Doctor of Philosophy, The Ohio State University, 2016, Electrical and Computer Engineering

Characteristic Mode Theory is one of the very few numerical methods that provide a great deal of physical insight because it allows us to determine the natural modes of the radiating structure. The key feature of these modes is that the total induced antenna current, input impedance/admittance and radiation pattern can be expressed as a linear weighted combination of individual modes. Using this decomposition method, it is possible to study the behavior of the individual modes, understand them and therefore control the antennas behavior; in other words, control the currents induced on the antenna structure. This dissertation advances the topic of antenna design by carefully controlling the antenna currents over the desired frequency band to achieve the desired performance specifications for a set of constraints. Here, a systematic method based on the Theory of Characteristic Modes (CM) and lumped reactive loading to achieve the goal of current control is developed. The lumped reactive loads are determined based on the desired behavior of the antenna currents. This technique can also be used to impedance match the antenna to the source/generator connected to it. The technique is much more general than the traditional impedance matching. Generally, the reactive loads that properly control the currents exhibit a combination of Foster and non-Foster behavior. The former can be implemented with lumped passive reactive components, while the latter can be implemented with lumped non-Foster circuits (NFC). The concept of current control is applied to design antennas with a wide band (impedance/pattern) behavior using reactive loads. We successfully applied this novel technique to design multi band and wide band antennas for wireless applications. The technique was developed to match the antenna to resistive and/or complex source impedance and control the radiation pattern at these frequency bands, considering size and volume constraints. A wide band pat (open full item for complete abstract)

Committee: Roberto Rojas Prof (Advisor); Fernando Teixeira Prof (Committee Member); Robert Burkholder Prof (Committee Member) Subjects: Electrical Engineering; Electromagnetics; Engineering

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

In recent years, there has been a great interest in wide-band small antennas for wireless communication in both ground and airborne applications. Electrically small antennas; however, are narrow bandwidth since they exhibit high a quality factor (Q). Therefore, matching networks are required to improve their input impedance and radiation characteristics. Unfortunately, due to gain-bandwidth restrictions, wideband matching cannot be achieved using passive networks unless a high order matching network is used. Fortunately, the so-called non-Foster circuits (NFCs) employ active networks to bypass the well-known gain-bandwidth restrictions derived by Bode-Fano. Although NFCs can be very useful in numerous microwave and antenna applications, they are difficult to design because they are potentially instable. Consequently, an accurate and efficient systematic stability assessment is necessary during the design process to predict any undesired behavior. In this dissertation, the design, stability, and measurement of two non-Foster matching networks for two different small monopole antennas, a non-Foster circuit embedded within half loop antenna, a combination of Foster and non-Foster matching network for small monopole antenna are presented. A third circuit; namely, a non-Foster coupling network for a two-element monopole array is also presented for phase enhancement applications. It all examples, the NFCs substantially improve the antenna's performance over a wide frequency band. First, the stability properties of NFCs are discussed with a time-domain technique that computes the largest Lyapunov exponent for time series signals. In case of instability, it is shown how the circuit can be stabilized with different controllers. The proposed stability approach has been successfully applied to a negative capacitor to match a 3" electrically small monopole receiver antenna. Measured results verified that the system is stable and the non-Foster matching networks improve bot (open full item for complete abstract)

Committee: Roberto Rojas Prof. (Advisor); Patrick Roblin Prof. (Committee Member); Fernando Teixeira Prof. (Committee Member) Subjects: Electrical Engineering; Electromagnetics

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

In this dissertation, the Theory of Characteristic Modes is used as a framework for the design, optimization, and benchmarking of electrically small radiating systems. The foundation of this work is in the theory of Characteristic Modes, an eigenvalue equation of the Method of Moments impedance matrix [Z], that leads to derive the fundamental radiation modes of arbitrary-shaped bodies. After an overview of small antenna theory, we derive a new method for computing the Q factor of arbitrary-shaped radiating bodies using CMs using only the Method of Moments impedance matrix [Z]. Following this derivation, we present a new method for computing the fundamental limits on Q (and thus bandwidth) for arbitrary-shaped antennas. As a by-product of this method, we extract the optimal current distribution as a function of antenna shape for design guidelines. We further extend this theory to find the Q limits of arbitrary-shaped antennas and antenna-platform systems, subject to specific radiation pattern requirements. In the second part of the thesis, we use the Theory of Characteristic Modes to optimize the location and excitation of single and multiple in-situ ESAs mounted on finite, sub-wavelength platforms as relates to unmanned aerial vehicles (UAVs). By properly analyzing the CMs of the supporting platform, we show that a complex, multivariate optimization problems can by radically simplified using CMs. Based on this capability, we present a new, systematic design methodology for location optimization of small antennas on-board finite platforms. The approach is shown to drastically reduce the time, computational cost, and complexity of a multi-element in-situ antenna design, as well as providing significant performance improvements in comparison to a typical single-antenna implementations.

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

Doctor of Philosophy, The Ohio State University, 2014, Electrical and Computer Engineering

As technology progresses, mobile devices such as laptops, tablets, cell phones, and two-way radios have become smaller in size. Consequently antennas become electrically small to fit inside aggressive packaging requirements with rapidly changing real and imaginary impedances. As such, these antennas are very narrow in bandwidth with high-Q and input impedance which is very sensitive to environmental effects. The radiation efficiency of the device is drastically decreased as the antenna is detuned and signal quality is degraded. As the number of mobile devices we use increases, adaptive impedance tuners have and will become a bigger necessity, especially as more radios are integrated into a single device. This dissertation presents novel improvements to closed loop tuning topologies from a system level perspective addressing impedance tuners, sensing techniques, and how they apply to different antennas. The biggest design hindrance to impedance tuners are losses due to small signal resistance, and loss due to circuit resonances and radiation. A detailed explanation of these loss mechanisms is developed, providing designers with the knowledge to minimize the impact of said losses and improve system efficiency. By exploiting loss mechanisms, a novel small and low cost VHF impedance synthesizer is presented to characterize impedance tuners in load pull measurements. With full consideration of circuit loss mechanisms, a new directional coupler based tuning topology is presented. Traditional tuning topologies aim to minimize |S11| of the matching network. As demonstrated in this work, such a method has the potential to maximize losses in the circuit, especially in multi-stage tuners. Alternative directional coupler based topologies are presented which maximize the system transducer gain. Furthermore, a novel method of sensing a tuned state through the use of a near field probe that detects far field radiated power is introduced. A design guide is detailed with sev (open full item for complete abstract)

Committee: John Volakis Professor (Advisor); Chi-Chih Chen Professor (Committee Member); Chris Baker Professor (Committee Chair) Subjects: Electrical Engineering; Electromagnetics; Electromagnetism

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, The Ohio State University, 2011, Electrical and Computer Engineering

The demand for wide-band small antennas is steadily increasing for both civilian and military applications due to the explosive growth of wireless communications systems. Linearly polarized electrically small antennas can be generally classified as TM10 and TE10 mode antennas. For a TM10 mode antenna, the input impedance of the antenna is considerably reactive with a small real part. In contrast, the input admittance of a TE10 mode antenna is characterized by a high susceptance and a small conductance, i.e. the input impedance is almost a short. It is therefore critical to match the antenna to a receiver (or transmitter) to optimize the transfer of power in the frequency range of interest. With conventional passive matching networks, the antennas can be only matched over narrow frequency bands. However, Non-Foster matching networks composed of negative capacitors and/or inductors can in principle match the antenna over wide frequency bands because Non-Foster matching networks can overcome the gain-bandwidth restrictions derived by Bode-Fano. In this dissertation, the design, implementation, and measurement of two Non-Foster matching networks for a TM10 mode antenna and a Non-Foster loading network for a TE10 mode antenna are the topics to be discussed, which improve performance of both types of electrically small antennas over broad frequency ranges. These devices take advantage of the unique property of Non-Foster impedances, counter-clock wise rotation on the Smith chart as the frequency increases. First, a systematic methodology is introduced to design a Non-Foster matching network for an electrically small antenna. Key steps in the proposed methodology are presented to demonstrate how to realize a fabricated Non-Foster capacitor for a 3′′ electrically small monopole receiver antenna. Based on experimental results, it is verified that Non-Foster matching networks will improve both the antenna gain and the signal to noise ratio. Second, a Non-Foster matching (open full item for complete abstract)

Committee: Roberto Rojas Ph.D. (Advisor); Fernando Teixeira Ph.D. (Committee Member); Patrick Roblin Ph.D. (Committee Member) Subjects: Electrical Engineering

Master of Science, The Ohio State University, 2011, Electrical and Computer Engineering

With the rapid growth of wireless communications and associated demand for high data-rates, many future antennas must be ultra-wideband (UWB). A single antenna is certainly a most attractive solution to handling the large bandwidth requirements. Designing such an antenna that is concurrently of low-profile is necessary for mounting on a variety of grounds and airborne vehicles. This thesis presents a new low-profile UWB antenna concept. The concept is based on a shorted low-profile patch strategically fed on the open side to obtain wide bandwidth. For miniaturizing the proposed antenna and widening its bandwidth, it was loaded with ferrite bars placed between the patch plate and the ground plane. Much of the design effort for this antenna is focused on the location and shape of the ferrite bars. Minimization of antenna weight is also performed. The developed ferrite-loaded shorted-patch concept is compared with a low-profile monopole to demonstrate its superior bandwidth and gain performance over the frequency range from 30 MHz to 400 MHz. Prototype versions of various ferrite-loaded shorted-patch antennas were fabricated and measured for validation. A version of these was the design that has a planar patch as small as λ/16 in diameter and a height of only λ/200 at the lowest operational frequency. We note that the ferrite loading design led to 5 to 14 dB gain improvement in the low frequency range below 50 MHz. The resulting gain was actually close to the optimal Fano-Bode limit. An important aspect of the ferrite loading design was the use of commercially available materials. Although these were lossy at high frequencies, their strategic placement resulted in minimal gain reduction.

Committee: John Volakis Prof. (Advisor); Chi-Chih Chen Dr. (Advisor) Subjects: Electrical Engineering; Electromagnetics; Electromagnetism

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, 2013, Electrical and Computer Engineering

In the first part of this dissertation, we explore the metric invariance of Maxwell's equations to design metamaterial blueprints for three novel electromagnetic devices. The metric invariance of Maxwell's equations here means that the effects of an (hypothetical) distortion of the background spatial domain on the electromagnetic fields can be mimicked by properly chosen material constitutive tensors. The exploitation of such feature of Maxwell's equations to derive metamaterial devices has been denoted as `transformation optics' (TO). The first device proposed here consists of metamaterial blueprints of waveguide claddings for (waveguide) miniaturization. These claddings provide a precise control of mode distribution and frequency cut-off. The proposed claddings are distinct from conventional dielectric loadings as the former do not support hybrid modes and are impedance-matched to free-space. We next derive a class of metamaterial blueprints designed for low-profile antenna applications, whereby a simple spatial transformation is used to yield uniaxial metamaterial substrate with electrical height higher than its physical height and surface waves are not supported, which is an advantage for patch antenna applications. We consider the radiation from horizontal wire and patch antennas in the presence of such substrates. Fundamental characteristics such as return loss and radiation pattern of the antennas are investigated in detail. Finally, transformation optics is also applied to design cylindrical impedance-matched absorbers. In this case, we employ a complex-valued transformation optics approach (in the Fourier domain) as opposed to the conventional real-valued approach. A connection of such structures with perfectly matched layers and recently proposed optical pseudo black-hole devices is made. In the second part of this dissertation, we move from the derivation of metamaterial blueprints to the application of pre-defined unit-cell metamaterial structures for (open full item for complete abstract)

Committee: Fernando Teixeira (Advisor) Subjects: Electrical Engineering; Electromagnetics