Theradio spectrum is the part of theelectromagnetic spectrum withfrequencies from 3 KHz to 3,000 GHz (3 THz). Electromagnetic waves in this frequency range, calledradio waves, are widely used in modern technology, particularly intelecommunication. To preventinterference between different users, the generation andtransmission of radio waves is strictly regulated by national laws, coordinated by an international body, theInternational Telecommunication Union (ITU).^[1]

Different parts of the radio spectrum are allocated by the ITU for different radio transmission technologies and applications; some 40radiocommunication services are defined in the ITU'sRadio Regulations (RR).^[2] In some cases, parts of the radio spectrum are sold or licensed to operators of private radio transmission services (for example, cellular telephone operators or broadcast television stations). Ranges of allocated frequencies are often referred to by their provisioned use (for example, cellular spectrum or television spectrum).^[3] Because it is a fixed resource which is in demand by an increasing number of users, the radio spectrum has become increasingly congested in recent decades, and the need to utilize it more effectively is driving modern telecommunications innovations such astrunked radio systems,spread spectrum,ultra-wideband,frequency reuse,dynamic spectrum management, frequency pooling, andcognitive radio.

Thefrequency boundaries of the radio spectrum are a matter of convention in physics and are somewhat arbitrary. Since radio waves are the lowest frequency category ofelectromagnetic waves, there is no lower limit to the frequency of radio waves.^[4] Radio waves are defined by the ITU as: "electromagnetic waves of frequencies arbitrarilylower than 3000 GHz, propagated in space without artificial guide".^[5] At the high frequency end the radio spectrum is bounded by theinfrared band. The boundary between radio waves and infrared waves is defined at different frequencies in different scientific fields. Theterahertz band, from 300 gigahertz to 3 terahertz, can be considered either as microwaves or infrared. It is the highest band categorized as radio waves by theInternational Telecommunication Union.^[4] but spectroscopic scientists consider these frequencies part of thefar infrared and mid infrared bands.

Because it is a fixed resource, thepractical limits and basic physical considerations of the radio spectrum, the frequencies which are useful forradio communication, are determined by technological limitations which are impossible to overcome.^[6] So although the radio spectrum is becoming increasingly congested, there is no possible way to add additional frequencybandwidth outside of that currently in use.^[6] The lowest frequencies used for radio communication are limited by the increasing size of transmittingantennas required.^[6] The size of antenna required to radiate radio power efficiently increases in proportion towavelength 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. A second limit is the decreasingbandwidth available at low frequencies, which limits thedata rate that can be transmitted.^[6] Below about 30 kHz, audio modulation is impractical and only slow baud rate data communication is used. The lowest frequencies that have been used for radio communication are around 80 Hz, inELFsubmarine communications systems built by a few nations' navies to communicate with their submerged submarines hundreds of meters underwater. These employ hugeground dipole antennas 20–60 km long excited by megawatts of transmitter power, and transmit data at an extremely slow rate of about 1 bit per minute (17millibits per second, or about 5 minutes per character).

The highest frequencies useful for radio communication are limited by the absorption of microwave energy by the atmosphere.^[6] As frequency increases above 30 GHz (the beginning of themillimeter wave band), atmospheric gases absorb increasing amounts of power, so the power in a beam of radio waves decreases exponentially with distance from the transmitting antenna. 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 theterahertz band above 300 GHz, the radio waves are attenuated to zero within a few meters due to theabsorption of electromagnetic radiation by the atmosphere (mainly due toozone,water vapor andcarbon dioxide), which is so great that it is essentially opaque toelectromagnetic emissions, until it becomes transparent again near thenear-infrared andoptical window frequency ranges.^[7][8]

Aradio band is a smallfrequency band (a contiguous section of the range of the radio spectrum) in whichchannels are usually used or set aside for the same purpose. To prevent interference and allow for efficient use of the radio spectrum, similar services are allocated in bands. For example, broadcasting, mobile radio, or navigation devices, will be allocated in non-overlapping ranges of frequencies.

For each radio band, the ITU has aband plan (orfrequency plan) which dictates how it is to be used and shared, to avoidinterference and to setprotocol for thecompatibility oftransmitters andreceivers.^[9]

Each frequency plan defines the frequency range to be included, howchannels are to be defined, and what will be carried on those channels. Typical definitions set forth in a frequency plan are:

• numbering scheme – which channel numbers or letters (if any) will be assigned
• center frequencies – how far apart thecarrier wave for each channel will be
• bandwidth and/ordeviation – how wide each channel will be
• spectral mask – how extraneoussignals will beattenuated by frequency
• modulation – what type will be used or are permissible
• content – what types of information are allowed, such asaudio orvideo,analog ordigital
• licensing – what the procedure will be to obtain abroadcast license

The actual authorized frequency bands are defined by theITU^[10] and the local regulating agencies like the USFederal Communications Commission (FCC)^[11] and voluntary best practices help avoid interference.^[12]

As a matter of convention, the ITU divides the radio spectrum into 12 bands, each beginning at awavelength which is a power of ten (10ⁿ) metres, with corresponding frequency of 3×10⁸⁻ⁿ hertz, and each covering a decade of frequency or wavelength. Each of these bands has a traditional name. For example, the termhigh frequency (HF) designates the wavelength range from 100 to 10 metres, corresponding to a frequency range of 3 to 30 MHz. This is just a symbol and is not related to allocation; the ITU further divides each band into subbands allocated to different services. Above 300 GHz, the absorption ofelectromagnetic radiation byEarth's atmosphere is so great that the atmosphere is effectively opaque, until it becomes transparent again in thenear-infrared andoptical window frequency ranges.

TheseITU radio bands are defined in theITURadio Regulations. Article 2, provision No. 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".^[13]

The table originated with a recommendation of the fourthCCIR meeting, held in Bucharest in 1937, and was approved by the International Radio Conference held at Atlantic City, NJ in 1947. The idea to give each band a number, in which the number is the logarithm of the approximate geometric mean of the upper and lower band limits in Hz, originated with B. C. Fleming-Williams, who suggested it in a letter to the editor ofWireless Engineer in 1942. For example, the approximate geometric mean of band 7 is 10 MHz, or 10⁷ Hz.^[14]

The band name "tremendously low frequency" (TLF) has been used for frequency and wavelength of 1–3 Hz | 300,000–100,000 km (1000 Mm),^[15] but the term has not been defined by the ITU.^[16]

Frequency bands in themicrowave range are designated by letters. This convention began around World War II with military designations for frequencies used inradar, which was the first application of microwaves. There are several incompatible naming systems for microwave bands, and even within a given system the exact frequency range designated by a letter may vary somewhat between different application areas. One widely used standard is theIEEE radar bands established by the USInstitute of Electrical and Electronics Engineers.

A frequency of 1–3 Hz^[15] has been called TLF but the term has not been defined by the ITU.^[25]

Radio has many practical applications, which include broadcasting, voice communication, data communication, radar, radiolocation, medical treatments, and remote control.

• ITU-R Recommendation V.431:Nomenclature of the frequency and wavelength bands used in telecommunications.International Telecommunication Union, Geneva.
• IEEE Standard 521-2002: Standard Letter Designations for Radar-Frequency Bands
• AFR 55-44/AR 105-86/OPNAVINST 3430.9A/MCO 3430.1, 27 October 1964 superseded by AFR 55-44/AR 105-86/OPNAVINST 3430.1A/MCO 3430.1A, 6 December 1978: Performing Electronic Countermeasures in the United States and Canada, Attachment 1,ECM Frequency Authorizations.

• UnwantedEmissions.com A reference to radio spectrum allocations.
• "Radio spectrum: a vital resource in a wireless world" European Commission policy.

• Radio spectrum

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