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

Many high performance wireless applications continue to be integrated onto increasingly small platforms, such as satellites, UAVs, and handheld devices. Lowprofile and ultra-wideband antenna arrays have emerged as a potential solution, by allowing many disparate functions to be consolidated into a shared, multi-functional aperture. Simultaneously, the demand for high data rate communications has driven these applications to higher frequencies, with many now exploring the use of the millimeter-wave spectrum. However, existing UWB arrays often utilize complex feed structures which cannot scale to these frequencies. The development of wideband millimeter-wave arrays compatible with low-cost commercial fabrication processes is critical to enabling these small and highly connected platforms. Tightly Coupled Arrays are one family of low-profile and wideband arrays which have demonstrated superior bandwidth and wide scanning capability. However, the feed design of these arrays is limited to operation below 5 GHz, and suffers from reduced efficiency when scanning. In this work, the feed is modified to improve efficiency by eliminating a Wilkinson power divider, and mitigating the resultant cavity resonances with the application of shorting pins. Likewise, strenuous fabrication requirements are relaxed, allowing fabrication at higher frequencies. This effort is approached initially through the intermediate frequencies in the X-, Ku- and Ka-bands, and is demonstrated to allow the new design to scale up 49 GHz. An 8x8 prototype operating over 3.5–18.5 GHz is fabricated and measured to validate the design. Infinite array simulations show VSWR < 2 across this band at broadside, with scanning to ±45deg in the H-plane (VSWR < 2.6) and as far 70deg in E-plane (VSWR < 2). At millimeter-wave frequencies, planar co-fabrication of the entire array is critical to achieving repeatable fabrication, by eliminating the need for complex assembly at such small scales. Simultaneously, (open full item for complete abstract)

Committee: John Volakis (Advisor); Robert Burkholder (Committee Member); Kubilay Sertel (Committee Member) Subjects: Electrical Engineering; Electromagnetics; Electromagnetism

Master of Science in Electrical Engineering (MSEE), Wright State University, 2019, Electrical Engineering

This study focuses on the use of microstrip antenna technology for designing an ultra-wideband antenna to meet low size, weight and power requirements. Based on the recent literature for such antennas, a quasi-log periodic microstrip antenna array is designed to operate from 8 to 40 GHz (radar bands X, Ku, K and Ka). The array consists of 33 co-linear, inset-fed, square patches on a Roger's Duroid substrate, and is modeled using the Advanced Design System software from Keysight. The simulated results show the antenna has pass-band gains greater than 5 dB, a half-power beamwidth of 30 degrees, and linear polarization with a broadside radiation pattern. In addition, the fractional voltage standing wave ratio is less than 1.8 for 18 GHz of the pass-band, and the antenna has an efficiency greater than 60 percent over the entire pass band.

Committee: Michael A. Saville Ph.D., P.E. (Advisor); Yan Zhuang Ph.D. (Committee Member); Saiyu Ren Ph.D. (Committee Member); Josh Ash Ph.D. (Committee Member) Subjects: Electrical Engineering; Electromagnetics; Technology

Doctor of Philosophy, The Ohio State University, 2015, Electrical and Computer Engineering

Ultra-wideband (UWB) antennas and arrays are essential for high data rate communications and for addressing spectrum congestion. Tightly coupled dipole arrays (TCDAs) are of particular interest due to their low-profile, bandwidth and scanning range. But existing UWB (>3:1 bandwidth) arrays still suffer from limited scanning, particularly at angles beyond 45° from broadside. Almost all previous wideband TCDAs have employed dielectric layers above the antenna aperture to improve scanning while maintaining impedance bandwidth. But even so, these UWB arrays have been limited to no more than 60° away from broadside. In this work, we propose to replace the dielectric superstrate with frequency selective surfaces (FSS). In effect, the FSS is used to create an effective dielectric layer placed over the antenna array. FSS also enables anisotropic responses and more design freedom than conventional isotropic dielectric substrates. Another important aspect of the FSS is its ease of fabrication and low weight, both critical for mobile platforms (e.g. unmanned air vehicles), especially at lower microwave frequencies. Specifically, it can be fabricated using standard printed circuit technology and integrated on a single board with active radiating elements and feed lines. In addition to the FSS superstrate, a modified version of the stripline-based folded Marchand balun is presented. As usual the balun serves to match the 50-Ohm coaxial cable to the high input impedance (~200-Ohm) at the terminals of array elements. Doing so, earlier Wilkinson power dividers, which degrade efficiency during E-plane scanning, are eliminated. To verify the proposed array concept, 12x12 TCDA prototype was fabricated using the modified balun and the new FSS superstrate layer. The design and experimental data showed an impedance bandwidth of 6.1:1 with VSWR<3.2. The latter VSWR was achieved even when scanning down to ±60° in the H-plane, ±70° in the D-plane and ±75° in the E-plane. All array comp (open full item for complete abstract)

Committee: John L. Volakis Prof (Advisor); Nima Ghalichechian Dr (Committee Member); Chi-Chih Chen Dr (Committee Member); Fernando L. Teixeira Prof (Committee Member) Subjects: Electrical Engineering; Electromagnetics

Doctor of Philosophy, The Ohio State University, 2015, Electrical and Computer Engineering

This dissertations focuses on the development of a novel Ultra-Wideband (UWB) circularly polarized dielectric rod antenna (CPDRA) which yields a constant gain, pattern, and phase center. These properties are important in many applications. Within radar systems a constant phase center is desirable to avoid errors within downrange and crossrange measurements. In a reflector antenna the illumination, spillover, and phase efficiencies will remain the same over an ultra-wideband. Lastly, near field probes require smooth amplitude and phase patterns over frequency to avoid errors during the calibration process of the antenna under test. In this dissertation a novel CP feeding network has been developed for an ultra-wideband dielectric rod antenna. Circularly-polarized antennas have a major advantage over its linearly-polarized counterpart in that the polarization mismatch loss caused by misalignment between the polarizations of the incident fields and antenna can be avoided. This is important in satellite communications and broadcasts where signal propagation through the ionosphere can experience Faraday Rotation. A circularly polarized antenna is also helpful in mobile radar and communication systems where the receiving antenna's orientation is not fixed. Previous research on UWB dielectric rod antenna designs has focused on Dual linear feeds. Each polarization within the dual linear feed is excited by a pair of linear launcher arms fed with a 00-1800 hybrid balun. The proposed CPDRA design does not require the 00-1800 hybrid baluns or 00-900 hybrid for achieving CP operation. These hybrids will increase the antenna's size, weight, cost, and reduce operational bandwidth. A design technique has been developed for an UWB multilayer dielectric waveguide used in a CPDRA antenna. This design technique uses near-field Electric field data from inside the waveguide, in conjunction with a genetic algorithm optimization to yield a wideband waveguide with a near fie (open full item for complete abstract)

Committee: Chi-Chih Chen (Advisor); Fernando Teixeira (Committee Member); Patrick Roblin (Committee Member) Subjects: Electrical Engineering; Electromagnetics

Doctor of Philosophy, The Ohio State University, 2012, Electrical and Computer Engineering

There is much interest in developing body-centric wireless communication systems (BWCS) for mobile health care systems. However, the realization of a BWCS is challenging due to the body's interference with the antenna's operation. More specifically, body-worn antennas suffer from impedance detuning, pattern deformation, and gain reduction caused by the body. Therefore, it is important to consider these effects in evaluating body-worn antennas. In this regard, a diversity technique is proposed to improve body-worn antenna performance. More specifically, a channel decomposition method (CDM) is proposed and used to evaluate body-worn antenna systems. The CDM significantly reduces computation time when evaluate body-worn antennas and is applicable to various surrounding environments without recalculation of the more complex interaction. A second contribution of this dissertation is design of a diversity systems which automatically determines the minimum number of antennas while maximizing performance. This approach is employed to design body-worn antenna diversity systems for given communication scenarios. The results obtained via this process demonstrated that this simple method can substantially reduced computation time in designing body-worn antenna diversity system. As a demonstration of the proposed methodology, a vest-mounted UHF body-worn antenna diversity system (BWADS) is developed using 4 light-weight antennas. The proposed BWADS is transparent and unobtrusive to the users but provides performance superior to commercial antennas. A variety of tests were performed to validate the proposed BWADS. It was found that the proposed BWADS provided 7 dB (outdoor) to 16.5 dB (indoor) of higher gain as compared to commercial antennas. The dissertation concludes by proposing other applications of the developed body-worn antennas and design methods.

Committee: John Volakis PhD (Committee Chair); Chi-Chih Chen PhD (Advisor); Fernando Teixeria PhD (Committee Member); Dimitris Psychoudakis PhD (Committee Member) Subjects: Electrical Engineering; Electromagnetics

Doctor of Philosophy, The Ohio State University, 2010, Electrical and Computer Engineering

Emerging broadband applications with market pressures for miniaturized communication devices have encouraged the use of electrically small antennas (ESA) and highly integrated RF circuitry for high volume low cost mobile devices. This research work focuses on developing a novel scheme to design wideband electrical small antennas that incorporates active and passive loading as well as passive matching networks. Several antennas designed using the proposed design technique and built and measured to assess their performance and to validate the design methodology. Previously, the theory of Characteristic Modes (CM) has been used mostly for antennas analysis. However; in this chapter a design procedure is proposed for designing wide band (both the input impedance bandwidth and the far field pattern bandwidth) electrically small to mid size antennas using the CM in conjunction with the theory of matching networks developed by Carlin. In order to increase the antenna gain, the antenna input impedance mismatch loss needs to be minimized by carefully exciting the antenna either at one port or at multiple ports and/or load the antenna at different ports along the antenna body such that the Q factor in the desired frequency range is suitable for wideband matching network design. The excitation (feeding structure), the loading of the antenna and/or even small modifications to the antenna structure can be modeled and understood by studying the eigenvalues and their corresponding eigencurrents obtained from the CM of the antenna structure. A brief discussion of the theory of Characteristic Modes (CM) will be presented and reviewed before the proposed design scheme is introduced. The design method will be used to demonstrate CM applications to widen the frequency bandwidth of the input impedance of an electrically small Vee shape Antenna and to obtain vertically polarized Omni-directional patterns for such antenna over a wide bandwidth. A loading technique based on the CM to eith (open full item for complete abstract)

Committee: Roberto G. Rojas PhD (Advisor); Garbacz Robert PhD (Committee Member); Teixeira Fernando PhD (Committee Member) Subjects: Electrical Engineering; Engineering; Experiments

Doctor of Philosophy, The Ohio State University, 2019, Electrical and Computer Engineering

Utility of wireless connectivity has been steadily increasing as broadband internet becomes widely available and having low-cost technology leads to more devices built with Wi-Fi capabilities and sensors. As the traditional radio-frequency (RF) bands (sub 3 GHz) become congested, the mmW band offering vast amount of spectrum, is poised to be the backbone of 5G wireless networks. Particularly, thanks to much smaller wavelengths, antenna-integrated transceivers are viable solutions for the future 5G wireless networks. However, key challenges still remain for on-chip implementation of efficient radiators at such high frequencies. Namely, poor antenna bandwidths, severely low radiation efficiencies, as well as laborious and expensive antenna-transceiver integration (wire bonds, flip-chip, ball grid arrays, etc.) limit the utility of truly-integrated on-chip antennas. To overcome these prevailing obstacles we present an ultra-wideband (UWB), low-profile, high efficiency, tightly-coupled array topology which is adopted from RF-frequency realizations and modified as a multilayered structure suitable for standard micro-fabrication process. Through this work, we show that on-chip radiation efficiency is well above 60% over the entire impedance bandwidth. The proposed array exhibits wideband performance, covering 35-75 GHz, achieving an unprecedented coverage that spans most of the bands allocated for mobile communications. Utilization of low-loss materials in such designs can address the substrate coupling issues and improve the radiation efficiency. Moreover, the structural support and packaging materials that exhibit low loss are indispensable for cost-effective realization of integrated high frequency systems. To effectively address these requirements, polymers are a natural, low-cost choice for structural support and packaging of microchips due to their favorable chemical, thermal, and mechanical properties. However, many polymers have not been studied for mmW and TH (open full item for complete abstract)

Committee: Kubilay Sertel (Advisor); Niru Nahar (Committee Member); Fernando Teixeira (Committee Member) Subjects: Electrical Engineering; Electromagnetics; Electromagnetism

Master of Science, The Ohio State University, 2017, Electrical and Computer Engineering

Differential RF front-end components provide greater immunity to ground noise and distortion by suppressing external interference. Recent advancements in differential RF front-end components offer high dynamic range, high input linearity, and low noise in the transceiver chain. However, differential phased arrays are not inherently ultra-wideband (UWB). To create a wideband differentially fed array, a novel approach is introduced by building on the well-established UWB Tightly Coupled Dipole Array (TCDA). This differential TCDA (D-TCDA) is proposed for S-Ku band (3-18 GHz) communications with emphasis on dual-linear polarization and wide-angle scanning. The array achieves 6:1 bandwidth for VSWR < 3 at broadside with scanning down to 45° from boresight in the E and H planes. It also exhibits very high polarization purity of 55 dB and port to port isolation exceeding 60 dB. Array simulations are verified with measured results of a similar 8×8 prototype.

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

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

Master of Science, University of Akron, 2017, Electrical Engineering

Radio frequency (RF) antenna array beamforming based on electronically steerable wideband phased-array apertures find applications in communications, radar, imaging and radio astronomy. High-bandwidth requirements for wideband RF applications necessitate hundreds of MHz or GHz frame-rates for the digital array processor. Systolic array architectures are often employed in multi-dimensional (MD) signal processing for linear and rectangular antenna arrays. Thus, this research used a FPGA hardware platform, the ROACH-2, which is equipped with a Xilinx Virtex-6 SX475T FPGA chip, and which is widely used in the field of radio astronomy. The research concentrated on the prospects of implementation of systolic array based MD beamformers on the ROACH-2, and on methods of extending the operating frequency to GHz range by using polyphase structures. The proposed systolic array architectures employ a differential form 2-D IIR frequency planar beam filter structure which is low in hardware utilization. The study highlights techniques that can be used to overcome the limitations of the ROACH-2 signal processing platform to achieve high operating frequencies.

Committee: Arjuna Madanayake (Advisor); Subramaniya Hariharan (Committee Member); Joan Carletta (Committee Member) Subjects: Communication; Electrical Engineering; Engineering

Doctor of Philosophy, University of Toledo, 2015, Electrical Engineering

The ever-growing demand toward designing microwave front-end components with enhanced access to the radio spectrum (e.g., multi-/wideband functionality) and improved physical features (e.g., miniaturized circuitry, ease and cost of fabrication) is becoming more paramount than ever before. This dissertation proposes new design methodologies, simulations, and experimental validations of passive front-ends (i.e., antennas, couplers, dividers) at microwave frequencies. The presented design concepts optimize both electrical and physical characteristics without degrading the intended performance. The developed designs are essential to the upcoming wireless technologies. The first proposed component is a compact ultra-wideband (UWB) Wilkinson power divider (WPD). The design procedure is accomplished by replacing the uniform transmission lines in each arm of the conventional single-frequency divider with impedance-varying profiles governed by a truncated Fourier series. While such non-uniform transmission lines (NTLs) are obtained through the even-mode analysis, three isolation resistors are optimized in the odd-mode circuit to achieve proper isolation and output ports matching over the frequency range of interest. The proposed design methodology is systematic, and results in single-layered and compact structures. For verification purposes, an equal split WPD is designed, simulated, and measured. The obtained results show that the input and output ports matching as well as the isolation between the output ports are below –10 dB; whereas the transmission parameters vary between –3.2 dB and –5 dB across the 3.1–10.6 GHz band. The designed divider is expected to find applications in UWB antenna diversity, multiple-input-multiple-output (MIMO) schemes, and antenna arrays feeding networks. The second proposed component is a wideband multi-way Bagley power divider (BPD). Wideband functionality is achieved by replacing the single-frequency matching uniform microstrip lines (open full item for complete abstract)

Committee: Vijaya Devabhaktuni (Committee Chair); Mansoor Alam (Committee Member); Junghwan Kim (Committee Member); Daniel Georgiev (Committee Member); Douglas Nims (Committee Member); Mohammad Almalkawi (Committee Member); Abdelrazik Sebak (Committee Member) Subjects: Electrical Engineering; Electromagnetics; Engineering

Doctor of Philosophy, The Ohio State University, 2013, Electrical and Computer Engineering

Advanced wireless communication and sensing systems have created a growing need for high performance, compact antennas. Low-profile wideband phased arrays are of particular interest, and have recently been shown to be capable of extremely large bandwidths. However, the size, weight, and cost of phased arrays still makes them impractical for many applications. The development of thinner, lightweight, and inexpensive wideband arrays is critical to improving the capabilities of small platforms such as small unmanned aerial vehicles. Like all antennas, phased arrays are limited by a fundamental compromise between size and performance. Although the theoretical limitations of electrically small antennas have been well known for over 60 years, similarly general limits have not yet been developed for periodic antenna arrays. In the first part of this thesis, we derive a new fundamental bandwidth limit for any periodic array that is backed by a conducting ground plane and constructed from passive and reciprocal materials. This limit is related to several critical design factors, including the array's thickness, polarization, scan angle, materials used, as well as the overall complexity of the array design. We also consider the common case when all radiating currents are confined to a thin planar sheet placed above the ground plane. We show here that such planar phased arrays have a fundamental impedance bandwidth limit of 8.3:1 (with VSWR ≤ 2:1), in the absence of material loading. This bandwidth may be further improved by adding dielectric superstrate or magnetic substrate material layers. Knowledge of such fundamental bandwidth limits is extremely useful in the design of practical wideband arrays, which is the focus of the second part of this thesis. A key challenge with many wideband arrays is developing a feed circuit that supports extremely wide bandwidths without significantly adding to the size, weight, and cost of the design. Here, we demonstrate a novel (open full item for complete abstract)

Committee: John Volakis Dr (Advisor); Kubilay Sertel Dr (Advisor); Chris Baker Dr (Committee Member) Subjects: Electrical Engineering; Electromagnetics; Physics