Electromagnetic radiation: 5. Radio
The existence of radio waves was predicted by James Clerk Maxwell in 1864. Their properties were investigated by Heinrich Hertz (after whom the hertz is named) in 1885-9.
With the splitting-off of microwaves as a separate band, radio waves are now regarded as having wavelengths exceeding 30cm, frequencies below 1GHz. There is no upper limit to the wavelength hence no lower limit to the frequency. The standard divisions are:
- Ultra high frequency (UHF): Frequencies of 300MHz to 3GHz, hence wavelengths of 10cm to 1m. This overlaps with the microwave region.
- Very high frequency (VHF): Frequencies of 30MHz to 300MHz, hence wavelengths of 1m to 10m.
- High frequency (HF): Frequencies of 3MHz to 30MHz, hence wavelengths of 10m to 100km.
- Medium frequency (MF): Frequencies of 300khz to 3MHz, hence wavelengths of 100m to 1km. These are also called hectometric waves (hectometre = 100m).
- Low frequency (LF): Frequencies of 30khz to 300kHz, hence wavelengths of 1km to 10km.
- Very low frequency (VLF): Frequencies of 10khz to 30kHz, hence wavelengths of 10km to 30km.
- Ultra low frequency (ULF): Frequencies of 300Hz to 10khz, hence wavelengths of 30km to 1000km; these are mainly used for long-distance underwater transmission.
- Extremely low frequency (ELF): The lowest frequencies (below 300 Hz; wavelength > 1000km).
Sometimes a division by wavelength is used:
- Short wave (SW): Wavelengths of 10-200m hence frequencies of 1.5-33MHz; there are several sub-bands.
- Medium wave (MW): Wavelengths of 200-1000m hence frequencies of 300kHz-1.5MHz.
- Long wave (LW): Wavelengths > 1000m hence frequencies < 300kHz.
Other terms found are:
- Frequency modulation (FM): This is not a frequency band, but a way of coding the audio signal onto a radio wave. However, quite a high frequency is necessary for FM to work well, and in practice frequencies of close to 100MHz (wavelength 3m, the middle of the VHF band) are used.
- Digital radio: Again, this is not a frequency band. Typically, frequencies of the order of 200MHz (wavelength 1.5m, towards the short wavelength or high frequency end of the VHF band) are used.
- Decametric: Wavelengths of 10-30m (decametre = 10m) hence frequencies of 10-33MHz. The planet Jupiter radiates strongly at these wavelengths.