Characteristics of different frequencies
The electromagnetic spectrum covers a huge range of frequencies and wavelengths, from the audible “mains hum” generated in some mains-powered electrical equipment, to X-rays and gamma rays. Visible light is part of the spectrum, towards the upper end of the frequency range (from 384,000 GHz to 769,000 GHz).
Radio waves have much lower frequencies, from about 3kHz to 30GHz, and their wavelengths range from several kilometres to a few centimetres. Different frequencies have different characteristics, which make them suitable for different purposes, so they are often subdivided into different “bands”, with descriptive names such as VLF (Very Low Frequency) and SHF (Super High Frequency).
In general, lower frequencies are more susceptible to the effects known as diffraction and refraction, in which the waves are bent when they pass through or along the boundary between different media (such as between air and water).
This means that low frequencies (VLF and LF) are good for long range broadcasting and radio navigation systems, because they are easily diffracted, so they can curve around obstructions or over the horizon.
Medium frequencies (MF) are slightly less good at following the Earth’s curvature, so they are used for some medium-range navigational aids, such as aeronautical radio beacons, for regional broadcasting, and for medium range communications.
High frequencies (HF) don’t bend over the horizon to any significant extent. The earth‘s atmosphere, however, contains a number of electrically-charged layers, known as the ionosphere. These layers are capable of refracting HF radio waves to such an extent that they bounce back to ground level. By carefully choosing the right frequency to make best use of the ionosphere, HF can be used for world-wide broadcasting and communication.
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