1-1 COMMUNICATION WITH RADIO WAVES 《天线与电波传播》课件.pptVIP

Electromagnetics and Communication Lab. Robert E. Collin Electromagnetics and Communication Lab. School of Electronics Information Engineering Xi’an Jiaotong University, P.R.China Prof. Jiang Yansheng References 1、Antenne Theory —— analysis and design  (second edition) Constantine A. Balanis 2 、《天线》 John D. Kraus 章文勋译 电子工业出版社 2005年 3、《天线工程手册》 林昌禄 电子工业出版社 2002年 4、《近代天线设计》林昌禄主编 人民邮电出版社 CHAPTER ONE COMMUNICATION WITH RADIO WAVES Communication by electrical means began with the introduction of telegraphy in 1844, followed by telephony in 1878. In these systems electrical signals are sent over two-wire transmission lines that connect the sender and recipient. It was not until 1897 that Marconi first patented a complete wireless telegraphy system based on the use of electromagnetic radiation (radio waves) that had been predicted theoretically by Maxwell 43 years earlier. 1.1 Types of communications There are two broad categories of communication systems: those that utilize transmission lines in an interconnected network and those that rely on electromagnetic radiation with an antenna at both the transmitting and receiving sites. Twisted-pair transmission lines will introduce an attenuation of around 2 to 3dB/km at frequencies around 10kHz. Coaxial transmission line’s loss is around 4 to 5dB/km.Thus a loss of 5dB/km becomes a loss of 100dB over a 20-km path. If the path is doubled to 40-km an additional loss of 100dB is suffered.When it is recalled that a 100-dB loss is a reduction in received signal power by a factor of , it is readily appreciated that the exponential attenuation law ultimately places a severe restriction on the distance over which communication can take place without the use of repeater amplifiers. Coaxial transmission line’s loss is around 4 to 5dB/km.Thus a loss of 5dB/km becomes a loss of