天线基本原理PPT.ppt

邢洪波 2009年8月12日 概述 1.天线基本概念－辐射原理 天线所辐射的电磁波中电 场矢量在空间运动的轨迹 称为极化； 基站天线属于线极化，极 化方向与振子轴线平行。 双极化天线阵子 即一个天线单元中有两个排列正交的振子 两个振子所辐射的电磁波是相互独立的 极化分集可以有效避免多径传输效应对天线接收的影响 2.天线辐射参数－交叉极化比 2.天线辐射参数－方向图 辐射参数 主瓣 副瓣 半功率波束宽度 交叉极化鉴别率 增益 前后比 前后比是指扇形天线(定向天线)的前向辐射功率和后向辐射功率之比 3.天线电路参数--驻波比，隔离度，无源交调 驻波比为传输线上的电压最大值与电压最小值之比。 当天线端口没有反射时，就是理想匹配，驻波比为1； 当天线端口全反射时，驻波比为无穷大。 3.天线电路参数－驻波比 3.天线电路参数-隔离度 隔离度是某一极化接收到的另一极化信号的比例。 3.天线电路参数-无源交调 对称振子组阵能够控制辐射能构成“扁平的面包圈” 增益是指在输入功率相等的条件下，实际天线与理想的辐射单元在空间同一点处所产生的场强的平方之比，即功率之比。增益一般与天线方向图有关，方向图主瓣越窄，后瓣、副瓣越小，增益越高。 Note that the plane of oscillation, ie the polarisation, is the same as that of the dipole launching the wave.无线电波在空间传播时，其电场方向是按一定的规律而变化的，这种现象称为无线电波的极化。无线电波的电场方向称为电波的极化方向。如果电波的电场方向垂直于地面，我们就称它为垂直极化波。如果电波的电场方向与地面平行，则称它为水平极化波 Dual polarised antennas are used to provide “diversity” on the radio path from the mobile to the basestation. Traditionally this is done by having 2 receive antennas at the basestation mounted a few metres apart. When radio waves are transmitted they get reflected off buildings and other objects and can arrive at the receiver having travelled along several different paths. This can cause them to cancel each other out, or fade. However it is very likely that while fading is happening at one antenna location, it will not be fading a few metres away, so another antenna there can still receive a good signal. The base station chooses the best signal and so provides a more reliable service. This is called space diversity. The same effect happens to waves arriving with different polarisations, because the polarisation of a wave changes when it is reflected. In this case the basestation chooses the best polarisation. This is called polarisation diversity.当来波的极化方向与接收天线的极化方向不一致时，在接收过程中通常都要产生极化损失，例如：当用圆极化天线接收任一线极化波，或用线极化天线接收任一圆极化波时，都要产生３分贝的极化损失，即只能接收到来波的一半能量； 当接收天线的极化方向（例如水平或右旋圆极化）与来波的极化方向（相应为垂直或左旋圆极化）完全正交时，接收天线也就完全接收不到来波的能量，这时称来波与接收天线极化是隔离的。 Dual polarised antennas are used to provide “diversity” on the radio path from the mobile