Phenomenology and Technology in Support of Sub-Terahertz Radar Systems

Abstract

Sub-terahertz (Sub-THz) systems operate at the border of electronics and photonics, and have profited from the development of both fields. One of the main driving forces to develop such systems originates from security applications and, in particular, stand-off imaging of persons and hidden objects. Other important applications include very high speed, short distance, line-of-sight communication links, as well as short-range navigation radars used in vehicles and aircrafts. These systems have the potential to approach the resolution of optical imaging systems while operating under adverse conditions of weather. Based on these applications and the potential for improved performance, the main theme of this thesis is to investigate and characterize the unique advantages as well as performance limitations of such systems working around 240 GHz in typical outdoor environments. The research has three main directions. The first direction is the development of novel scattering models that can accurately describe the propagation of electromagnetic waves in realistic communication channels. Specifically, a semi-numerical propagation model is developed to predict the attenuation as well as the back scattered power from 2D, very long, discrete, sparse random media. Such sparse random media can be used, in general, to model environments like rain, snow, or dust. The model is named Statistical S-matrix Approach for Wave Propagation in Spectral Domain, or SSWaP-SD in short. The advantage of this model is that it is a coherent one, and it can capture the multiple interactions of the electromagnetic wave with the scattering particles to all orders. Being a coherent model, it is then extended to study the degradation in imaging resolution for a Synthetic Aperture Radar (SAR) working in such rainy environments. The second direction is related to the characterization of different synthetic and artificial targets found in typical radar environments. In this part, a free-space, transmission-only, dielectric characterization setup is custom built to measure the dielectric constant of different fabric materials (wool, polyester, jeans, etc). Such measurements are important in the context of concealed object detection algorithms. In addition, a fully polarimetric, network analyzer-based instrumentation radar system working at 222 GHz is built to characterize the back scattering coefficient of different natural and synthetic rough surfaces found in typical outdoor driving environments like asphalt, concrete, grass, etc. The third direction of the research is related to the development of a major component of an electronic beam-steering radar which is the phase shifter. Specifically, a rectangular waveguide-based phase shifter is designed to provide 360 degree variable phase shift at 240 GHz. The proposed structure relies on mechanically actuating a tunable perfect magnetic conductor (PMC) inside a modified TE10 rectangular waveguide. A prototype is manufactured using silicon micro-fabrication techniques and the PMC is actuated electrically using an external piezoelectric actuator. The proposed phase shifter shows very small insertion loss which is important for realizing high efficiency electronic beam-steering antennas at sub-THz frequencies.