多頻段平面倒F型天線之微小化研究

Abstract

為了滿足多頻通訊系統的發展，手機天線不僅要小尺寸還要具有多頻帶的特性。意謂著天線要在有限尺寸下具有多重的共振路徑,由於每個頻帶的輻射單元被縮小，導致輻射效率變低。市面上的手機多頻微小天線，大都是利用PIFA (planar inverted-F antenna) 結構來減小天線的物理長度。對傳統的PIFA來說，有兩個方法可達到多頻帶的需求，利用寄生輻射單元，增加天線的共振模態，但單元間偶合複雜，難以設計且增加天線面積;另一種方法是利用U型狹縫在主輻射體切割出多重共振路徑，但此方法會導致頻寬和輻射效率降低。本論文提出一種能讓PIFA維持小尺寸且具有多頻段的技術，使其每個頻帶重覆使用相同的輻射單元，不僅維持小尺寸且輻射效能也能保持一定的水準。改良短路棒在傳統式PIFA中的功用，不再是靠近饋入點的電容性負載，而是等效成短路式負載接在天線的末端，再配合接地板的特殊設計，使得天線等效成短路式半波長阻抗轉換器。為了滿足多頻帶的需求，在天線中引進寄生轉折單元來增加共振路徑。隨著頻率的改變，等效阻抗轉換器會利用短路附載來切換共振電流路徑，逕而輻射。同時，利用輻射阻抗來做阻抗匹配。由於此設計中，短路棒作為短路式負載且輻射單元重複使用使得天線體積縮小至30mm×20mm×4mm且接地板只需要30mm×43mm，覆蓋頻帶為GSM900、DCS、PCS、UMTS、Bluetooth/WiFi、WiMAX、HiperLAN/2 and additional band (4600~4800MHz)。天線輻射場型近似全向性且具有合理的增益值，量測各頻帶最大值介於1dBi ~ 4.8dBi之間。

For the multisystem applications, the antenna of a handset not only needs small size but also covers multi bands, which means that the antenna must have multi-resonating paths in a limitary size. Due to each radiating element of each frequency band is reduced such that the radiation efficiency is not good and the bandwidth is narrow. These critical parameters are trade-off one another, therefore, it is difficult to keep good performance in a compact antenna of multi-band. PIFA ( planar inverted-F antenna) is the popular type of antenna in the mobile phone market. PIFA can reduce effectively physical length of the antenna with a shorted pin which acts capacitance load and is near the feed. In conventional PIFA design, there are two popular techniques for the requirement of multi-band. Firstly, use parasitic element to increase the resonating modes, but this way would to increase the area of the antenna. Secondly, use the embedded U-shaped slits to create multi-resonating paths in main radiating element, but the radiating element is reduced, so the radiation efficiency is not good. In this paper, a novel miniaturization technique of multi-band PIFA is proposed, which makes the antenna to reuse the radiating element to radiate every frequency band. It not only reaches multi-band but also keeps good performances and small size. The modification is that the shorted pin acts short-circuit load and locates at the end of antenna instead of the function of the shorted pin in conventional PIFA. The proposed PIFA can be modeled by a half-wave transmission line impedance transformer (TILT) of short-circuited load form. For the requirement of multi-band, it adds parasitic folded branches to increase resonating current paths. Therefore, the antenna is equivalent to the parallel multi-TLITs. With changing resonate frequency, the parallel multi-TLIT is similar as a current switch to change the current path with the short-circuited load and radiate. The size of the proposed PIFA and the ground plane are 30mm×20mm×4mm and 30mm×43mm. The covered frequency bands included GSM900, DCS, PCS, UMTS, Bluetooth/WLAN, WiMAX, HiperLAN/2 bands, and an additional band (4600~4800MHz). Moreover, the antenna has a nearly omni-directional radiation pattern and a reasonable peak gain of 1dBi to 4.8dBi.