ABSTRACT
Reflection occurs when a plane wave meets a plane object that is large relative to the wavelength of the signal. In such cases the wave is reflected back with minimal distortion and without any change in velocity. The effect is similar to the reflection of a beam of light when it arrives at a mirrored surface. Refraction occurs when a wave moves from one medium into another in which it travels at a different speed. For example, when moving from a more dense to a less dense medium the wave is bent away from the normal (i.e. an imaginary line constructed at right angles to the boundary). Conversely, when moving from a less dense to a more dense medium, a wave will bend towards the normal. The effect is similar to that experienced by a beam of light when it encounters a glass prism. Diffraction occurs when a wave meets an edge (i.e. a sudden impenetrable surface discontinuity) which has dimensions that are large relative to the wavelength of the signal. In such cases the wave is bent so that it follows the profile of the discontinuity. Diffraction occurs more readily at lower frequencies (typically VHF and below). An example of diffraction is the bending experienced by VHF broadcast signals when they encounter a sharply defined mountain ridge. Such signals can be received at some distance beyond the ‘knife edge’ even though they are well beyond the normal LOS range. Scattering occurs when a wave encounters one or more objects in its path having a size that is a fraction of the wavelength of the signal. When a wave encounters an obstruction of this type it will become fragmented and re-radiated over a wide angle. Scattering occurs more readily at higher frequencies (typically VHF and above) and regularly occurs in the troposphere at UHF and EHF. Radio signals can also be directed upwards (by suitable choice of antenna) so that signals enter
Figure 1.6 Constituents of a ground wave
