Date of Award

2018

Document Type

Open Access Dissertation

Department

Electrical Engineering

Sub-Department

College of Engineering and Computing

First Advisor

Mohammod Ali

Abstract

There has been recent interest in the development of multifunction structures for weight-critical applications. A multifunction structure is a load-bearing structure that also allows one or more additional functions such as RF communication, energy storage, sensing etc. The focus of this dissertation is to analyze, design, develop, and test new high performance (broadband, high gain, circularly polarized) internal antennas that are structural and integral to the aircraft. It is demonstrated that antennas with more bandwidth and higher efficiency could be developed if the space and materials available in an aircraft structure could be judiciously exploited for multifunctional usage. This is improbable with bolt-on approaches, such blade antennas or antennas housed within a wing pod.

Firstly, a method called Characteristic Mode Analysis (CMA) is studied and used both for a dipole antenna and a VHF airfoil integrated antenna. Although computationally intensive, it provides fundamental insights on the significance of each mode, modal interactions, and overall achievable bandwidth. The CMA of a dipole antenna loaded with a thin coating of DNG material is undertaken. The presented analysis considers the MoM Galerkin formulation. The analyses presented demonstrate that when the relative permittivity and permeability are greater than -1 but less than 0, the configuration shows potential for antenna size reduction. For example, a 25% size reduction is achieved when the relative permittivity and permeability are equal to -0.3.

Secondly, the study, design, and development of a broadband (2:1 frequency ratio), positive gain (> 0 dBi), VHF antenna integrated within a composite airfoil structure are undertaken to overcome the limitations of very low gain (-20 dBi typical at low VHF frequencies) associated with resistively matched, electrically small, broadband airborne blade antennas. It is demonstrated that a broadband antenna operating from 89-220 MHz can be incorporated into composite structures. Simulation and experimental results clearly show that such antennas can be built using structural composite materials, such as fiberglass or cyanate-ester/quartz, Rohacell foam and conductive mesh with appropriate thicknesses commensurate with the frequency band of operation. Additionally, the antenna is studied with CMA to understand the contributions of various modes to antenna performance and to asses the performance impact of composite materials as a result of structural integration. The proposed sandwich structure antenna was also studied for possible MIMO application in an inverted V-tail UAV configuration. The two antennas in that configuration clearly show excellent performance based on their ECC and simulated radiation patterns.

Finally, fundamental studies and innovations are made in the topic area of structurally integrated, broadband, circularly polarized spiral antennas on EBG structures. To allow directional radiation, spirals require a quarter of a wavelength separation when placed on a reflecting surface (e.g. the aircraft’s ground). This thickness (as much as 6 inches or more at 450 MHz) is a significant challenge from a structural integration perspective and is unacceptable at UHF frequencies. While RF absorbing materials have been proposed, they significantly reduce antenna efficiency. To our knowledge, no work on spirals on EBGs has been reported that addresses either the broadband EBG design challenges in the UHF frequency band or the integration of such structures with composite aircraft platforms. Therefore, the investigation, design, and development of an equiangular spiral antenna on an EBG are conducted for 425-800 MHz satellite communication applications. Starting from a mushroom EBG structure, analysis and simulations are undertaken to determine the dependency of antenna gain bandwidth, impedance bandwidth, pattern bandwidth, and axial ratio on the EBG geometry, materials, and height. A structural integration scheme is proposed, and a corresponding antenna plus EBG with nearly an octave bandwidth is designed, built, and tested that demonstrate good circularly polarized performance (gain greater than 4 dBi RHCP and axial ratio less than 3 dB). While further optimization of gain versus axial ratio versus EBG geometry and height is quite possible, the findings demonstrate the clear feasibility of a RHCP spiral antenna on a planar, tapered EBG with half the thickness of a traditional spiral on a reflector for composite structural integration.

Available for download on Sunday, May 12, 2019

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