Radio spectrum

[6] The lowest frequencies used for radio communication are limited by the increasing size of transmitting antennas required.

[6] The size of antenna required to radiate radio power efficiently increases in proportion to wavelength or inversely with frequency.

Below about 10 kHz (a wavelength of 30 km), elevated wire antennas kilometers in diameter are required, so very few radio systems use frequencies below this.

At 30 GHz, useful communication is limited to about 1 km, but as frequency increases the range at which the waves can be received decreases.

In the terahertz band above 300 GHz, the radio waves are attenuated to zero within a few meters due to the absorption of electromagnetic radiation by the atmosphere (mainly due to ozone, water vapor and carbon dioxide), which is so great that it is essentially opaque to electromagnetic emissions, until it becomes transparent again near the near-infrared and optical window frequency ranges.

2.1 states that "the radio spectrum shall be subdivided into nine frequency bands, which shall be designated by progressive whole numbers in accordance with the following table".

This convention began around World War II with military designations for frequencies used in radar, which was the first application of microwaves.

Since VHF and UHF frequencies are desirable for many uses in urban areas, in North America some parts of the former television broadcasting band have been reassigned to cellular phone and various land mobile communications systems.

From the very early days of radio, large oceangoing vessels carried powerful long-wave and medium-wave transmitters.

High-frequency allocations are still designated for ships, although satellite systems have taken over some of the safety applications previously served by 500 kHz and other frequencies.

Marine VHF radio is used in coastal waters and relatively short-range communication between vessels and to shore stations.

The ISM bands were initially reserved for non-communications uses of RF energy, such as microwave ovens, radio-frequency heating, and similar purposes.

However, in recent years the largest use of these bands has been by short-range low-power communications systems, since users do not have to hold a radio operator's license.

Cordless telephones, wireless computer networks, Bluetooth devices, and garage door openers all use the ISM bands.

Bands of frequencies, especially in the VHF and UHF parts of the spectrum, are allocated for communication between fixed base stations and land mobile vehicle-mounted or portable transceivers.

Police radio and other public safety services such as fire departments and ambulances are generally found in the VHF and UHF parts of the spectrum.

The demand for mobile telephone service has led to large blocks of radio spectrum allocated to cellular frequencies.