藉由合併雙諧振器與切半型零階諧振天線提升縮小化天線頻寬與效率

Abstract

本論文致力於尋找方法去同時改善微小型天線的缺點:窄頻與低效率。本論文分別嘗試用兩種方法去改進天線效率與頻寬。在論文的第一部份，利用一高效率的諧振器去補償低效率的諧振器，藉由將兩頻帶合併，低效率的諧振器頻寬與效率都大幅提升，同時，高效率諧振器在效率近乎不變的情況下，頻寬也大幅提升。為實踐此機制，本論文提出一個由開口環式諧振器(提供高效率)與接地螺旋形諧振器(造成低效率)所共同組成的雙層微小天線。最上層為開口環式諧振器與接地螺旋形諧振器，作為其輻射體，中間層為一附有曲折型阻抗匹配網路的矩形饋入線。對比於傳統式的貼片天線，其面積的縮小幅度為87%，相較於原先的接地型螺旋式單頻天線，其頻寬與輻射效率皆提升了3倍以上。 在第二部分研究中，將目標轉向於零階諧振天線的效能提升。因為零階諧振天線具有無限長波長的特性，因此廣泛被應用於天線縮小化的設計中。然而，低效率與窄頻寬限制了其用途。 在此章節中，藉由將對稱型零階諧振天線切半，天線的尺寸與品質因數皆大幅下降。本論文藉由萃取等效電路的參數，驗證了品質因數的下降結果。因此，天線的效率與頻寬皆大幅的提升。 在此，本論文提出一個具有加強頻寬與輻射效率，切半型電容負載零階諧振天線。此切半型電容負載零階諧振天線採用共平面結構，其組成為:兩個切半的指叉式電容構成輻射體，並於輻射體單端接地，還有一曲折型阻抗匹配網路。對比於切半前的天線，其頻寬增為原來2.7倍，效率提升1.5倍。本論文分析且驗證了將對稱型的零階諧振天線切半，不但能縮小天線尺寸，同時也提高了輻射效率與頻寬。

In this thesis, we work on finding methods to improve the disadvantages of electrically small antenna (ESA) such as narrow bandwidth and low efficiency at the same time. The research is divided into two parts. The first branch investigates merging two resonances which have higher and lower efficiency, respectively. After merging the two resonances, the efficiency and bandwidth of the one with lower efficiency increased, and the bandwidth of higher one also extended while efficiency is almost fixed. To practice this mechanism, a small planar microstrip antenna which is composed of a split ring resonator (SRR) [higher efficiency] and a shorted spiral [lower efficiency] etched on a upper dielectric slab. The pin drills through a bottom dielectric layer on which a feed line comprising a rectangular microstrip and a meander line for matching is printed. The increase of in bandwidth and efficiency is by more than triple that of the single band counterpart. Compared to conventional patch antennas, the size reduction is by 87%. The second part of the work focuses on improving the performances of zeroth order resonance (ZOR) antennas. The ZOR has infinite wavelength characteristic which causes ZOR can be used for antenna miniaturization. However, the narrow bandwidth and low efficiency limit its applications. By bisecting symmetric ZOR antenna, the antenna dimension and quality factor (Q) are decreased remarkably. We prove it by calculating Q which is extracted from the parameters of equivalent circuit model. Therefore, the bandwidth and efficiency are increased. A bisected ZOR antenna based on bisected inter-digital capacitor-loaded transmission line (B-CL-TL), with extended bandwidth and higher radiation efficiency is proposed. The antenna consists of two cascaded bisected co-planar inter-digital capacitors as series capacitors shorted to the ground plane at one end. A meander-line inductor is implemented for matching. Compared to the CL-TL ZOR antenna, the ratio of increase of bandwidth and efficiency is 2.7 and 1.5, respectively. In this thesis, we prove from theorem and experiment that bisecting symmetric ZOR antenna not only reduce antenna size, but also increase efficiency and bandwidth.