Radio spectrum in the context of Electromagnetic interference

Radar is a system that uses radio waves to determine the distance (ranging), direction (azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations and terrain. The term RADAR was coined in 1940 by the United States Navy as an acronym for "radio detection and ranging". The term radar has since entered English and other languages as an anacronym, a common noun, losing all capitalization.

A radar system consists of a transmitter producing electromagnetic waves in the radio or microwave domain, a transmitting antenna, a receiving antenna (often the same antenna is used for transmitting and receiving) and a receiver and processor to determine properties of the objects. Radio waves (pulsed or continuous) from the transmitter reflect off the objects and return to the receiver, giving information about the objects' locations and speeds. This device was developed secretly for military use by several countries in the period before and during World War II. A key development was the cavity magnetron in the United Kingdom, which allowed the creation of relatively small systems with sub-meter resolution.

PKS 1127−145 is a radio-loud quasar located in the constellation of Crater. This is a Gigahertz Peaked Spectrum object with a redshift of (z) 1.187, first discovered by astronomers in 1966. Its radio spectrum appears to be flat making it a flat-spectrum radio quasar, or an FSRQ in short.

Amateur radio, also known as ham radio, is the use of the radio spectrum for non-commercial communication, technical experimentation, self-training, recreation, radiosport, contesting, and emergency communications. In 1927 a radio amateur was defined as "a duly authorized person interested in radioelectric practice with a purely personal aim and without pecuniary interest," meaning without monetary or similar reward. The definition distinguished amateur activity from commercial broadcasting, public safety services, and professional two-way radio uses such as maritime, aviation, and taxi communication.

The amateur radio service (amateur service and amateur-satellite service) is established by the International Telecommunication Union (ITU) through its Radio Regulations. National governments set technical and operational rules for transmissions and issue individual station licences with unique call signs. Call signs must be used in transmissions, at least every ten minutes and at the end. Amateur radio operators must hold an amateur radio licence, granted after an examination that tests knowledge of radio theory, electronics, and national regulations.

Extremely high frequency (EHF) is the International Telecommunication Union (ITU) designation for the band in the electromagnetic spectrum from 30 to 300 gigahertz (GHz). It is in the microwave part of the radio spectrum, between the super high frequency band and the terahertz band. Radio waves in this band have wavelengths from ten to one millimeter, so it is also called the millimeter band and radiation in this band is called millimeter waves, sometimes abbreviated MMW or mmWave. Some define mmWaves as starting at 24 GHz, thus covering the entire FR2 band (24.25 to 71 GHz), among others.

Compared to lower bands, radio waves in this band have high atmospheric attenuation: they are absorbed by the gases in the atmosphere. Absorption increases with frequency until at the top end of the band the waves are attenuated to zero within a few meters. Absorption by humidity in the atmosphere is significant except in desert environments, and attenuation by rain (rain fade) is a serious problem even over short distances. However the short propagation range allows smaller frequency reuse distances than lower frequencies. The short wavelength allows modest size antennas to have a small beam width, further increasing frequency reuse potential. Millimeter waves are used for military fire-control radar, airport security scanners, short range wireless networks, and scientific research.

A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or parabolic dish. The main advantage of a parabolic antenna is that it has high directivity. It functions similarly to a searchlight or flashlight reflector to direct radio waves in a narrow beam, or receive radio waves from one particular direction only. Parabolic antennas have some of the highest gains, meaning that they can produce the narrowest beamwidths, of any antenna type. In order to achieve narrow beamwidths, the parabolic reflector must be much larger than the wavelength of the radio waves used, so parabolic antennas are used in the high frequency part of the radio spectrum, at UHF and microwave (SHF) frequencies, at which the wavelengths are small enough that conveniently sized reflectors can be used.

Parabolic antennas are used as high-gain antennas for point-to-point communications, in applications such as microwave relay links that carry telephone and television signals between nearby cities, wireless WAN/LAN links for data communications, satellite communications, and spacecraft communication antennas. They are also used in radio telescopes.

Digital television (DTV) is the transmission of television signals using digital encoding, in contrast to the earlier analog television technology which used analog signals. In the 2000s it was represented as the first significant evolution in television technology since color television in the 1950s. Modern digital television is transmitted in high-definition television (HDTV) with greater resolution than analog TV. It typically uses a widescreen aspect ratio (commonly 16:9) in contrast to the narrower format (4:3) of analog TV. It makes more economical use of scarce radio spectrum space; it can transmit up to seven channels in the same bandwidth as a single analog channel, and provides many new features that analog television cannot. A transition from analog to digital broadcasting began around 2000. Different digital television broadcasting standards have been adopted in different parts of the world; below are the more widely used standards:

• Digital Video Broadcasting (DVB) uses coded orthogonal frequency-division multiplexing (OFDM) modulation and supports hierarchical transmission. This standard has been adopted in Europe, Africa, Asia and Australia, for a total of approximately 60 countries.
• Advanced Television System Committee (ATSC) standard uses eight-level vestigial sideband (8VSB) for terrestrial broadcasting. This standard has been adopted by 9 countries: the United States, Canada, Mexico, South Korea, Bahamas, Jamaica, the Dominican Republic, Haiti and Suriname.
• Integrated Services Digital Broadcasting (ISDB) is a system designed to provide good reception to fixed receivers and also portable or mobile receivers utilizing OFDM and two-dimensional interleaving. It supports hierarchical transmission of up to three layers and uses MPEG-2 video and Advanced Audio Coding. This standard has been adopted in Japan and the Philippines. ISDB-T International is an adaptation of this standard using H.264/MPEG-4 AVC, which has been adopted in most of South America as well as Botswana and Angola. 1seg (1-segment) is a special form of ISDB. Each channel is further divided into 13 segments. Twelve are allocated for HDTV and the other for narrow-band receivers such as mobile televisions and cell phones.
• Digital Terrestrial Multimedia Broadcast (DTMB) adopts time-domain synchronous (TDS) OFDM technology with a pseudo-random signal frame to serve as the guard interval (GI) of the OFDM block and the training symbol. The DTMB standard has been adopted in China, including Hong Kong and Macau.
• Digital Multimedia Broadcasting (DMB) is a digital radio transmission technology developed and adopted in South Korea as part of the national information technology project for sending multimedia such as TV, radio and datacasting to mobile devices such as mobile phones, laptops and GPS navigation systems.

The IEEE K-band is a portion of the radio spectrum in the microwave range of frequencies from 18 to 27 gigahertz (GHz). The range of frequencies in the center of the K-band between 18 and 26.5 GHz are absorbed by water vapor in the atmosphere due to its resonance peak at 22.24 GHz, 1.35 cm (0.53 in). Therefore these frequencies experience high atmospheric attenuation and cannot be used for long-distance applications. For this reason, the original K-band has been split into three bands: Ka-band, K-band, and Ku-band as detailed below.

The K stands for Kurz, the German word for 'short'.

Centaurus A (also known as NGC 5128 or Caldwell 77) is a galaxy in the constellation of Centaurus. It was discovered in 1826 by Scottish astronomer James Dunlop from his home in Parramatta, in New South Wales, Australia. There is considerable debate in the literature regarding the galaxy's fundamental properties such as its Hubble type (lenticular galaxy or a giant elliptical galaxy) and distance (11–13 million light-years). It is the closest radio galaxy to Earth, as well as the closest BL Lac object, so its active galactic nucleus has been extensively studied by professional astronomers. The galaxy is also the fifth-brightest in the sky, making it an ideal amateur astronomy target. It is only visible from the southern hemisphere and low northern latitudes.

The center of the galaxy contains a supermassive black hole with a mass of 55 million solar masses, which ejects a relativistic jet that is responsible for emissions in the X-ray and radio wavelengths. By taking radio observations of the jet separated by a decade, astronomers have determined that the inner parts of the jet are moving at about half of the speed of light. X-rays are produced farther out as the jet collides with surrounding gases, resulting in the creation of highly energetic particles. The X-ray jets of Centaurus A are thousands of light-years long, while the radio jets are over a million light-years long.

Digital radio is the use of digital technology to transmit or receive across the radio spectrum. Digital transmission by radio waves includes digital broadcasting, and especially digital audio radio services. This should not be confused with Internet radio which also is digital but not transmitted by radio waves in the radio spectrum.

A broadcast license is a type of spectrum license granting the licensee permission to use a portion of the radio frequency spectrum in a given geographical area for broadcasting purposes. The licenses generally include restrictions, which vary from band to band.

Spectrum may be divided according to use. As indicated in a graph from the National Telecommunications and Information Administration (NTIA), frequency allocations may be represented by different types of services which vary in size. Many options exist when applying for a broadcast license; the FCC determines how much spectrum to allot to licensees in a given band, according to what is needed for the service in question.

The digital television transition, also called the digital switchover (DSO), the analogue switch/sign-off (ASO), the digital migration, or the analogue shutdown, is the process in which older analogue television broadcasting technology is converted to and replaced by digital television. Conducted by individual nations on different schedules, this primarily involves the conversion of analogue terrestrial television broadcasting infrastructure to Digital terrestrial television (DTT), a major benefit being extra frequencies on the radio spectrum and lower broadcasting costs, as well as improved viewing qualities for consumers.

The transition may also involve analogue cable conversion to digital cable or Internet Protocol television, as well as analogue to digital satellite television. Transition of land based broadcasting had begun in some countries around 2000. By contrast, transition of satellite television systems was well underway or completed in many countries by this time. It is an involved process because the existing analogue television receivers owned by viewers cannot receive digital broadcasts; viewers must either purchase new digital TVs, or digital converter boxes which have a digital tuner and change the digital signal to an analogue signal or some other form of a digital signal (i.e. HDMI) which can be received on the older TV. Usually during a transition, a simulcast service is operated where a broadcast is made available to viewers in both analogue and digital at the same time. As digital becomes more popular, it is expected that the existing analogue services will be removed. In most places this has already happened, where a broadcaster has offered incentives to viewers to encourage them to switch to digital. Government intervention usually involves providing some funding for broadcasters and, in some cases, monetary relief to viewers, to enable a switchover to happen by a given deadline. In addition, governments can also have a say with the broadcasters as to what digital standard to adopt – either DVB-T2 ATSC Governments can also require all receiving equipment sold in a country to support the necessary digital 'tuner'.

The ISM radio bands are portions of the radio spectrum reserved internationally for industrial, scientific and medical (ISM) purposes, excluding applications in telecommunications. Examples of applications for the use of radio frequency (RF) energy in these bands include RF heating, microwave ovens, and medical diathermy machines. The powerful emissions of these devices can create electromagnetic interference and disrupt radio communication using the same frequency, so these devices are limited to certain bands of frequencies. In general, communications equipment operating in ISM bands must tolerate any interference generated by ISM applications, and users have no regulatory protection from ISM device operation in these bands.

Despite the intent of the original allocations, in recent years the fastest-growing use of these bands has been for short-range, low-power wireless communications systems, since these bands are often approved for such devices, which can be used without a government license, as would otherwise be required for transmitters; ISM frequencies are often chosen for this purpose as they already must tolerate interference issues. Cordless phones, Bluetooth devices, near-field communication (NFC) devices, garage door openers, baby monitors, and wireless computer networks (Wi-Fi) may all use the ISM frequencies, although these low-power transmitters are not considered to be ISM devices.

In radio, longwave (also spelled long wave or long-wave and commonly abbreviated LW) is the part of the radio spectrum with wavelengths longer than what was originally called the medium-wave (MW) broadcasting band. The term is historic, dating from the early 20th century, when the radio spectrum was considered to consist of LW, MW, and short-wave (SW) radio bands. Most modern radio systems and devices use wavelengths which would then have been considered 'ultra-short' (i.e. VHF, UHF, and microwave).

In contemporary usage, the term longwave is not defined precisely, and its intended meaning varies. It may be used for radio wavelengths longer than 1,000 m i.e. frequencies smaller than 300 kilohertz (kHz), including the International Telecommunication Union (ITU) low frequency (LF, 30–300 kHz) and very low frequency (VLF, 3–30 kHz) bands. Sometimes the upper limit is taken to be higher than 300 kHz, but not above the start of the medium wave broadcast band at 520 kHz.

The L band is the Institute of Electrical and Electronics Engineers (IEEE) designation for the range of frequencies in the radio spectrum from 1 to 2 gigahertz (GHz). This is at the top end of the ultra high frequency (UHF) band, at the lower end of the microwave range.