全向性水平極化天線

Abstract

近年來消費者對於無線通訊產品的傳輸速度和品質的需求越來越高，許多產商紛紛採用天線分集技術以提高信噪比(S/N ratio)和分集增益(diversity gain)。有鑑於市面上空間分集(space diversity)的技術已相當普遍，而極化分集(polarization diversity)可望成為下一波網通產品通訊速度提升之關鍵。本申請案提出的全向性水平極化天線可以和垂直極化的單極天線相互搭配，讓不同極化方向上的信號變成相互獨立或接近相互獨立，使無線網路的性能獲得大幅度的改善。

本論文提出一新型之全向性水平極化天線。將兩個共平面波導餽入之槽孔天線互相正交，藉由適當調整兩餽入訊號的振幅和相位，可在天線側向合成出全向性輻射場型。此天線具有高輻射效率和高極化純度之優點。和其他平面式的全向性水平極化天線相比，本天線在外型上較為細長，可以和現有的垂直極化天線組合成極化分集系統，適用於無線路由器和無線基地台之應用。

Antennas with omnidirectional radiation pattern can ensure full coverage and are widely adopted for indoor and base station applications. Most existing systems are vertically polarized, and these antennas can be simply realized by vertical dipoles. But the polarization of propagating electromagnetic wave will change significantly due to the complicated surrounding environment. Recently, the requirement of horizontally polarized omnidirectional antennas has increased rapidly as they can serve as the diversity counterparts for vertically polarized ones.

In this dissertation, a novel horizontally polarized antenna with omnidirectional pattern is presented. Two CPW-fed slot dipoles are placed in a perpendicularly conjugate form. When the magnitude and phase of input signals are arranged properly, the omnidirectional pattern can be synthesized at broadside. A prototype is developed which offers a horizontally polarized omnidirectional radiation pattern with moderate gain and high e?ciency through the operating band. Furthermore, a 20-dB polarization purity is achieved in this design. Distinct from the other proposed horizontally polarized antennas provided with planar geometry, this antenna is slim in shape, and it can be readily integrated with vertical dipoles to form a polarization diversity system.

This study also presents a novel cloaking technique that renders a metallic block invisible from impinging electromagnetic wave inside a metallic waveguide. Di?erent from other cloaks using coordinate-transformation proposed in literature, this cloak consists of a stack of commonly used dielectric slabs. Numerical simulations have shown that the cloak can reroute the direction of the Poynting vector. The experiments verify this finding by measuring the reflection and transmission coe?cients in the waveguide, showing that the cloak can make a metallic object invisible within a broad bandwidth. The theory of mode conversion are proposed to explain the cloaking phenomenon.