Doctor of Philosophy, The Ohio State University, 2023, Electrical and Computer Engineering

A complete procedure of dealing with frequency-selective surfaces (FSS) in electromagnetic is addressed in this dissertation, including automatic design and fast numerical simulation. The design of the FSS contains the design of periodic structure and the arrangement of the periodic structure on an arbitrary platform. For the design of periodic structure, an adaptive artificial neural network (ANN) is presented herein to meet the desired frequency response automatically. Afterward, the designed periodic structure is arranged on an arbitrary platform by some seed points to approximate the locations, and the generation of seed points uses some electrons reaching an electrostatic distribution. For the fast numerical simulations of FSS, a generalized transition condition (GTC) is presented herein to reduce computation resources of both memory computation time due to the complex geometry. In this method, GTC serves as a field relationship to represent the electromagnetic interaction effect from the original FSS. The proposed GTC is inspired by the generalized sheet transition condition (GSTC) but gets rid of the zero-thickness assumption to build a relationship between the electric field and magnetic field on both sides of the FSS. The process of the proposed numerical simulation method of FSS includes three steps. The first is to obtain the reflection and transmission coefficients by the numerical simulation of FSS as an infinite periodic structure with periodic boundary conditions (PBC). Secondly, a self-consistent scheme is adopted to compute the coefficient in GTC with a required order. Finally, the constructed GTC can be used to replace the FSS in a real model. The collaboration of GTC with both surface integral equation (SIE) method and finite element method (FEM) are addressed in this dissertation. Especially for FEM, GTC is modified to bypass the edge effects and results in a conformal mesh on both sides of the transition condition. The accuracy of the pr (open full item for complete abstract)

Committee: Jin-Fa Lee (Advisor); Kubilay Sertel (Committee Member); Balasubramaniam Shanker (Committee Member) Subjects: Electrical Engineering; Electromagnetics

Doctor of Philosophy, The Ohio State University, 2017, Electrical and Computer Engineering

This dissertation deals with novel ways to reconfigure the bandwidth of tightly-coupled arrays (TCAs). TCAs constitute a class of phased antenna arrays that demonstrate ultra-wide bandwidth, high gain, wide scanning, compact size and low fabrication cost. These attributes render them attractive for a wide range of applications, including multiple-input multiple-output (MIMO) systems, synthetic aperture radar (SAR) and software defined radio (SDR). However, TCAs, like all wideband systems, suffer from signal-to-interference-plus-noise ratio (SINR) degradation, reducing channel capacity and quality of communication. Existing spatial and digital filtering techniques fail to provide a comprehensive solution suppressing both noise and interference simultaneously. Therefore, bandwidth reconfiguration techniques, implemented in analog and at the RF frequency, are highly attractive. In this work, a low loss reconfiguration approach, using variable capacitors within a TCAs balun feed structure, is proposed. Specifically, a rejection notch, tunable both in center frequency and bandwidth, is created, rejecting noise and interference within the entire TCA scanning volume. The proposed reconfigurable array exhibits strong advantages over the use of stand-alone band rejection antennas or the alternative use of tunable band rejection filters, placed after the antenna. The proposed scheme is validated experimentally via fabrication and testing of reconfigurable balun prototypes, both in isolation and within an array environment. Digital MEMS capacitors were utilized for the practical implementation of the tunable band rejection. Measurement results demonstrate tunability with >2:1 frequency tuning range and rejection magnitude in excess of 30dB. The scheme can be expanded to multiple rejection notches, providing complete control over the bandwidth. An alternative way of reconfiguring the bandwidth of a TCA is also examined. This method incorporates a reconfigurable fre (open full item for complete abstract)

Committee: John Volakis (Advisor); Robert Burkholder (Committee Member); Asimina Kiourti (Committee Member) Subjects: Electrical Engineering

Doctor of Philosophy, The Ohio State University, 2011, Electrical and Computer Engineering

There is strong interest to combine many antenna functionalities within a single, wideband aperture. However, size restrictions and conformal installation requirements are major obstacles to this goal (in terms of gain and bandwidth). Of particular importance is bandwidth; which, as is well known, decreases when the antenna is placed closer to the ground plane. Hence, recent efforts on EBG and AMC ground planes were aimed at mitigating this deterioration for low-profile antennas. In this dissertation, we propose a new class of tightly coupled arrays (TCAs) which exhibit substantially broader bandwidth than a single patch antenna of the same size. The enhancement is due to the cancellation of the ground plane inductance by the capacitance of the TCA aperture. This concept of reactive impedance cancellation was motivated by the ultrawideband (UWB) current sheet array (CSA) introduced by Munk in 2003. We demonstrate that as broad as 7:1 UWB operation can be achieved for an aperture as thin as λ/17 at the lowest frequency. This is a 40% larger wideband performance and 35% thinner profile as compared to the CSA. Much of the dissertation's focus is on adapting the conformal TCA concept to small and very low-profile finite arrays. Three particular designs are presented. One is a 6x6 patch array occupying a λ/3 x λ/3 small aperture (mid-frequency is at 2.1 GHz). Remarkably, it is only λ/42 thick yet delivers 5.6% impedance bandwidth (|S11| < -10dB), 4.4dB realized gain (87% efficiency) and 23% gain bandwidth (3dB drop). The second finite TCA consists of 4x2 patches and occupies a λ/3.2 x λ/3.2 aperture on a λ/26 thick substrate (mid-frequency is at 2 GHz). This antenna delivers 17.3% impedance bandwidth, 4.8dB realized gain (95% efficiency) and 30% gain bandwidth. That is, more than twofold impedance bandwidth is delivered as compared to a single patch antenna of the same size on conventional or EBG substrate. The third array being considered consists of 3x2 patches occu (open full item for complete abstract)

Committee: John L. Volakis PhD (Advisor); Kubilay Sertel PhD (Advisor); Robert J. Burkholder PhD (Committee Member); Fernando L. Teixeira PhD (Committee Member) Subjects: Electrical Engineering; Electromagnetics

Doctor of Philosophy in Engineering, University of Toledo, 2013, College of Engineering

This dissertation proposes three novel bandpass filter structures to protect systems exposed to damaging levels of electromagnetic (EM) radiation from intentional and unintentional high-power microwave (HPM) sources. This is of interest because many commercial microwave communications and sensor systems are unprotected from high power levels. Novel technologies to harden front-end components must maintain existing system performance and cost. The proposed concepts all use low-cost printed circuit board (PCB) fabrication to create compact solutions that support high integration. The first proposed filter achieves size reduction of 46% using a technology that is suitable for low-loss, narrowband filters that can handle high power levels. This is accomplished by reducing a substrate-integrated waveguide (SIW) loaded evanescent-mode bandpass filter to a half-mode SIW (HMSIW) structure. Demonstrated third-order SIW and HMSIW filters have 1.7 GHz center frequency and 0.2 GHz bandwidth. Simulation and measurements of the filters utilizing combline resonators prove the underlying principles. The second proposed device combines a traditional microstrip bent hairpin filter with encapsulated gas plasma elements to create a filter-limiter: a novel narrowband filter with integral HPM limiter behavior. An equivalent circuit model is presented for the ac-coupled plasma-shell components used in this dissertation, and parameter values were extracted from measured results and EM simulation. The theory of operation of the proposed filter-limiter was experimentally validated and key predictions were demonstrated including two modes of operation in the on state: a constant output power mode and constant attenuation mode at high power. A third-order filter-limiter with center frequency of 870 MHz was demonstrated. It operates passively from incident microwave energy, and can be primed with an external voltage source to reduce both limiter turn-on threshold power and output power v (open full item for complete abstract)

Committee: Vijay Devabhaktuni Ph.D. (Advisor); Mansoor Alam Ph.D. (Committee Member); Mohammad Almalkawi Ph.D. (Committee Member); Matthew Franchetti Ph.D. (Committee Member); Daniel Georgiev Ph.D. (Committee Member); Telesphor Kamgaing Ph.D. (Committee Member); Roger King Ph.D. (Committee Member) Subjects: Electrical Engineering; Electromagnetics; Plasma Physics