PhD Oral Exam - Oludayo Sokunbi, 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

Millimeter wave (mm-wave) antenna arrays with high gain are explored to meet the user requirements of high throughput, with a very close distance between the antenna elements, which enables miniaturization. However, high-gain antenna array design at mm-wave bands has been a growing concern among antenna engineers because of the inevitable, undesired mutual coupling between the antenna elements. Therefore, different methods of improving isolation and designing high-gain antenna arrays are introduced in this work.

First, various decoupling techniques are explored to increase isolation between patch and ME-dipole MIMO antenna arrays at 30 and 60 GHz. The performance of the MIMO antennas is evaluated through simulations. Then, a novel customized π-shaped split-ring resonator (SRR) metasurface is designed. The SRR is arranged in two configurations to decouple 1×2 and 1×4 millimeter-wave (mm-wave) magneto-electric dipole (ME-dipole) in the H-plane with 1.6 mm (0.32λ at 60 GHz) inter-element spacing. The metamaterial unit cell is designed to inhibit the propagation of coupled surface waves within the bandwidth of interest. The antenna performances are verified experimentally.

Third, an efficient method is used to design a large dual-polarized finite planar array and its corporate feed network. The procedure is verified by an 8×8 and16×16 array of metallic ME dipoles fed by a network of Microstrip Ridge Gap Waveguide (MRGW). The ME-dipole used here is purely metallic to avoid using dielectric substrate and vias. Thus, it eliminates dielectric losses, surface waves, and the complexity of using vias. The procedure is based on designing the corporate feeding network by replacing the elements’ ports with each element’s corresponding effective input impedance that accounts for the mutual coupling between the antenna elements. In addition, the array parameters’ far-field characteristics, such as the directivity, gain, and radiation patterns, are predicted using pattern multiplication, including the mutual coupling effects. The results are verified with the full-wave numerical solution. The procedure requires limited resources and speeds up the design cycle. The MRGW helps the feeding network lines tighter than the ridge gap technology. Thus, allowing the distance between the radiating elements to become smaller than one wavelength to avoid grating lobes. More importantly, the isolation between the two polarizations is confirmed to be more than 40 dB. For an array multiple of 4×4, the feeding network layout introduces a gap larger between the 4×4 arrays than the distance between neighbouring elements. Thus, parasitic elements are added, increasing the 8×8 array to a10×10 array, improving the array aperture distribution, and increasing the gain.

A 10×10 array is fabricated and measured. The array bandwidth is similar to the element bandwidth. The simulated results are confirmed with measurements. The dual-layer feed network is invented to solve the problem of the gap between the sub-arrays so that each polarization is on a different layer. The performance of the single and dual layers is also compared.

Finally, the same technique is used to design a dual circularly polarized metallic magneto-electric dipole with excellent radiation characteristics. Just like the linear polarization above, the layout of the feeding network also introduces a larger gap between the 4×4 arrays. Thus, parasitic elements are added, increasing the 8×8 array to a 9×9 array and improving the gain. Different scenarios are explored, such as element failure, specific element excitation, and MIMO configuration. The antenna is also fabricated to confirm the simulated results. The single-layer, single-slot design is also introduced to save the cost of fabricating dual slots. The two-layer feeding network is introduced to solve the problem of extra gaps introduced by the feeding network. The performances are compared with the single-layer excitation.