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 30 cm, frequencies below 1 GHz. 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 300 MHz to 3 GHz, hence wavelengths of 10 cm to 1m. This overlaps with the microwave region.
- Very high frequency (VHF): Frequencies of 30 MHz to 300 MHz, hence wavelengths of 1m to 10m.
- High frequency (HF): Frequencies of 3 MHz to 30 MHz, hence wavelengths of 10m to 100 km.
- Medium frequency (MF): Frequencies of 300 kHz to 3 MHz, hence wavelengths of 100m to 1 km. These are also called hectometric waves (hectometre = 100m).
- Low frequency (LF): Frequencies of 30 kHz to 300 kHz, hence wavelengths of 1 km to 10 km.
- Very low frequency (VLF): Frequencies of 10 kHz to 30 kHz, hence wavelengths of 10 km to 30 km.
- Ultra low frequency (ULF): Frequencies of 300 Hz to 10 kHz, hence wavelengths of 30 km to 1000 km; these are mainly used for long-distance underwater transmission.
- Extremely low frequency (ELF): The lowest frequencies (below 300 Hz; wavelength > 1000 km).
Sometimes a division by wavelength is used:
- Short wave (SW): Wavelengths of 10-200m hence frequencies of 1.5-33 MHz; there are several sub-bands.
- Medium wave (MW): Wavelengths of 200-1000m hence frequencies of 300 kHz-1.5 MHz.
- Long wave (LW): Wavelengths > 1000m hence frequencies < 300 kHz.
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 100 MHz (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 200 MHz (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 10-33 MHz. The planet Jupiter radiates strongly at these wavelengths.