of this chapter are: Radio signals are generally considered to be electro- to help you understand why certain methods and magnetic signals which are broadcast or radiated through techniques are used for radio frequency (r.f.) and space. They vary in frequency from several kilohertz¹ to high frequency communication circuits; well over 100 GHz (1011 Hz). They include some well to appreciate the need for modulation; known public broadcasting bands: long-wave (155- 280kHz), medium-wave (522-1622kHz), short-wave to understand the basic principles of modulation and (3-30 MHz), very high frequency (VHF) FM band (88- demodulation; 108 MHz), ultrahigh frequency (UHF) television band to understand the basic principles of signal propaga- (470-890 MHz) and the satellite television band (11.6 to tion using antennas; 12.4GHz). The frequencies 2 quoted above are approximate to introduce radio receivers; figures and are only provided to give an indication of some of to introduce you to the requirements of selectivity the frequency bands used in Radio and TV broadcasting. and bandwidth in radio communication circuits. 2.1.1.1 Aims 2.1.2 Radio communication The aims of this chapter are to introduce you to some systems basic radio communications principles and methods. These include modulation (impressing signal information 2.1.2.1 Stages in communication on to radio carrier waves), propagation (transmission of radio cartier waves) and demodulation (detection of Let's commence with a simple communications example radio carrier waves) to recover the original signal and analyse the important stages necessary for commu- information. nication. This is shown diagramatically in Figure 2.1-1. One hertz (Hz) means 1 cyclic vibration per second: 1 kHz = 1000 cyclic vibrations per second, 1 MHz = 1 1000000 cyclic vibrations per second, and 1 GHz = 1 000 000 000 cyclic vibrations per second. The word Hertz is named after Heinrich Hertz, one of the early pioneers of physics. The frequencies quoted are for Europe. Other countries do not necessa