Abstract
摘要：本研究計畫內容包含兩部分，分年進行。計畫第一年將先研製一款新型波束可掃瞄之圓極化反射陣列，其單位元係由一對正交之微帶偶極所構成，分別提供水平及垂直線性極化分量所需之反射相位，並於特定方向上合成出圓極化主波束。每一個微帶偶極的中心均有一變容二極體作為負載，藉由改變其逆向偏壓調整該微帶偶極之反射相位，而分別調整各單位元之水平和垂直極化分量之反射相位，即可實現圓極化且波束可掃瞄之特性。此外，吾人所提出之單位元之晶格排列形狀為正六邊形，可提高單位元密度、降低因離散化造成之相位誤差，並抑制接地平面之鏡向反射。最後，結合類比訊號輸出模組、微處理器、系統控制程式及預先建立之查找表等，便可完成一波束可自動掃瞄之圓極化反射陣列系統。計畫第二年預計將前述反射陣列應用於微波成像系統中，配合所推導之探針天線補償成像公式，改良微波成像架構，以波束掃瞄特性取代傳統架構中的機械式掃瞄平台，由於吾人所研製之反射陣列係透過微處理器自動控制其主波束方向，掃瞄速度極快，可大幅縮短掃瞄時間。此外，利用主波束之圓極化特性，可同時擷取兩正交極化分量之散射場資訊，減少因散射體幾何形狀或擺設之不同所造成之成像限制與失真，改善成像品質。
Abstract: This two-year research project consists of two parts. In the first year, a novel beam-steerable circularly-polarized (CP) reflectarray will be developed. The unit cell of the proposed reflectarray is composed of a pair of orthogonal microstrip dipoles, responsible for the tuning of the reflection phases of vertical and horizontal polarization wave components, respectively. Each microstrip dipole is loaded with a varactor diode at its center. By properly adjusting the reverse bias voltage of the varactor, the reflection phase of the associated wave component can be tuned such that a CP main beam can be steered or formed at any specified direction. Besides, the hexagonal lattice of the proposed reflectarray increases the number of unit cells per unit area, which in turn lowers the quantization phase error and suppresses the specular reflection caused by the bottom ground plane. Lastly, by integrating the multi-channel analog output board, a microprocessor, system control code, and a lookup table containing the experimentally characterized data, an electronically beam-steerable CP reflectarray can be realized. In the second year of this project, the aforementioned reflectarray will be applied to microwave imaging application to simplify the system architecture. The associated imaging formulas with probe compensation will also be derived analytically and verified numerically. Thanks to the beam-steering characteristics of the reflectarray, the commonly used mechanical scanner is no longer necessary in the proposed setup, and the total time of scanning can be reduced drastically. Moreover, the CP radiation of the proposed reflectarray enables us to obtain simultaneously the two orthogonal components of the scattered fields. In this manner, the limitations on imaging and image distortion associated with the orientation or aspect ratio of the scatterer of interest could be mitigated, and therefore, the quality of the imaging results can be improved.
Keyword(s)
波束掃瞄
圓極化
逆散射
微波成像
反射陣列
Beam steering
circular polarization
inverse scattering
microwave imaging
reflectarrays
