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我们知道,一个完整的无线电广播电视系统必须由以下三个部分组成:发射电视信号的发射设备、传播电波的媒质和接收电视信号的接收设备。其方框图如图1所示。无线电波是通过发射天线与接收天线之间的媒质进行传播的。所谓媒质指的是地球表面和大气层。地球表面是复杂的,有平原、丘陵、高山、森林、湖面和海面,有僻静的山村,也有工业集中和高楼林立的城市;大气层又包括对流层、平流层、流星余迹(一般在离地面70～130Km高空)和电离层。
We know that a complete radio and television system must consist of three components: the transmitting equipment that transmits the television signal, the medium that transmits the electric wave, and the receiving equipment that receives the television signal. The block diagram shown in Figure 1. Radio waves are transmitted through the medium between the transmitting antenna and the receiving antenna. The so-called medium refers to the earth’s surface and the atmosphere. The surface of the earth is complex with plains, hills, mountains, forests, lakes and seas, secluded mountain villages, and cities with industrial concentration and skyscrapers; the atmosphere also includes the troposphere, the stratosphere, the meteor trail (usually 70 ~ 130Km altitude) and the ionosphere.