Free-space waves
Ionospheric waves
Tropospheric waves
Ground waves
Radio waves are a form of electromagnetic radiation which lies at the bottom end of the spectrum below visible light. Radio waves have frequencies ranging from almost zero Hertz up to hundreds of Giga Hertz. The spectrum is usually divided up into bands, some of which are: Very Low Frequency (VLF); Low Frequency (LF); Medium Frequency (MF); High Frequency (HF); Very High Frequency (VHF); Ultra High Frequency (UHF). There are additional bands both above and below these ranges, but these will not be covered here.
In general, VLF is used for long distance communications over thousands of miles or for special-purpose short distance applications such as for hearing aid inductive loops (public address systems can couple to hearing aids using VLF). LF is used in Europe for longwave broadcasting. MF, also known as mediumwave, is used for broadcasting throughout the world. HF, or shortwave, has many applications including broadcasting, long distance communications and over the horizon radar. It is capable of supporting worldwide communications. VHF and UHF is used for broadcasting and mobile communications mainly over short distances (typically up to 100 kms or more).
Conventional radios cover the spectrum from VLF to UHF. This part of the electromagnetic spectrum contains most of the man-made transmissions that exist. Specialist receivers and aerial systems are needed for the extreme limits of the radio spectrum; issues raised by this kind of operation will not be explored here.
Like all electromagnetic radiation, radio is affected by the medium it propagates through. The type of propagation that can be supported depends on the frequency and other circumstances. This has a bearing on what you can hear with a radio receiver.
The known modes of propagation are as follows:
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