e to provide coverage even in hilly or mountainous terrain where signals at VHF and higher would not. 28 Newnes Guide to Radio and Communications Technology Direction of incident wave Medium 1 Some of refractive incident wave index 1 is reflected Medium 2 refractive Direction of reflected wave index 2 Figure 2.9 Refraction of an electromagnetic wave at the boundary between two areas of differing refractive index Signal diffracted at reduced strength Shadow area Hill approximating to a 'knife edge' Figure 2.10 Diffraction of a radio signal around an obstacle Radio waves and propagation 29 Reflected signals Signals that travel near to other objects suffer reflections from a variety of objects. One is the earth itself, but others may be local buildings, or in fact anything that can reflect or partially reflect radio waves. As a result the received signal is the sum of a variety of signals from the transmitter that have reached the receiving antenna via a variety of paths. Each will have a slightly different path length and this will mean that the signals will not reach the receiver with the same phase. As a result some will reinforce the strength of the overall signal while others will interfere and reduce the overall level. This effect can be noticed when an aircraft flies overhead and the overall strength of a signal varies as the aircraft moves and the path length of the signal reflected from it changes. This causes the signal to flutter. Tall building Reflected signals Transmitter Receiver Direct signal Tall building Direct signal Reflected signal Figure 2.11 Multiple paths lengths of a received signal arising form reflections 30 Newnes Guide to Radio and Communications Technology Not only are signals reflected by visible objects, but areas like the atmosphere have a significant effect on signals, reflecting and refracting them, and enabling them to travel over distances well beyond the line of sight. Before investigating the different ways in which this can happen it is first necessary to take a look at the atm