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, 2011, Electrical and Computer Engineering

Unmanned aerial vehicles (UAVs) have become increasingly popular for scientific research, remote sensing, transportation of goods, search and rescue as well as military applications. UAVs have several key advantages over piloted aircrafts including low cost and the ability to penetrate unattainable areas that would be classified as unsafe. Technological advances and miniaturization allow communication devices to be placed on small UAVs. To improve aerodynamics it is thus necessary to design antennas conformal to host structure of UAVs. However, at VHF/UHF the UAVs often become electrically small, making it challenging to design wideband communication antennas. In this research, the theory of characteristic modes (CMs) is used for the analysis and design of complex conformal antennas. Traditionally, CM theory is used as an analysis tool. However, research efforts in this dissertation are focused on expanding characteristic mode theory for the design of antennas. First, two systematic simplification procedures are developed which reduce the number of characteristic modes considered for complex antenna structures. This lays the foundation for simplifying the analysis of complex antenna structures allowing the designer to focus on a small subset of critical modes. Later, thorough analysis of the input admittance of CMs is conducted. It is shown that if a mode contributes to the conductance, it ultimately contributes to the radiated pattern. For higher order modes this is typically undesirable. Thus, it will be shown how to suppress the effects of higher order modes by proper feed placement. By suppressing a higher order mode's admittance, the bandwidth of an antenna can be increased. Using the simplification procedures, an investigation of electrically small square and rectangular ground planes is conducted. The analysis identified four important modes and allowed thorough analysis of the eigen properties of each mode. Results showed that antenna elements with electri (open full item for complete abstract)

Committee: Roberto Rojas (Advisor); Fernando Teixeira (Committee Member); Robert Garbacz (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