Reflector Antennas for Therahertz Imaging Applications

The term terahertz has been ubiquitous in the arena of new technological advances during the past couple of years. New applications are emerging every day that are exploiting the promises of terahertz: its small wavelength; its ability to penetrate dust, clouds, and fog; and the possibility of having a large instantaneous bandwidth for high-speed communication channels. Until very recently, space-based instruments for astrophysics, planetary science, and Earth science missions have been the primary motivator for the development of terahertz antennas, sensors, sources, and systems. In recent years, however, emerging areas such as imaging from space platforms, surveillance of person-borne hidden weapons or contraband from a safe stand-off distance and reconnaissance, medical imaging and DNA sequencing, and the world of high-speed communications have been the driving force for this area of research. In this chapter, the main properties of terahertz reflector systems for imaging applications are addressed in detail. Figure 4.2 shows a conceptual sketch of such an imaging system. The backend electronics, that is, the receiver (in the case of passive systems) or the transceiver (in the case of active systems), are located in the focal plane of the reflector system, whereas the reflector system is used to generated an image of the target plane. This plane is located at a target distance Rf from the reflector system aperture. The different pixels in the image can be measured by creating a multibeam antenna using a focal plane array (FPA) with many beams pointing to different pointing directions θ, or by mechanically rotating the pointing direction of the reflector antenna, or by a combination of both. The image will be generated over a certain field of view (FoV) defined by the maximum pointing angle θ of the beam in each plane. The different antenna pointing directions and pattern shapes create a footprint at the target plane. This footprint defines the resolution of the image and has to be optimized to achieve high image acquisition speeds. In this chapter, we address the properties and typical configurations of terahertz imaging reflector systems for both FPAs and mechanical scanners. To study the properties of these systems, we derive an analytical expression of the vectorial field in the focal plane of a general focusing system. (Appendix 4A provides the detailed derivation for this field.) This analytical expression helps us to study the properties of the field at both the focal plane and target plane.