Design of Ultra-Wideband Reflectors.

摘要

It is well-known that reflectarrays typically have a narrow bandwidth which is commonly attributed to the resonant nature of the antenna elements used and the narrow bandwidth of the phase shifting networks used. Typical approaches to increase the bandwidth include the use of true-time-delay (TTD) devices, coupling multiple resonances together, stacking multiple layer of scatterers or the use of numerical optimization techniques. While these approaches have been shown to improve the bandwidth of reflectarrays, the upper bound remains at approximately 40 in fractional bandwidth. This thesis investigates how ultra-wideband reflectors can be designed using novel approaches going beyond the conventional approaches listed above. Specifically, we investigate two new methods to design reflectarrays. In the first method, the reflector is designed as a Transformation Optics (TO) device and in the second method, the reflector is designed as a metasurface. The TO method amounts to manipulating of electromagnetic waves using an appropriate set of material parameters such that the wave propagation inside the material behaves in a desired manner. We use TO as a means to design an ultra-wideband reflector and draw insights into the practical considerations associated with designing such a reflector. In the second method, we design a reflectarray as a metasurface. Traditionally, reflectarrays are considered to be an array of individual antenna elements with no surface properties typically defined. A metasurface often has homogenized surface properties such as surface impedance and admittance which are realized by sub-wavelength elements that make up the reflector. A novel method to design ultra-wideband reflector using a metasurface is derived from first principles and it is shown that the designed reflector exhibits excellent characteristics over a very wide band of frequencies. The advantages and disadvantages of the reflectors designed using each of these two methods are discussed. A comparison of our metasurface is made to a state-of-the-art wideband reflectarray.