Frequency band designators.
Abbreviation Frequencies Wavelengths
VLF 3-30 kHz 100-10 km
LF 30-300 kHz 10000-1000m
MF 300-3000 kHz 1000-100m
HF 3-30 MHz 100-10m
VHF 30-300 MHz 10-1m
UHF 300-3000 MHz 100-10 cm
SHF 3-30 GHz 10-1 cm
EHF 30-300 GHz 100-10 mm
Radio waves of VHF bands and above are generally reflected by solid material, although lower frequencies are absorbed. However, the surface of the earth also affects waves travelling parallel or almost parallel to it. At low frequencies, some of the wave energy is lost in inducing currents in the surface, which also slows the wave down. The amount lost depends on the material and its condition affecting its conductivity; dry sand produces greater attenuation and speed reduction than wet loam, and sea water produces less attenuation than either. Obstacles in the path of a radio wave also affect its path. Radio waves tend to be reflected by objects larger than about half their wavelength. At higher frequencies, most obstacles will cause reflection, or absorption, and therefore shadows behind them, but at lower frequencies the waves will curve around a small obstacle, even a hill. This is called ‘diffraction’, and can be considered as the obstacle creating friction in the part of the wave close to it, causing the wave to curve towards it as it passes. The amount of diffraction is inversely proportional to the frequency.
The lower atmosphere
The molecules of gas in the atmosphere absorb some of the energy in the radio wave. This attenuation depends on the wavelength of the signal. The shorter the wavelength (or the higher the frequency) the greater the atmospheric absorption. The molecules of gas in the air can also reflect some radio energy, especially in the UHF band and above. A directional receiver over the horizon can collect any scattered signal which continues in its direction. However, this is not usually employed for aviation purposes. The density of air reduces with pressure, but increases with temperature. The radio signal travels faster in a less dense medium, and if a wave passes through gas of changing density at an angle, it will curve towards the higher density. Density normally reduces slowly with altitude in the troposphere, where the pressure reduction has more effect than the temperature reduction, and in the stratosphere an increasing temperature with altitude reduces the density further. This effect increases the bending of radio waves around the earth s surface, and can also produce more spectacular results. The speed of radio waves also changes with the different gases. Water vapour is less dense than dry air, and changes in humidity suggest a similar bending towards less humid air. However, a high water vapour content actually encourages refraction (bending) towards it.
Above the tropopause lies the stratosphere, and above that a region called tne Ionosphere. Here radiation from the sun has a considerable effect on the molecules of a thin atmosphere, and electrons are set free from their atoms. The free electrons provide several electrically charged layers in this ionosphere, but their existence depends on excitation from the sun’s rays. The number of free electrons, and their distribution, depend on the angle at which the sun’s rays meet the ionosphere, as well as the intensity of the rays themselves. As the density of free electrons changes, it changes the ‘refractive index’ of the air. Electromagnetic waves passing through the layers in the ionosphere at an angle are refracted, or bent away from areas of higher electron density, which happen to be in the higher part of the ionosphere. Therefore radio waves are bent towards the earth. The amount of refraction depends on three factors, the frequency of the waves, the change in electron density, and the angle at which the waves hit the layer. The waves are also attenuated, by an amount depending on the electron density and the frequency.
The waves reaching a’ receiver in a straight line (line if sight) are called direct waves. All frequencies can be received along direct waves. Signals are attenuated by spreadout in accordance with the inverse square law.
Direct waves are regarded as the sole means of propagation of all signals in the VHF band and higher frequencies, and allow lower frequency signals to be received at short range.
Waves can be reflected by any object whose size is more than half their wavelength. This is usually a hindrance to efficient propagation, but radar of course uses the principle of reflection to work. Direct waves and waves reflected from the ground are together called ‘space waves’.
Surface waves are those which are bent around the surface of the earth. At HF frequencies or lower, the waves are refracted sufficiently to follow the curvature of the earth. However, there is considerable absorption by the earth’s surface, and the higher the frequency the more absorption takes place. The range of a signal therefore is indirectly proportional to its frequency, or directly proportional to its wavelength, as well as being directly proportional to the power at the transmitter. Surface waves and space waves are together called ‘ground waves.
The waves refracted by the ionosphere are called sky waves. The ionosphere absorbs and refracts signals by an amount directly proportional to their wavelength.