以創新雙頻帶拒共振結構強化帶拒能力之三頻帶微型槽孔天線

Abstract

利用在寬頻天線上加入不同形式、種類的槽線，或是加入金屬寄生貼片來在天線共振頻率造成阻抗不匹配，形成帶拒(Band-stop)的效果，稱為帶拒天線。帶拒天線除了要求小尺寸外，帶拒能力也是其關心的議題，從相關雙頻帶拒天線文獻得出，為了要縮小帶拒天線的面積，其帶拒結構必須受限於面積而使帶拒效果受影響，而為了增強帶拒效果，則需要使用成對的對稱帶拒結構，以及將帶拒結構設計至天線能量集中處，但往往這樣的要求將使天線面積增大。 為了達成雙頻帶拒效果，相關論文使用兩個以上的帶拒結構，使得天線所需面積受限於帶拒結構而增大，因此為了使帶拒天線面積縮小，相關研究使用四分之一波長的共振帶拒結構，相較傳統共振器只有一半的大小，以減少帶拒結構使用的面積，但非對稱的帶拒結構會使輻射場型產生擠壓效果，造成天線增益上升，使帶拒效果變差，而帶拒結構之間彼此靠近耦合，也導致增益壓抑能力不足。 而為了增強帶拒天線增益壓抑的能力，相關研究使用成對的對稱帶拒結構來提供更充足的能量集中效果，使帶拒能力上升，但是成對帶拒結構需要較大的面積，造成天線面積增加。 因此為了同時達到微小化帶拒天線以及增強雙頻帶拒的能力，帶拒結構方面，本論文提出一具有雙頻帶拒效果的挖槽式對稱帶拒結構，利用共用開路端的方式整合雙頻帶拒結構，以縮小帶拒結構使用面積，在各別帶拒頻段共振時，其等效為一對四分之一波長的共振器，使能量更加集中，且利用挖槽式與共用開路端的設計，使結構彼此間影響減小，帶拒頻段皆獨立可調整。本論文天線設計方面則使用共平面波導饋入之寬頻槽孔天線產生2.36-6.17 GHz的寬頻效果，並透過接地面的縮減與加入挖槽線以及特殊饋入結構的匹配，來達成面積微小與寬頻匹配的效果。 此外，為了能夠將帶拒結構設計於天線共振能量集中處以增強帶拒效果，在不增加整體寬頻槽孔天線面積的情況下，增加天線饋入端的面積，使帶拒結構能夠整合於天線能量集中處，來獲得雙頻帶拒皆較佳的增益壓抑能力。 本天線整體大小為21×27×0.8mm3，使用FR4基板，為單面結構，低成本且製作十分容易。經實作量測後，在2.36-6.1 GHz有不錯的匹配程度(VSWR≦2)，除了帶拒頻段2.78-3.36 GHz與4.03-4.76 GHz之外，天線增益在操作通帶維持2~3.5 dBi，場型皆為全向性，雙頻帶拒頻帶的增益壓抑則可大於14 dB，產生之三頻操作頻帶皆包含WLAN(2.4/5.2/5.8)、WiMAX(2.5/3.5/5.5)頻段規範。本天線的設計細節及實驗結果在論文中皆有詳細討論。

By embedding slots on the broadband antenna with different shapes, or utilizing parasitic elements near the radiator. Antennas could achieve band-notched function. Band-notched antennas are not only focusing on pursuing small-sizes, but also concerning the performance of gain suppression. In order to achieve dual band-notched function, most literatures used more than two notched structures. These uses lead to increase the size of band-notched antenna. Therefore, some literatures used quarter-wavelength notched structures to reduce the use of area, because the sizes of quarter-wavelength structures are only half of general resonators. However, these asymmetric notched structures may increase the directivity of radiation pattern, which lead to increase the antenna gain at the notched frequency and lower the performance of gain suppression. The effect of mutual coupling on these band-notched structures by being close to each other, also lead to the insufficiency on gain suppression. To enhance the gain suppression in the notched band, some literatures used pairs of symmetric notched structures, which could concentrate more energy at the notched frequency and increase the gain suppression. However, more areas to implement are needed for more notched structures that could lead to increase the size of antenna. Accordingly, in order to minimize the size of dual band-notched antenna and to enhance the gain suppression in each notched band simultaneously. A compact CPW-fed tri-band slot antenna with high level dual stopband rejection is proposed. Firstly, a compact slot antenna is obtained by reducing ground plane size and designing proper geometrical shapes of the slot and feeding patch to yield required bandwidth. Secondly, to yield high gain suppression in dual notched bands, a fork shaped feeding patch, which coincides with the current concentration spot, has been introduced without increasing the overall antenna size that could accommodate suitable band-notched elements. Next, a novel dual band-notched symmetrical slot structure could be properly fitted in the small fork shaped feeding patch to yield high level dual stopband rejection. Slots have been utilized as notched resonators because of mutual coupling between metal strips/stubs is stronger than slots that may limit the performance of gain suppression. Therefore, by integrating two symmetrical slot resonators using a common open-ended point of the slot, the novel notched structure with dual band-notched characteristics and size miniaturization is achieved. Furthermore, the novel symmetrical band-notched slot structure is equal to a pair of quarter-wavelength resonators in each notched band that could concentrate more notched energy. With these designs, good gain suppression ability and compact antenna size are obtained. The proposed antenna has been fabricated on a FR4 substrate with a single plane for verifying the design concepts, and the overall size of this antenna is only 21×27×0.8 mm3. The measured results show the impedance bandwidths from 2.36-6.1 GHz, and two rejected bands from 2.78-3.36 GHz and 4.03-4.76 GHz are achieved, which generate three separate operating bands. Moreover, antenna with flat gain frequency response, stable radiation patterns and over 14 dB gain suppression in the notched bands are also obtained. The purposed antenna feature compact size, good tri-band operating bandwidths, and a high level of signal rejection in dual notched bands, which is suitable for WLAN and WiMAX applications.