PhD Oral Exam - Muhammad Mustafa Tahseen, 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.
Reflectarray (RA) antennas are introduced as an alternative to the conventional parabolic reflectors and antenna arrays. The RA provides the advantages of both reflectors and arrays. The narrow bandwidth is one of the distinct disadvantages of RA. The RA bandwidth is primarily limited by two factors, the element bandwidth, and the spatial phase delay. As the RA size is fixed with a proper focal distance, the second factor is hard to be used in the RA bandwidth optimization. It is impossible to design a broadband RA using narrowband elements, so wideband elements are developed in a unit cell of a periodic structure to further provide wider overall antenna bandwidth. In the literature review, 10-15 % RA bandwidth has been observed, which is mainly because of the used narrowband elements. Several RA antennas operating in the Ka-Band are designed and fabricated to learn an understanding of the RA with wideband characteristics. Novel wideband elements for linear polarization (LP) and circular polarization (CP) are utilized. A bandwidth of 22 % in LP-RA antennas is achieved using the proposed wideband elements. Over 50 % gain, and 50 % axial ratio bandwidth, and 57-62 % maximum aperture efficiency are experimentally measured for the CP. The effect of the edge diffraction on the phase correction is also investigated. The initial study showed little effect, but the considered RA size is small to highlight this effect. The matching of the feed is deteriorated when placed in front of the reflector. Therefore, few methods are implemented to improve the antenna reflection coefficients. The experimental results show possible improvements. The proposed work is categorized into three Parts. Part A provides a comprehensive study of the wideband element designs for both LP- and CP-RA. Part B utilizes the wideband elements developed in Part A, in the RA environment and introduces the performance of the antenna. In part C, textile-reflectarray (TRA) is proposed using conductive thread and shielded fabric, which is the first to be considered as a new trend in the field of RA. The element analysis and measurements show promising results. However, when implemented in the RA, the shielded fabric does not perform as expected. This problem is resolved by introducing a dual-sided embroidered reflectarray antenna where a flexible frequency selective surface (FSS) embroidered at the textile material using a conductive thread is used at the back side of the radiating elements replacing the ground plane. The measured results of the first ever built flexible, portable textile-RA show very good antenna performance. An FSS based flexible portable and rollable circularly polarized TRA design is also presented using the wideband cross Bowtie elements. The radiating elements are embroidered using conductive thread.
The initial results exhibit good performance of the proposed CP TRA antenna.
For RA with small f/D, the spatial delay problem is the main reason for limiting the bandwidth even with wideband RA elements. This issue is solved by introducing a novel split aperture into panels that reduce the difference in the path length between the center and those away towards the RA edge. This method shows a significant improvement in the RA bandwidth.