing different heat treatment atmospheres to heat the powders. We have obtained a magnetization of 9500 (G), 1.8 times higher than the magnetization in microwave and space industries. This will open a new doorway for the microwave and space industry supporting various future applications in the mm-wave spectrum. From the industrial point of view, only the standard ferrite diameter and thicknesses are available in the market, and a custom-made disk requires much time and is expensive. A simple technique is described to fabricate the ferrite disk. Later, the same ferrite disk is deployed with a circulator operating in the mm-wave range with excellent measured isolation and return loss below -20 dB at the center frequency of 28 GHz.