Radio

Source: Wikipedia, the free encyclopedia.
A variety of radio antennas on Sandia Peak near Albuquerque, New Mexico, US

Radio is the technology of

gigahertz (GHz). They are generated by an electronic device called a transmitter connected to an antenna which radiates the waves. They are received by another antenna connected to a radio receiver. In addition to communication, radio is used for radar, radio navigation, remote control, remote sensing
, and other applications.

In radio communication, used in

keyless entry systems
, radio signals transmitted from a controller device control the actions of a remote device.

The existence of radio waves was first proven by German physicist

Harding-Cox presidential election were broadcast by Westinghouse Electric and Manufacturing Company in Pittsburgh, under the call sign KDKA.[8]

The emission of radio waves is regulated by law, coordinated by the International Telecommunication Union (ITU), which allocates frequency bands in the radio spectrum for various uses.

Etymology

The word "radio" is derived from the Latin word "radius", meaning "spoke of a wheel, beam of light, ray". It was first applied to communications in 1881 when, at the suggestion of French scientist Ernest Mercadier [fr], Alexander Graham Bell adopted "radiophone" (meaning "radiated sound") as an alternate name for his photophone optical transmission system.[9][10]

Following

telegraph signals by radio waves,[4] so radio communication was first called "wireless telegraphy". Up until about 1910 the term "wireless telegraphy" also included a variety of other experimental systems for transmitting telegraph signals without wires, including electrostatic induction, electromagnetic induction and aquatic and earth conduction, so there was a need for a more precise term referring exclusively to electromagnetic radiation.[12][13]

The French physicist Édouard Branly, who in 1890 developed the radio wave detecting coherer, called it in French a radio-conducteur.[14][15] The radio- prefix was later used to form additional descriptive compound and hyphenated words, especially in Europe. For example, in early 1898 the British publication The Practical Engineer included a reference to "the radiotelegraph" and "radiotelegraphy".[14][16]

The use of "radio" as a standalone word dates back to at least 30 December 1904, when instructions issued by the British Post Office for transmitting telegrams specified that "The word 'Radio'... is sent in the Service Instructions."[14][17] This practice was universally adopted, and the word "radio" introduced internationally, by the 1906 Berlin Radiotelegraphic Convention, which included a Service Regulation specifying that "Radiotelegrams shall show in the preamble that the service is 'Radio'".[14]

The switch to "radio" in place of "wireless" took place slowly and unevenly in the English-speaking world. Lee de Forest helped popularize the new word in the United States—in early 1907, he founded the DeForest Radio Telephone Company, and his letter in the 22 June 1907 Electrical World about the need for legal restrictions warned that "Radio chaos will certainly be the result until such stringent regulation is enforced."[18] The United States Navy would also play a role. Although its translation of the 1906 Berlin Convention used the terms "wireless telegraph" and "wireless telegram", by 1912 it began to promote the use of "radio" instead. The term started to become preferred by the general public in the 1920s with the introduction of broadcasting.

History

See History of radio, Invention of radio, Timeline of radio, History of broadcasting

spark gap transmitter to send Morse code over long distances. By December 1901, he had transmitted across the Atlantic ocean.[4][5][6][7] Marconi and Karl Ferdinand Braun shared the 1909 Nobel Prize in Physics "for their contributions to the development of wireless telegraphy".[20]

During radio's first two decades, called the

Technology

Radio waves are radiated by electric charges undergoing acceleration.[21][22] They are generated artificially by time varying electric currents, consisting of electrons flowing back and forth in a metal conductor called an antenna.[23][24]

As they travel farther from the transmitting antenna, radio waves spread out so their

high-gain antenna transmits radio waves in a beam in a particular direction, or receives waves from only one direction.[25][26][27]

Radio waves travel at the speed of light in vacuum.[28][29]

The other types of

gamma rays, can also carry information and be used for communication. The wide use of radio waves for telecommunication is mainly due to their desirable propagation properties stemming from their large wavelength.[24]

Radio communication

Radio communication. Information such as sound is converted by a transducer such as a microphone to an electrical signal, which modulates a radio wave produced by the transmitter. A receiver intercepts the radio wave and extracts the information-bearing modulation signal, which is converted back to a human usable form with another transducer such as a loudspeaker.
Comparison of AM and FM modulated radio waves

In radio communication systems, information is carried across space using radio waves. At the sending end, the information to be sent is converted by some type of

radio transmitter. In the transmitter, an electronic oscillator generates an alternating current oscillating at a radio frequency, called the carrier wave because it serves to "carry" the information through the air. The information signal is used to modulate the carrier, varying some aspect of the carrier wave, impressing the information on the carrier. Different radio systems use different modulation methods:[31]

  • AM (amplitude modulation) – in an AM transmitter, the amplitude (strength) of the radio carrier wave is varied by the modulation signal;[31]: 3 
  • FM (frequency modulation) – in an FM transmitter, the frequency of the radio carrier wave is varied by the modulation signal;[31]: 33 
  • FSK (frequency-shift keying) – used in wireless digital devices to transmit digital signals, the frequency of the carrier wave is shifted between frequencies.[31]: 58 
  • OFDM (
    digital audio broadcasting (DAB) to transmit digital data using a minimum of radio spectrum bandwidth. It has higher spectral efficiency and more resistance to fading than AM or FM. In OFDM, multiple radio carrier waves closely spaced in frequency are transmitted within the radio channel, with each carrier modulated with bits from the incoming bitstream so multiple bits are being sent simultaneously, in parallel. At the receiver, the carriers are demodulated and the bits are combined in the proper order into one bitstream.[32]

Many other types of modulation are also used. In some types, a carrier wave is not transmitted but just one or both modulation sidebands.[33]

The modulated carrier is amplified in the transmitter and applied to a transmitting antenna which radiates the energy as radio waves. The radio waves carry the information to the receiver location.[34] At the receiver, the radio wave induces a tiny oscillating

video signal is converted to images by a display, while a digital signal is applied to a computer or microprocessor, which interacts with human users.[31]

The radio waves from many transmitters pass through the air simultaneously without interfering with each other because each transmitter's radio waves oscillate at a different rate, in other words, each transmitter has a different

resonant frequency at which it oscillates. The resonant frequency of the receiver's tuned circuit is adjusted by the user to the frequency of the desired radio station; this is called "tuning". The oscillating radio signal from the desired station causes the tuned circuit to resonate, oscillate in sympathy, and it passes the signal on to the rest of the receiver. Radio signals at other frequencies are blocked by the tuned circuit and not passed on.[35]

Bandwidth

Frequency spectrum of a typical modulated AM or FM radio signal. It consists of a component C at the carrier wave frequency with the information (modulation) contained in two narrow bands of frequencies called sidebands (SB) just above and below the carrier frequency.

A modulated radio wave, carrying an information signal, occupies a range of

bits per second) regardless of where in the radio frequency spectrum it is located, so bandwidth is a measure of information-carrying capacity. The bandwidth required by a radio transmission depends on the data rate of the information (modulation signal) being sent, and the spectral efficiency of the modulation method used; how much data it can transmit in each kilohertz of bandwidth. Different types of information signals carried by radio have different data rates. For example, a television (video) signal has a greater data rate than an audio signal.[31][37]

The

economic good which has a monetary cost and is in increasing demand. In some parts of the radio spectrum, the right to use a frequency band or even a single radio channel is bought and sold for millions of dollars. So there is an incentive to employ technology to minimize the bandwidth used by radio services.[37]

A slow transition from

noise immunity than analog, digital signal processing chips have more power and flexibility than analog circuits, and a wide variety of types of information can be transmitted using the same digital modulation.[31]

Because it is a fixed resource which is in demand by an increasing number of users, the

ITU frequency bands

The ITU arbitrarily divides the radio spectrum into 12 bands, each beginning at a wavelength which is a power of ten (10n) metres, with corresponding frequency of 3 times a power of ten, and each covering a decade of frequency or wavelength.[3][40] Each of these bands has a traditional name:[41]

Band name Abbreviation Frequency Wavelength
Extremely
low frequency
ELF 3–30 Hz 100,000–
10,000 km
Super
low frequency
SLF 30–300 Hz 10,000 –
1,000 km
Ultra
low frequency
ULF 300–
3,000 Hz
1,000–
100 km
Very
low frequency
VLF 3–30 kHz 100–10 km
Low
frequency
LF 30–300 kHz 10–1 km
Medium
frequency
MF 300–
3,000 kHz
1,000–
100 m
Band name Abbreviation Frequency Wavelength
High
frequency
HF 3–30 MHz 100–10 m
Very
high frequency
VHF 30–300 MHz 10–1 m
Ultra
high frequency
UHF 300–
3,000 MHz
100–10 cm
Super
high frequency
SHF 3–30 GHz 10–1 cm
Extremely
high frequency
EHF 30–300 GHz 10–1 mm
Tremendously
high frequency
THF 300–3,000 GHz
(0.3–3.0 THz)
1.0–0.1 mm

It can be seen that the bandwidth, the range of frequencies, contained in each band is not equal but increases exponentially as the frequency increases; each band contains ten times the bandwidth of the preceding band.[42]

The term "tremendously low frequency" (TLF) has been used for wavelengths from 1–3 Hz (300,000–100,000 km),[43] though the term has not been defined by the ITU.[41]

Regulation

The airwaves are a resource shared by many users. Two radio transmitters in the same area that attempt to transmit on the same frequency will interfere with each other, causing garbled reception, so neither transmission may be received clearly.

Interference with radio transmissions can not only have a large economic cost, but it can also be life-threatening (for example, in the case of interference with emergency communications or air traffic control).[44][45]

To prevent interference between different users, the emission of radio waves is strictly regulated by national laws, coordinated by an international body, the International Telecommunication Union (ITU), which allocates bands in the radio spectrum for different uses.[36][3] Radio transmitters must be licensed by governments, under a variety of license classes depending on use, and are restricted to certain frequencies and power levels. In some classes, such as radio and television broadcasting stations, the transmitter is given a unique identifier consisting of a string of letters and numbers called a call sign, which must be used in all transmissions.[46] In order to adjust, maintain, or internally repair radiotelephone transmitters, individuals must hold a government license, such as the general radiotelephone operator license in the US, obtained by taking a test demonstrating adequate technical and legal knowledge of safe radio operation.[47]

Exceptions to the above rules allow the unlicensed operation by the public of low power short-range transmitters in consumer products such as cell phones,

ISM bands, a series of frequency bands throughout the radio spectrum reserved for unlicensed use. Although they can be operated without a license, like all radio equipment these devices generally must be type-approved before the sale.[48]

Applications

Below are some of the most important uses of radio, organized by function.

Broadcasting

commercial advertising or governments. In subscription systems like satellite television and satellite radio the customer pays a monthly fee. In these systems, the radio signal is encrypted and can only be decrypted by the receiver, which is controlled by the company and can be deactivated if the customer does not pay.[51]

Broadcasting uses several parts of the radio spectrum, depending on the type of signals transmitted and the desired target audience.

shortwave bands have a greater potential range but are more subject to interference by distant stations and varying atmospheric conditions that affect reception.[52][53]

In the very high frequency band, greater than 30 megahertz, the Earth's atmosphere has less of an effect on the range of signals, and line-of-sight propagation becomes the principal mode. These higher frequencies permit the great bandwidth required for television broadcasting. Since natural and artificial noise sources are less present at these frequencies, high-quality audio transmission is possible, using frequency modulation.[54][55]

Audio: Radio broadcasting

Radio broadcasting means transmission of audio (sound) to radio receivers belonging to a public audience. Analog audio is the earliest form of radio broadcast. AM broadcasting began around 1920. FM broadcasting was introduced in the late 1930s with improved fidelity. A broadcast radio receiver is called a radio. Most radios can receive both AM and FM.[56]

1100 W AM broadcasting transmitter
Mast radiator antenna of AM radio station
Panasonic AM radio from 1964
  • AM (
    clear-channel stations) are allowed to transmit at 50 kW. AM stations broadcast in monaural audio; AM stereo broadcast standards exist in most countries, but the radio industry has failed to upgrade to them, due to lack of demand.[58]
  • shortwave bands by legacy radio stations. Since radio waves in these bands can travel intercontinental distances by reflecting off the ionosphere using skywave or "skip" propagation, shortwave is used by international stations, broadcasting to other countries.[58][59]
car radio
's interface display
  • FM (
    audio distortion than AM. In the US, radiated power (ERP) of FM stations varies from 6–100 kW.[60]
  • Digital radio involves a variety of standards and technologies for broadcasting digital radio signals over the air. Some systems, such as HD Radio and DRM, operate in the same wavebands as analog broadcasts, either as a replacement for analog stations or as a complementary service. Others, such as DAB/DAB+ and ISDB_Tsb, operate in wavebands traditionally used for television or satellite services.[61]
"Roberts" radio for DAB
  • Digital Audio Broadcasting (DAB) debuted in some countries in 1998. It transmits audio as a digital signal rather than an analog signal as AM and FM do.[62] DAB has the potential to provide higher quality sound than FM (although many stations do not choose to transmit at such high quality), has greater immunity to radio noise and interference, makes better use of scarce radio spectrum bandwidth and provides advanced user features such as electronic program guides. Its disadvantage is that it is incompatible with previous radios so that a new DAB receiver must be purchased.[63] Several nations have set dates to switch off analog FM networks in favor of DAB / DAB+, notably Norway in 2017[64] and Switzerland in 2024.[65]
A single DAB station transmits a 1,500 kHz bandwidth signal that carries from 9–12 channels of digital audio modulated by OFDM from which the listener can choose. Broadcasters can transmit a channel at a range of different bit rates, so different channels can have different audio quality. In different countries DAB stations broadcast in either Band III (174–240 MHz) or L band (1.452–1.492 GHz) in the UHF range, so like FM reception is limited by the visual horizon to about 40 miles (64 km).[66][63]
  • audio data compression algorithm.[70]
The DRM system is designed to be as compatible as possible with existing AM and FM radio transmitters, so that much of the equipment in existing radio stations can continue in use, augmented with DRM modulation equipment.[71][59]
Volkswagen's RNS-510 receiver supports Sirius Satellite Radio.
  • radios in vehicles. Satellite radio uses the 2.3 GHz S band in North America, in other parts of the world, it uses the 1.4 GHz L band allocated for DAB.[72][73]

Video: Television broadcasting

video) signals occupy a wider bandwidth than broadcast radio (audio) signals. Analog television, the original television technology, required 6 MHz, so the television frequency bands are divided into 6 MHz channels, now called "RF channels".[74]

The current television standard, introduced beginning in 2006, is a digital format called

digital cliff" effect. Unlike analog television, in which increasingly poor reception causes the picture quality to gradually degrade, in digital television picture quality is not affected by poor reception until, at a certain point, the receiver stops working and the screen goes black.[75][76]

Television studio control room, Celebro Studios, London
A television broadcasting antenna
A modern flatscreen television receiver
  • fringe reception areas typically require an outdoor antenna mounted on the roof to get adequate reception.[79]
(left) DISH Network's Super Dish 121 mounted on a rooftop. (right) A residential tower block with TV satellite dishes used by various users
  • geostationary orbit 22,200 miles (35,700 km) above the Earth's equator transmits many channels (up to 900) modulated on a 12.2 to 12.7 GHz Ku band microwave downlink signal to a rooftop satellite dish antenna on the subscriber's residence. The microwave signal is converted to a lower intermediate frequency at the dish and conducted into the building by a coaxial cable to a set-top box connected to the subscriber's TV, where it is demodulated and displayed. The subscriber pays a monthly fee.[81][82]

Time

Government

GPS satellites and the Internet Network Time Protocol (NTP) provide equally accurate time standards.[85]

Two-way voice communication

Cellphones typical of Japan in the early 21st century.
Cellular phone tower shared by antennas belonging to 3 different networks.

A

full duplex, a bidirectional link using two radio channels so both people can talk at the same time, as in a cell phone.[86]

(left) 5G millimeter wave antenna, Germany (right) Polish 5G smartphones
  • millimeter wave band, around 28 and 39 GHz. Since these frequencies have a shorter range than previous cellphone bands, the cells will be smaller than the cells in previous cellular networks which could be many miles across. Millimeter-wave cells will only be a few blocks long, and instead of a cell base station and antenna tower, they will have many small antennas attached to utility poles and buildings.[91][92]
Satellite phones, showing the large antennas needed to communicate with the satellite
  • cell towers. They are more expensive than cell phones; but their advantage is that, unlike a cell phone which is limited to areas covered by cell towers, satphones can be used over most or all of the geographical area of the Earth. In order for the phone to communicate with a satellite using a small omnidirectional antenna, first-generation systems use satellites in low Earth orbit, about 400–700 miles (640–1,100 km) above the surface. With an orbital period of about 100 minutes, a satellite can only be in view of a phone for about 4 – 15 minutes, so the call is "handed off" to another satellite when one passes beyond the local horizon. Therefore, large numbers of satellites, about 40 to 70, are required to ensure that at least one satellite is in view continuously from each point on Earth. Other satphone systems use satellites in geostationary orbit in which only a few satellites are needed, but these cannot be used at high latitudes because of terrestrial interference.[93][94]
  • landline telephone in which the handset is portable and communicates with the rest of the phone by a short-range full duplex radio link, instead of being attached by a cord. Both the handset and the base station have low-power radio transceivers that handle the short-range bidirectional radio link.[95] As of 2022, cordless phones in most nations use the DECT transmission standard.[96]
Motorola SCR-536 from WW2, the first walkie-talkie
  • CB, FRS, GMRS, and MURS. Modern digital systems, called trunked radio systems, have a digital channel management system using a control channel that automatically assigns frequency channels to user groups.[97]
Firefighter using modern walkie-talkie
    • Walkie-talkie – a battery-powered portable handheld half-duplex two-way radio, used in land mobile radio systems.[98]
  • air corridors a VHF-AM system using channels between 108 and 137 MHz in the VHF band is used. This system has a typical transmission range of 200 miles (320 km) for aircraft flying at cruising altitude.[99][100] For flights in more remote areas, such as transoceanic airline flights, aircraft use the HF band or channels on the Inmarsat or Iridium satphone satellites.[101] Military aircraft also use a dedicated UHF-AM band from 225.0 to 399.95 MHz.[102]
VHF marine radio on a ship
  • full-duplex, to be compatible with the telephone network, to allow users to make telephone calls through a marine operator.[103]

One-way voice communication

One way, unidirectional radio transmission is called

simplex
.

  • Baby monitor – a crib-side appliance for parents of infants that transmits the baby's sounds to a receiver carried by the parent, so they can monitor the baby while they are in other parts of the house.[105] The wavebands used vary by region, but analog baby monitors generally transmit with low power in the 16, 9.3–49.9 or 900 MHz wavebands, and digital systems in the 2.4 GHz waveband.[106] Many baby monitors have duplex channels so the parent can talk to the baby, and cameras to show video of the baby.[107]
  • ISM bands.[109] European standards also support wireless multichannel audio systems (WMAS) that can better support the use of large numbers of wireless microphones at a single event or venue. As of 2021, U.S. regulators were considering adopting rules for WMAS.[110]

Data communication

  • VoIP), audio, and video content (called streaming media). Security is more of an issue for wireless networks than for wired networks since anyone nearby with a wireless modem can access the signal and attempt to log in. The radio signals of wireless networks are encrypted using WPA.[111]
A laptop (with Wi-Fi module) and a typical home wireless router (on the right) connecting it to the Internet. The laptop shows its own photo
    • ISM bands with OFDM (orthogonal frequency-division multiplexing) modulation to transmit data at high rates. The transmitters in Wi-Fi modems are limited to a radiated power of 200 mW to 1 watt, depending on country. They have a maximum indoor range of about 150 ft (50 m) on 2.4 GHz and 50 ft (20 m) on 5 GHz.[112]
Neighborhood wireless WAN router on telephone pole
    • cell towers; satellite internet access; and lower frequencies in the UHF band, which have a longer range than Wi-Fi frequencies. Since WWAN networks are much more expensive and complicated to administer than Wi-Fi networks, their use so far has generally been limited to private networks operated by large corporations.[112]
    • ISM band. This allows Bluetooth networks to operate in the presence of noise, other wireless devices and other Bluetooth networks using the same frequencies, since the chance of another device attempting to transmit on the same frequency at the same time as the Bluetooth modem is low. In the case of such a "collision", the Bluetooth modem just retransmits the data packet on another frequency.[113]
    • data packets are exchanged between computer-controlled radio modems (transmitter/receivers) called nodes, which may be separated by miles, and maybe mobile. Each node only communicates with neighboring nodes, so packets of data are passed from node to node until they reach their destination using the X.25 network protocol. Packet radio systems are used to a limited degree by commercial telecommunications companies and by the amateur radio community.[114]
  • Short Message Service (SMS) which transmits using spare bandwidth on the control radio channel used by cell phones to handle background functions like dialing and cell handoffs. Due to technical limitations of the channel, text messages are limited to 160 alphanumeric characters.[115]
Parabolic antennas of microwave relay links on tower in Australia
  • microwaves to another dish antenna and receiver. Since the antennas must be in line-of-sight, distances are limited by the visual horizon to 30–40 miles (48–64 km). Microwave links are used for private business data, wide area computer networks (WANs), and by telephone companies to transmit long-distance phone calls and television signals between cities.[116][117]
  • Telemetry – automated one-way (simplex) transmission of measurements and operation data from a remote process or device to a receiver for monitoring. Telemetry is used for in-flight monitoring of missiles, drones, satellites, and weather balloon radiosondes, sending scientific data back to Earth from interplanetary spacecraft, communicating with electronic biomedical sensors implanted in the human body, and well logging. Multiple channels of data are often transmitted using frequency-division multiplexing or time-division multiplexing.[118] Telemetry is starting to be used in consumer applications such as:
    • water meters, and gas meters that, when triggered by an interrogation signal, transmit their readings by radio to a utility reader vehicle at the curb, to eliminate the need for an employee to go on the customer's property to manually read the meter.[119]
    • Electronic toll collection – on toll roads, an alternative to manual collection of tolls at a toll booth, in which a transponder in a vehicle, when triggered by a roadside transmitter, transmits a signal to a roadside receiver to register the vehicle's use of the road, enabling the owner to be billed for the toll.[120]
RFID tag from a DVD
  • ISM bands and have a short range. Active tags, powered by a battery, are larger but can transmit a stronger signal, giving them a range of hundreds of meters.[121]
  • Submarine communication – When submerged, submarines are cut off from all ordinary radio communication with their military command authorities by the conductive seawater. However radio waves of low enough frequencies, in the VLF (30 to 3 kHz) and ELF (below 3 kHz) bands are able to penetrate seawater. Navies operate large shore transmitting stations with power output in the megawatt range to transmit encrypted messages to their submarines in the world's oceans. Due to the small bandwidth, these systems cannot transmit voice, only text messages at a slow data rate. The communication channel is one-way, since the long antennas needed to transmit VLF or ELF waves cannot fit on a submarine. VLF transmitters use miles long wire antennas like umbrella antennas. A few nations use ELF transmitters operating around 80 Hz, which can communicate with submarines at lower depths. These use even larger antennas called ground dipoles, consisting of two ground (Earth) connections 23–60 km (14–37 mi) apart, linked by overhead transmission lines to a power plant transmitter.[122][123]

Space communication

Satellite Communications Center Dubna in Russia[124]

This is radio communication between a

uplink, while a link that transmits data from the spacecraft to the ground is called a downlink.[125]

Communications satellite belonging to Azerbaijan
  • satellite ground station a microwave transmitter and large satellite dish antenna transmit a microwave uplink beam to the satellite. The uplink signal carries many channels of telecommunications traffic, such as long-distance telephone calls, television programs, and internet signals, using a technique called frequency-division multiplexing (FDM). On the satellite, a transponder receives the signal, translates it to a different downlink frequency to avoid interfering with the uplink signal, and retransmits it down to another ground station, which may be widely separated from the first. There the downlink signal is demodulated and the telecommunications traffic it carries is sent to its local destinations through landlines. Communication satellites typically have several dozen transponders on different frequencies, which are leased by different users.[127]
  • Direct broadcast satellite – a geostationary communication satellite that transmits retail programming directly to receivers in subscriber's homes and vehicles on Earth, in satellite radio and TV systems. It uses a higher transmitter power than other communication satellites, to allow the signal to be received by consumers with a small unobtrusive antenna. For example, satellite television uses downlink frequencies from 12.2 to 12.7 GHz in the ku band transmitted at 100 to 250 watts, which can be received by relatively small 43–80 cm (17–31 in) satellite dishes mounted on the outside of buildings.[128]

Radar

Military air traffic controller on US Navy aircraft carrier monitors aircraft on radar screen

Radar is a radiolocation method used to locate and track aircraft, spacecraft, missiles, ships, vehicles, and also to map weather patterns and terrain. A radar set consists of a transmitter and receiver.[129][130] The transmitter emits a narrow beam of radio waves which is swept around the surrounding space. When the beam strikes a target object, radio waves are reflected back to the receiver. The direction of the beam reveals the object's location. Since radio waves travel at a constant speed close to the speed of light, by measuring the brief time delay between the outgoing pulse and the received "echo", the range to the target can be calculated. The targets are often displayed graphically on a map display called a radar screen. Doppler radar can measure a moving object's velocity, by measuring the change in frequency of the return radio waves due to the Doppler effect.[131]

Radar sets mainly use high frequencies in the

monostatic radar. A radar which uses separate transmitting and receiving antennas is called a bistatic radar.[132]

ASR-8 airport surveillance radar antenna. It rotates once every 4.8 seconds. The rectangular antenna on top is the secondary radar.
  • Terminal Radar Approach Control), where air traffic controllers direct the aircraft by radio to maintain safe aircraft separation.[133]
    • Secondary surveillance radar – Aircraft carry radar transponders, transceivers which when triggered by the incoming radar signal transmit a return microwave signal. This causes the aircraft to show up more strongly on the radar screen. The radar which triggers the transponder and receives the return beam, usually mounted on top of the primary radar dish, is called the secondary surveillance radar. Since radar cannot measure an aircraft's altitude with any accuracy, the transponder also transmits back the aircraft's altitude measured by its altimeter, and an ID number identifying the aircraft, which is displayed on the radar screen.[134]
  • Electronic countermeasures (ECM) – Military defensive electronic systems designed to degrade enemy radar effectiveness, or deceive it with false information, to prevent enemies from locating local forces. It often consists of powerful microwave transmitters that can mimic enemy radar signals to create false target indications on the enemy radar screens.[135]
Rotating marine radar antenna on a ship
  • Marine radar – an S or X band radar on ships used to detect nearby ships and obstructions like bridges.[136] A rotating antenna sweeps a vertical fan-shaped beam of microwaves around the water surface surrounding the craft out to the horizon.
  • Doppler shift.[137]
  • Phased-array radar – a radar set that uses a phased array, a computer-controlled antenna that can steer the radar beam quickly to point in different directions without moving the antenna. Phased-array radars were developed by the military to track fast-moving missiles and aircraft. They are widely used in military equipment and are now spreading to civilian applications.[138]
  • Synthetic aperture radar (SAR) – a specialized airborne radar set that produces a high-resolution map of ground terrain. The radar is mounted on an aircraft or spacecraft and the radar antenna radiates a beam of radio waves sideways at right angles to the direction of motion, toward the ground. In processing the return radar signal, the motion of the vehicle is used to simulate a large antenna, giving the radar a higher resolution.[139]
  • Ground-penetrating radar – a specialized radar instrument that is rolled along the ground surface in a cart and transmits a beam of radio waves into the ground, producing an image of subsurface objects. Frequencies from 100 MHz to a few GHz are used. Since radio waves cannot penetrate very far into earth, the depth of GPR is limited to about 50 feet.[140]
  • LIDAR system on an automobile or vehicle that detects if the vehicle is about to collide with an object and applies the brakes to prevent the collision.[141]
  • Radar fuze – a detonator for an aerial bomb which uses a radar altimeter to measure the height of the bomb above the ground as it falls and detonates it at a certain altitude.[142]

Radiolocation

Radiolocation is a generic term covering a variety of techniques that use radio waves to find the location of objects, or for navigation.[143]

An early iPhone with its GPS navigation app in use.
A personal navigation assistant by Garmin, which uses GPS to give driving directions to a destination.
    • artillery shells to their target, and handheld GPS receivers are produced for hikers and the military.[145][146]
  • Radio beacon – a fixed location terrestrial radio transmitter which transmits a continuous radio signal used by aircraft and ships for navigation. The locations of beacons are plotted on navigational maps used by aircraft and ships.[147]
EPIRB emergency locator beacon on a ship
Wildlife officer tracking radio-tagged mountain lion
  • bearing of a radio signal, to determine the location of the transmitter. The location of a terrestrial transmitter can be determined by simple triangulation from bearings taken by two RDF stations separated geographically, as the point where the two bearing lines cross, this is called a "fix". Military forces use RDF to locate enemy forces by their tactical radio transmissions, counterintelligence services use it to locate clandestine transmitters used by espionage agents, and governments use it to locate unlicensed transmitters or interference sources. Older RDF receivers used rotatable loop antennas, the antenna is rotated until the radio signal strength is weakest, indicating the transmitter is in one of the antenna's two nulls. The nulls are used since they are sharper than the antenna's lobes (maxima). More modern receivers use phased array antennas which have a much greater angular resolution.[154][155]
    • GPS receivers which record and transmit a log of the animal's location.[156]

Remote control

MQ-1 Predator
drone flown remotely by a pilot on the ground

Radio remote control is the use of electronic

infrared light rather than radio waves, so are not examples of radio remote control. A security concern with remote control systems is spoofing, in which an unauthorized person transmits an imitation of the control signal to take control of the device.[157]
Examples of radio remote control:

  • automatic pilot systems that maintain stable flight and only require manual control to change directions.[158]
Remote keyless entry fob for a car
  • key fob transmitter, included with most modern cars, which can lock and unlock the doors of a vehicle from outside, eliminating the need to use a key. When a button is pressed, the transmitter sends a coded radio signal to a receiver in the vehicle, operating the locks. The fob must be close to the vehicle, typically within 5 to 20 meters. North America and Japan use a frequency of 315 MHz, while Europe uses 433.92 and 868 MHz. Some models can also remotely start the engine, to warm up the car. A security concern with all keyless entry systems is a replay attack, in which a thief uses a special receiver ("code grabber") to record the radio signal during opening, which can later be replayed to open the door. To prevent this, keyless systems use a rolling code system in which a pseudorandom number generator in the remote control generates a different random key each time it is used. To prevent thieves from simulating the pseudorandom generator to calculate the next key, the radio signal is also encrypted.[159]
    • Garage door opener – a short-range handheld transmitter which can open or close a building's electrically operated garage door from outside, so the owner can open the door upon arrival, and close it after departure. When a button is pressed the control transmits a coded FSK radio signal to a receiver in the opener, raising or lowering the door. Modern openers use 310, 315 or 390 MHz. To prevent a thief using a replay attack, modern openers use a rolling code system.[160][161]
Quadcopter, a popular remote-controlled toy
  • PCM or FSK.[162]
  • Wireless doorbell – A residential doorbell that uses wireless technology to eliminate the need to run wires through the building walls. It consists of a doorbell button beside the door containing a small battery powered transmitter. When the doorbell is pressed it sends a signal to a receiver inside the house with a speaker that sounds chimes to indicate someone is at the door. They usually use the 2.4 GHz ISM band. The frequency channel used can usually be changed by the owner in case another nearby doorbell is using the same channel.[163][164]

Jamming

Radio jamming is the deliberate radiation of radio signals designed to interfere with the reception of other radio signals. Jamming devices are called "signal suppressors" or "interference generators" or just jammers.[165]

During wartime, militaries use jamming to interfere with enemies' tactical radio communication. Since radio waves can pass beyond national borders, some

totalitarian countries which practice censorship use jamming to prevent their citizens from listening to broadcasts from radio stations in other countries. Jamming is usually accomplished by a powerful transmitter which generates noise on the same frequency as the target transmitter.[166][167]

US Federal law prohibits the nonmilitary operation or sale of any type of jamming devices, including ones that interfere with GPS, cellular, Wi-Fi and police radars.[168]

Scientific research

  • Very Long Baseline Interferometry (VLBI) radio telescopes on different continents are linked, which can achieve the resolution of an antenna thousands of miles in diameter.[169][170]
  • Remote sensing – in radio, remote sensing is the reception of electromagnetic waves radiated by natural objects or the atmosphere for scientific research. All warm objects emit microwaves and the spectrum emitted can be used to determine temperature. Microwave radiometers are used in meteorology and earth sciences to determine temperature of the atmosphere and earth surface, as well as chemical reactions in the atmosphere.[171][172]

See also

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General references

External links

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