寬頻及多頻介質共振器天線之設計

Abstract

本論文主要是設計寬頻及多頻介質共振器天線，可應用於寬頻無線通訊系統。 在此論文中，我們在介質共振器內插入一空隙，以增強電場強度，提高輻射效率，而使頻寬增加。或挖一溝槽，擾動原本的電磁場分佈，降低共振器之品質因子，而達到寬頻的效果。利用此概念設計一雙頻介質共振器天線，可符合WiMAX與WLAN之規格。 在此論文中，我們也設計一介質共振器天線，使共振器之TEy111、TEy112、與TEy113的共振頻率互相靠近，利用孔隙耦合的方式饋入，以結合三個共振頻帶而達到寬頻的效果，並在介質共振器內部挖一溝槽，以增加水平面上的天線增益，可使用在WLAN 802.11a 的應用。 我們亦在介質表面鍍上一層金屬，形成一共振器。該共振模態之電場與電流可產生全方向性的輻射場型。並利用共平面波導饋入，使表面金屬成為一單極天線，結合單極天線與共振器的頻帶，可以達到47%的頻寬，並具有全方向性的輻射場型。 在一介質共振器內挖一井狀空腔，或在介質共振器挖一矩形溝槽，能降低該共振器的品質因子，而有效的增加天線的阻抗頻寬，再結合孔隙天線模態的頻帶，可設計一寬頻介質共振器天線，符合WLAN 802.11a之規格。 將隧道狀的空腔橫向插入一矩形介質共振器內，可以改變TEy112模態的電場分佈，該電場在介質上方的分佈與TEy111模態的電場分佈相似，因此具有相似的輻射場型，設計空腔與介質共振器的大小，使兩模態的頻帶相連而達到20%的頻寬，並符合WLAN 802.11a之規格。

Various wideband and dual-band antenna designs using dielectric resonators (DR) have been proposed. A rectangular DR antenna generally operates in its TE111 mode with a typical bandwidth of 5-8%, depending on its permittivity and dimensions. The TE111 mode has its characteristic electric field distribution within the rectangular DR. Modification in shape will perturb the electric field distribution. By inserting a thin air gap normal to the electric field into the DR, the electric field will be enhanced by several times. Hence, the radiation becomes more efficient and the Q factor of the DR is reduced, rendering a wide impedance bandwidth. Notches or tunnels can also be carved off the DR to increase its bandwidth. The effects of shape modifications on the resonant frequencies are analyzed by using the reaction between the original and the perturbed field distributions. A rectangular DR with shape modification is designed to have dual-band characteristic with bandwidth covering 3.375-3.93 GHz (15%) and 5.08-5.415 GHz (6%), respectively.

Each resonant mode in the DR is associated with a unique radiation pattern. For example, a rectangular DR operating in its TE111 mode has a broadside radiation pattern on the H-plane, while that operating in the TE112 mode has a null radiation on the H-plane. The field distribution in a DR becomes the superposition of individual field distributions of different resonant modes when the DR operates at the frequency between the resonant frequencies of different modes. The impedance bandwidth can be increased by merging the bands associated with the TE111, TE112, and TE113 modes. The resonant frequencies of these three modes can be close by proper designing the DR dimensions, making the radiation pattern more consistent over the impedance bandwidth. Moreover, an asymmetric moat is caved off to tilt the beam and hence increase its directivity on the H-plane. A rectangular DR with an asymmetric moat is designed to provide a wide impedance bandwidth of 4.89-6.86 GHz, over which broadside radiation pattern is obtained

The interface between air and high-permittivity material can be approximated as a perfect magnetic condition (PMC) wall. If a DR is partially coated by metal, a new resonant mode appears. Coplanar waveguide is used to excite the new resonant mode of the metal-coated DR. The metal coating also acts as a monopole. Merging the bands of the DR and the monopole by proper designing their dimensions, a wide impedance bandwidth can be obtained (4.2-6.8 GHz), over which a nearly omnidirectional radiation pattern is achieved.