4 Project1 task 3 测量无线信号传播(双语)课件.ppt

Project 1 Foundation of mobile communications 移动通信基础Task 3 Measuring Radio Signal propagation测量无线信号的传播;教学内容 3.1 电波传播特性 3.2 天线的基本特性 3.3 基站天线的应用 3.4 天线分集技术 3.5 驻波比 3.6 天馈线安装与测量;3.1 Characteristics of Radio Propagation电波传播特性 当前陆地移动通信主要使用的频段为VHF和UHF，即150MHz，450MHz、900MHz、1800MHz、2.1GHz。 思考题1：移动通信有哪些主要的传播方式？ 移动通信中的传播方式主要有直射波、反射波、地表面波等传播方式，地表面波的传播损耗随着频率的增高而增大，传播距离有限。 ;The direct path A ground reflected propagation path The dispersion path; In a mobile radio channel, a signal direct path between the base station and a mobile is seldom the only physical means for propagation. ;3.1.1 自由空间电波传播方式 自由空间电波传播是指天线周围为无限大真空时的电波传播，它是理想传播条件。电波在自由空间传播时，可以认为是直射波传播，其能量既不会被障碍物所吸收，也不会产生反射或散射。 思考题2：为什么当电波经过一段路径传播后，能量仍会受到衰减？ 由于辐射能量的扩散而引起的。; The free space propagation(自由空间传播) model assumes a transmit antenna and a receive antenna to be located in an empty environment. Neither absorbing(吸收) obstacles (障碍物)nor reflecting surfaces are considered. In particular, the influence of the earth surface is assumed to be entirely absent. The free space propagation model assumes the ideal propagation condition that there is only one clear line-of-sight（视线） path between the transmitter and receiver. ; The propagation distances are much larger than the antenna size, so we are allowed to model the radiating antenna（发射天线） as a point source with negligible（可以忽略的） physical dimensions. In such case, the energy radiated by an omni-directional antenna is spread over the surface of a sphere（球体）. This allows us to analyze the effect of distance on the received signal power. ; 直射波传播损耗可看成自由空间的电波传播损耗Lbs： ;3.1.2 阴影效应 shadow/shading effect 1、什么是阴影效应？ 当电波在传播路径上遇到起伏地形、建筑物、植被（高大的树林）等障碍物的阻挡时，会产生电磁场的阴影。; Some object (such as a mountain or a tall building) between the points of transmission and reception may make the strength of an ultra-high-frequency signal reduce. ;2、盲区（Blind Area） 移动台在运动中通过不同障碍物的阴影时，存在阴影区。盲区定义为某些特定区域中，电波被吸收或被反射而使移动台接收不到信息的区域。