Microwave Shielding Structures with Applications to Ground-Penetrating Radar

Abstract

This thesis contains an overview and theory for a variety of antenna shielding structures, for the purpose of reducing interference and multipath issues due to the antenna's surrounding environment. Though such structures are relevant to a variety of applications, particular focus is given to ground-penetrating radar (GPR). For a GPR system, due to the high degree of loss an electromagnetic (EM) signal experiences as it travels underground, extraneous coupling due to multipath signals, such as those reflected from above-ground targets, can potentially overwhelm any measured signal of interest. Though this can be mitigated using metallic shields, any practical finite shield structure is going to suffer from diffraction effects, which cause additional back and sidelobes. Additionally, the shield itself will alter the antenna’s time-domain response, which can affect GPR system performance. To solve these issues, this work considers radar absorbing materials, which can be used to prevent reflection and dampen currents on a metal surface. This work also considers a variety of metallic shields, such as ground planes, cavities, and high-impedance or electromagnetic bandgap (EBG) surfaces, and mechanisms which either cause or suppress diffraction on these structures. These structures are also studied in the time domain, to elucidate how these shields perturb an antenna's response when excited by a broadband pulse. Generally, the excitation of surface waves is found to play a prominent role in diffraction around these shields, and the Sommerfeld half-space problem is considered as an analytical solution for the excitation of surface waves by a dipolar source. This solution can also readily extended for dipoles above multilayer absorbing materials. Finally, this work presents the in-depth study of a choke ring shield, together with the fabrication and measurement of a choke ring shield loaded with an absorber to suppress ringing effects within the shield's central cavity. Some potential further designs based off the principles from this thesis are also presented.