應用於室內Access Point的低剖面雙頻天線

Abstract

本論文設計出一種應用於室內無線存取點具有雙頻且低剖面的奧爾福德環形(Alford-Loop)架構天線，傳統的奧爾福德環形天線的設計有窄頻、低增益、大面積、高剖面、昂貴板材的缺點，經過改良後，使用FR4為基板達成不錯的增益與低剖面。最後頻帶範圍為2.4～2.4835GHz與5.150～5.850GHz滿足ISM (Industrial、Scientific、Medical)頻段。 一般來說，這些室內環境的天線會希望在水平面上為均勻輻射場型，以達到最佳的覆蓋率，偶極天線能滿足這樣的輻射場型且設計簡單，故常用來做這兩者應用的天線，但是缺點為天線需要垂直擺放造成體積較大、不美觀。其餘的低剖面平面式單極、平板、平面式倒F型天線天線通常都需要垂直擺放才能達成水平面上為均勻的輻射場型，造成縱向尺寸無法縮減，使得電路整合不易。 奧爾福德環形能解決以上問題，其能達成低剖面且有均勻的水平面輻射場型。本論文重點在改良奧爾福德環形天線，為了讓電流更均勻饋入，輻射體採用漸進式架構來減少不連續面，此設計也能有效增加共振長度。此外在上下金屬板末端會有重疊部分產生耦合，其功用為調整低頻頻帶與阻抗匹配，適度的調整能有效的縮小天線尺寸與提升增益。 接著在底板加入寄生金屬片，其效果為產生額外的低頻頻段，經由調整後能有效改善傳統架構的窄頻問題。寄生金屬片除了能增加頻寬也能提升增益，補償高頻的開槽造成低頻增益的下降，其增益提升原理為八木天線中的導向器能增加指向性的效果。在這裡需要注意的是由於二分之一波長會造成電流反向，因此在本天線的寄生金屬片加入指叉性電容結構來調整電流相位。 高頻為透過上下板的狹縫形成一個漸進式開槽天線，因而達成雙頻架構。槽線共振長度約為頻率的1/4波長，此外透過上板額外增加的1/4波長金屬段，能產生出高頻的第二個共振頻率，經由適當調整金屬段寬度與長度能達成阻抗匹配，而達成寬頻架構天線。 最後透過量測發現，在雙頻帶有不錯的增益與頻寬，也能達成雙頻為均勻的輻射場型。本天線的設計細節與實驗結果在論文中有詳細討論。

In this thesis,we propose a alford-loop antenna with low-profile and dual frequeny band characteristic applied in indoor access point. Traditional alford loop antenna had some disadvantage of narrow bandwidth,large area,high profile,expensive substrate cost.After modifying the traditional structure, we can use the fr4 substrate to achieve the goal to pretty good gain and low profile.Finally,the frequency band cover 2.4-2.483GHz and 5.15-5.85GHz satisfying the standard of industrial, scientific, medical frequency band. In general,in order to achieve the optimal signal coverage,this indoor environment antenna will hope that the radiation pattern is uniform and paralleled to ground.Dipole antenna can be designed easily to achieve this pattern.As a result,it was usually applied to indoor environment antenna.However,it must be placed vertically to meet the need of uniform radiation pattern.Dipole antenna has some disadvantage of large size and unattractive shape.In order to achieve uniform radiation pattern paralleling to ground,the other low profile antenna like monopole,patch,planar inverted F antenna need to be placed vertically,which will increase the longitudinal direction size. Consequently,it is hard to integrate with circuit. We can solve above problem through using alford loop antenna achieving low profile and uniform radiation pattern paralleling to ground.We focuse on modifying alford loop antenna in this thesis.In order to make current fed uniformly,the radiation portion use tapered structure to decrease discontinuous variation.We can increase resonance length effectively through this design. Besides, we use the overlapping area between top and bottom metal to produce coupling which is able to adjust low frequency band and impedance matching .We can reduce the antenna size and promote gain through moderate adjusting. Then,we add the parasitic metal strips into bottom ,which can produce another low frequency band to resolve narrow bandwidth problem in traditional structure effectively by adjusting the antenna moderately. In addition to increasing bandwidth,parasitic metal strips can also promote gain,which can compensate the decreasing of low frequency gain caused by the slot of high frequency.The theory of promoting gain is similar to the director in yagi antenna,which will increase the directivity.It must be taking note to that one-half wavelength will reverse the current.Thus,we would add a interdigital capacitor structure to adjust current phase. We can achieve the dual band structure through the slot between top and bottom.The resonance length of tapered slot antenna is about one-quarter wavelength.Besides,we add one-quarter wavelength metal strips line on top structure,which would produce another high frequency band.We can achieve the wideband antenna after adjusting the width and the length of metal strip moderately The measurement results show that the antenna has the pretty good gain and bandwidth in dual frequency band.Besides,the antenna can also achieve uniform radiation pattern parallelling to ground in dual frequency band.The design details of antenna and experimental results are discussed in detail in this thesis.