以等效電路法設計之縮小化及去耦合化印刷天線

Abstract

本論文旨在研究縮小化天線與多天線去耦合的設計。基於現代無線通訊產品尺寸日益縮小的趨勢，以及經常整合多種無線系統於其中，縮小化天線以及去耦合設計為兩大重要議題。本研究中所有的設計皆使用等效電路模型來解釋天線輸入阻抗的特性。內容主要在於小型天線的設計上提出了新的設計方式，採用電路合成的方式來合成天線，同時可達到共振頻率與阻抗匹配。而去耦合化的設計則提出兩種方式，包含了在電路端提高隔離度的方式，以及使用扼流器阻擋耦合電流的方式，兩者面積皆小於文獻中同類型設計。基於以縮小化為前提，本論文中所提出的架構皆適用於可攜性的無線通訊產品。 本研究將天線分成兩大類，第一類為利用足夠的長度產生波傳導自然共振的設計，例如：縮小化的四分之一波長倒F天線。第二類則是使用電路合成產生共振的設計，此為本論文所提出的重點。針對此兩種天線，本論文皆提出縮小化的設計。首先，本論文提出利用螺旋繞線的方式縮小傳統倒F天線，同時達到雙頻的效果，以及利用高介電系數的陶瓷材料縮小四股螺旋天線的方式。兩者分別為四分之一波長與一個波長的共振形式，各達到原始面積的50%及體積的2.7%的縮小化效果，並且都有等效電路分析其操作模式。另一方面，以電路合成的設計方式，提出了利用串連等長左右手傳輸線的概念來設計開路共振腔，並更進一步的利用短槽孔作為輻射單元，並採用兩顆總集電容，設計出小型天線且實現頻率可調整的功能，僅需要9 mm x 1.5 mm 的槽孔尺寸即可操作在1.35 GHz至2.45 GHz。此兩種設計都是從等效電路出發，再利用印刷元件來完成佈局實現天線功能，所提出的等效電路可解釋共振頻率以及阻抗匹配的機制。關於去耦合的設計，針對小尺寸的需求開發了縮小化的去耦合架構，使用地緣扼流器。地緣扼流器是使用印刷電路單元實現，其可等效為一個並聯諧振器，能阻擋流經地面邊緣的電流，利用此一特性可以幫助增加兩隻天線的隔離度。此一設計具有小型化以及高適應性的優點，且理論與實作皆已完備並可相互驗證。

This dissertation presents the miniaturized antennas and decoupling structures, which are two important issues in mobile wireless applications. All the proposed designs are developed by the equivalent circuit approach in order to understand the impedance features more clearly. The new design concept for antenna has been presented, which is to construct antennas through circuit synthesis. Methods of decoupling are also revealed, including blocking and canceling mechanism. All the proposed structures are compact and low cost that meets the requirement of portable devices nowadays. The presented miniaturized antennas are divided into two catalogs, the wave resonance type and circuit synthesis type. In natural resonance type, the printed dual-band inverted-F is miniaturized by spiraling the tail of the open-end strip that can simultaneously achieve dual-band operation and size reduction (50%). The dielectric loaded quaduature helix antenna using ceramic rod (relative permittivity = 40) dramatically reduces the volume to be 2.7% of air-loaded one. Both are analyzed by equivalent circuits. The designs of circuit approach utilize the short slot radiator as an inductive element to synthesize the equivalent circuit for antennas. A cascaded right/left-handed transmission line, with opposite phase delay, as a feed is used to create the resonance at operation frequency. An improved design of using two capacitors on a short slot radiator further shrinks the antenna size (9 mm by 1.5 mm for 2.4GHz) and gains more design flexibility. By using varactors it can become a frequency tunable antenna (9 mm by 1.5 mm for 1.35-2.45GHz, measured). Decoupling technology shows importance in integrated multiple systems and Multiple-Input-Multiple-Output (MIMO) system. For the small device, the decoupling design should also be compact. There is a decoupling method proposed. The current choke composed of printed elements with the function of blocking the induced current on the ground edge. The design is compact and easily integrated in most devices.