A variety of radio antennas on Sandia Peak near Albuquerque, New Mexico, US

Applications of radio waves that do not involve transmitting the waves significant distances, such as RF heating used in industrial processes and microwave ovens, and medical uses such as diathermy and MRI machines, are not usually called radio. The noun radio is also used to mean a broadcast radio receiver.

Radio waves were first identified and studied by German physicist Heinrich Hertz in 1886. The first practical radio transmitters and receivers were developed around 1895–1896 by Italian Guglielmo Marconi, and radio began to be used commercially around 1900. To prevent interference between users, the emission of radio waves is regulated by law, coordinated by an international body called the International Telecommunication Union (ITU), which allocates frequency bands in the radio spectrum for different uses.

## Technology

Radio waves are radiated by electric charges undergoing acceleration.[4][5] They are generated artificially by time varying electric currents, consisting of electrons flowing back and forth in a metal conductor called an antenna,[6][7] thus accelerating. In transmission, a transmitter generates an alternating current of radio frequency which is applied to an antenna. The antenna radiates the power in the current as radio waves. When the waves strike the antenna of a radio receiver, they push the electrons in the metal back and forth, inducing a tiny alternating current. The radio receiver connected to the receiving antenna detects this oscillating current and amplifies it.

Radio waves travel through a vacuum at the speed of light, and in the air at very close to the speed of light, so the wavelength of a radio wave, the distance in meters between adjacent crests of the wave, is inversely proportional to its frequency.

The other types of electromagnetic waves besides radio waves are infrared, visible light, ultraviolet, X-rays and gamma rays. The wide use of radio waves for telecommunication is mainly due to their desirable propagation properties stemming from their large wavelength.[7] Radio waves have the ability to pass through the atmosphere, foliage, and most building materials, and by diffraction can bend around obstructions, and unlike other electromagnetic waves, they tend to be scattered rather than absorbed by objects larger than their wavelength.

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 transducer to a time-varying electrical signal called the modulation signal.[7][8] The modulation signal may be an audio signal representing sound from a microphone, a video signal representing moving images from a video camera, or a digital signal consisting of a sequence of bits representing binary data from a computer. The modulation signal is applied to a 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:

• AM (amplitude modulation) – in an AM transmitter, the amplitude (strength) of the radio carrier wave is varied by the modulation signal.
• FM (frequency modulation) – in an FM transmitter, the frequency of the radio carrier wave is varied by the modulation signal.
• FSK (frequency-shift keying) – used in wireless digital devices to transmit digital signals, the frequency of the carrier wave is shifted periodically between two frequencies that represent the two binary digits, 0 and 1, to transmit a sequence of bits.
• OFDM (orthogonal frequency-division multiplexing) – a family of complicated digital modulation methods very widely used in high bandwidth systems such as Wi-Fi networks, cellphones, digital television broadcasting, and 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.

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

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.

At the receiver, the radio wave induces a tiny oscillating voltage in the receiving antenna which is a weaker replica of the current in the transmitting antenna.[7][8] This voltage is applied to the radio receiver, which amplifies the weak radio signal so it is stronger, then demodulates it, extracting the original modulation signal from the modulated carrier wave. The modulation signal is converted by a transducer back to a human-usable form: an audio signal is converted to sound waves by a loudspeaker or earphones, a 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.

## Bandwidth

Frequency spectrum of a typical modulated AM or FM radio signal. It consists of a component C at the carrier wave frequency ${\displaystyle f_{c}}$ 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 frequencies. See diagram. The information (modulation) in a radio signal is usually concentrated in narrow frequency bands called sidebands (SB) just above and below the carrier frequency. The width in hertz of the frequency range that the radio signal occupies, the highest frequency minus the lowest frequency, is called its bandwidth (BW).[9] For any given signal-to-noise ratio, an amount of bandwidth can carry the same amount of information (data rate in 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.

The radio spectrum, the total range of radio frequencies that can be used for communication in a given area, is a limited resource.[9][3] Each radio transmission occupies a portion of the total bandwidth available. Radio bandwidth is regarded as an 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.

In recent years there has been a transition from analog to digital radio transmission technologies. Part of the reason for this is that digital modulation can often transmit more information (a greater data rate) in a given bandwidth than analog modulation, by using data compression algorithms, which reduce redundancy in the data to be sent, and more efficient modulation. Other reasons for the transition is that digital modulation has greater 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.

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 use it more effectively is driving many additional radio innovations such as trunked radio systems, spread spectrum (ultra-wideband) transmission, frequency reuse, dynamic spectrum management, frequency pooling, and cognitive radio.

### 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][10] Each of these bands has a traditional name:

Band nameAbbreviationFrequencyWavelengthBand nameAbbreviationFrequencyWavelength
Extremely low frequencyELF3 – 30 Hz100,000–10,000 kmHigh frequencyHF3 – 30 MHz100–10 m
Super low frequencySLF30 – 300 Hz10,000–1,000 kmVery high frequencyVHF30 – 300 MHz10–1 m
Ultra low frequencyULF300 – 3000 Hz1,000–100 kmUltra high frequencyUHF300 – 3000 MHz100–10 cm
Very low frequencyVLF3 – 30 kHz100–10 kmSuper high frequencySHF3 – 30 GHz10–1 cm
Low frequencyLF30 – 300 kHz10–1 kmExtremely high frequencyEHF30 – 300 GHz10–1 mm
Medium frequencyMF300 – 3000 kHz1000–100 mTremendously high frequencyTHF300 – 3000 GHz1–0.1 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. The greater bandwidth available has motivated a continuing trend to exploit higher frequencies throughout radio's history.

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

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, cordless phones, wireless devices, walkie-talkies, citizens band radios, wireless microphones, garage door openers, and baby monitors. In the US, these fall under Part 15 of the Federal Communications Commission (FCC) regulations. Many of these devices use the 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.

## Applications

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

Television station

Broadcasting uses several parts of the radio spectrum, depending on the type of signals transmitted and the desired target audience. Longwave and medium wave signals can give reliable coverage of areas several hundred kilometers across, but have the more limited information-carrying capacity and so work best with audio signals (speech and music), and the sound quality can be degraded by radio noise from natural and artificial sources. The shortwave bands have a greater potential range but are more subject to interference by distant stations and varying atmospheric conditions that affect reception.

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.

• Shortwave broadcasting – AM broadcasting is also allowed in the 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.
FM broadcast transmitter of radio station KWNR, Las Vegas which transmits on 95.5 MHz with a power of 35 kW
• FM (frequency modulation) – in FM the frequency of the radio carrier signal is varied slightly by the audio signal. FM broadcasting is permitted in the FM broadcast bands between about 65 and 108 MHz in the very high frequency (VHF) range. Radio waves in this band travel by line-of-sight so FM reception is limited by the visual horizon to about 30–40 mi (48–64 km), and can be blocked by hills. However it is less susceptible to interference from radio noise (RFI, sferics, static) and has higher fidelity; better frequency response and less audio distortion, than AM. In the US, radiated power (ERP) of FM stations varies from 6 to 100 kW.
• 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.[11] 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. Most countries plan an eventual switchover from FM to DAB. The United States and Canada have chosen not to implement DAB.
A single DAB station transmits a 1,500 kHz bandwidth signal that carries from 9 to 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).
• Satellite radio is a subscription radio service that broadcasts CD quality digital audio direct to subscribers' receivers using a microwave downlink signal from a direct broadcast communication satellite in geostationary orbit 22,000 miles above the Earth. It is mostly intended for car 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.

Television broadcasting is the transmission of moving images by radio, which consist of sequences of still images, which are displayed on a screen on a television receiver (a "television" or TV) along with a synchronized audio (sound) channel. Television (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". The current television standard, introduced beginning in 2006, is a digital format called high-definition television (HDTV), which transmits pictures at higher resolution, typically 1080 pixels high by 1920 pixels wide, at a rate of 25 or 30 frames per second. Digital television (DTV) transmission systems, which replaced older analog television in a transition beginning in 2006, use image compression and high-efficiency digital modulation such as OFDM and 8VSB to transmit HDTV video within a smaller bandwidth than the old analog channels, saving scarce radio spectrum space. Therefore, each of the 6 MHz analog RF channels now carries up to 7 DTV channels – these are called "virtual channels". Digital television receivers have different behavior in the presence of poor reception or noise than analog television, called the "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.

• Terrestrial television, over-the-air (OTA) television, or broadcast television – the oldest television technology, is the transmission of television signals from land-based television stations to television receivers (called televisions or TVs) in viewer's homes. Terrestrial television broadcasting uses the bands 41 – 88 MHz (VHF low band or Band I, carrying RF channels 1–6), 174 – 240 MHz, (VHF high band or Band III; carrying RF channels 7–13), and 470 – 614 MHz (UHF Band IV and Band V; carrying RF channels 14 and up). The exact frequency boundaries vary in different countries. Propagation is by line-of-sight, so reception is limited by the visual horizon to 30–40 miles (48–64 km). In the US effective radiated power (ERP) of television, transmitters are limited to 35 kW in the VHF low band, 50 kW in the VHF high band, and 220 kW in UHF band; most TV stations operate below 75% of the limit. In most areas, viewers use a simple "rabbit ears" dipole antenna on top of the TV, but viewers in fringe reception areas more than 15 miles from a station usually have to use an outdoor antenna mounted on the roof to get adequate reception.
• Satellite television – a set-top box which receives subscription direct-broadcast satellite television, and displays it on an ordinary television. A direct broadcast satellite in 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.

#### Time

Government standard frequency and time signal services operate time radio stations which continuously broadcast extremely accurate time signals produced by atomic clocks, as a reference to synchronize other clocks. Examples are BPC, DCF77, JJY, MSF, RTZ, TDF, WWV, and YVTO. One use is in radio clocks and watches, which include an automated receiver that periodically (usually weekly) receives and decodes the time signal and resets the watch's internal quartz clock to the correct time, thus allowing a small watch or desk clock to have the same accuracy as an atomic clock. Government time stations are declining in number because GPS satellites and the Internet Network Time Protocol (NTP) provide equally accurate time standards.

### Two-way voice communication

(left) Cellphone from the early 21st century. (right) Cellular phone tower shared by antennas belonging to 3 different networks.

• Cell phone – a portable wireless telephone that is connected to the telephone network by radio signals exchanged with a local antenna at a cellular base station (cell tower).[12] The service area covered by the provider is divided into small geographical areas called "cells", each served by a separate base station antenna and multichannel transceiver. All the cell phones in a cell communicate with this antenna on separate frequency channels, assigned from a common pool of frequencies.

The purpose of cellular organization is to conserve radio bandwidth by frequency reuse. Low power transmitters are used so the radio waves used in a cell do not travel far beyond the cell, allowing the same frequencies to be reused in geographically separated cells. When a user carrying a cellphone crosses from one cell to another, his phone is automatically "handed off" seamlessly to the new antenna and assigned new frequencies. Cellphones have a highly automated full duplex digital transceiver using OFDM modulation using two digital radio channels, each carrying one direction of the bidirectional conversation, as well as a control channel that handles dialing calls and "handing off" the phone to another cell tower. Older 2G, 3G, and 4G networks use frequencies in the UHF and low microwave range, between 700 MHz and 3 GHz. The cell phone transmitter adjusts its power output to use the minimum power necessary to communicate with the cell tower; 0.6 W when near the tower, up to 3 W when farther away. Cell tower channel transmitter power is 50 W. Current generation phones, called smartphones, have many functions besides making telephone calls, and therefore have several other radio transmitters and receivers that connect them with other networks: usually a Wi-Fi modem, a Bluetooth modem, and a GPS receiver.

• 5G cellular network – next-generation cellular networks which began deployment in 2019. Their major advantage is much higher data rates than previous cellular networks, up to 10 Gbps; 100 times faster than the previous cellular technology, 4G LTE. The higher data rates are achieved partly by using higher frequency radio waves, in the higher microwave band 3–6 GHz, and 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.
Satellite phones, showing the large antennas needed to communicate with the satellite
• Satellite phone (satphone) – a portable wireless telephone similar to a cell phone, connected to the telephone network through a radio link to an orbiting communications satellite instead of through 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.
• Cordless phone – a 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 FM radio transceivers operating in the UHF band that handles the short-range bidirectional radio link.
Firefighter using walkie-talkie
• Land mobile radio system – short-range mobile or portable half-duplex radio transceivers operating in the VHF or UHF band that can be used without a license. They are often installed in vehicles, with the mobile units communicating with a dispatcher at a fixed base station. Special systems with reserved frequencies are used by first responder services; police, fire, ambulance, and emergency services, and other government services. Other systems are made for use by commercial firms such as taxi and delivery services. VHF systems use channels in the range 30–50 MHz and 150–172 MHz. UHF systems use the 450–470 MHz band and in some areas the 470–512 MHz range. In general, VHF systems have a longer range than UHF but require longer antennas. AM or FM modulation is mainly used, but digital systems such as DMR are being introduced. The radiated power is typically limited to 4 watts.[12] These systems have a fairly limited range, usually 3 to 20 miles (4.8 to 32 km) depending on terrain. Repeaters installed on tall buildings, hills, or mountain peaks are often used to increase the range when it is desired to cover a larger area than line-of-sight. Examples of land mobile systems are 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.
• Walkie-talkie – a battery-powered portable handheld half-duplex two-way radio, used in land mobile radio systems.
• Airband – Half-duplex radio system used by aircraft pilots to talk to other aircraft and ground-based air traffic controllers. This vital system is the main communication channel for air traffic control. For most communication in overland flights in air corridors a VHF-AM system using channels between 108 and 137 MHz in the VHF band are used. This system has a typical transmission range of 200 miles (320 km) for aircraft flying at cruising altitude. 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. Military aircraft also use a dedicated UHF-AM band from 225.0 to 399.95 MHz.
VHF marine radio on a ship
• Marine radio – medium-range transceivers on ships, used for ship-to-ship, ship-to-air, and ship-to-shore communication with harbormasters They use FM channels between 156 and 174 MHz in the VHF band with up to 25 watts power, giving them a range of about 60 miles (97 km). Some channels are half-duplex and some are full-duplex, to be compatible with the telephone network, to allow users to make telephone calls through a marine operator.

### 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. These transmit in FM on 49.300, 49.830, 49.845, 49.860, or 49.875 MHz with low power. Many baby monitors have duplex channels so the parent can talk to the baby, and video cameras to show a picture of the baby, this is called a baby cam.
• Wireless microphone – a battery-powered microphone with a short-range transmitter that is handheld or worn on a person's body which transmits its sound by radio to a nearby receiver unit connected to a sound system. Wireless microphones are used by public speakers, performers, and television personalities so they can move freely without trailing a microphone cord. Analog models transmit in FM on unused portions of the television broadcast frequencies in the VHF and UHF bands. Some models transmit on two frequency channels for diversity reception to prevent nulls from interrupting transmission as the performer moves around. Some models use digital modulation to prevent unauthorized reception by scanner radio receivers; these operate in the 900 MHz, 2.4 GHz or 6 GHz ISM bands.

### Data communication

• Wireless networking – automated radio links which transmit digital data between computers and other wireless devices using radio waves, linking the devices together transparently in a computer network. Computer networks can transmit any form of data: in addition to email and web pages, they also carry phone calls (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.
Laptop computer with Wi-Fi and typical home wireless router (right) connecting it to the Internet
• Wireless LAN (wireless local area network or Wi-Fi) – based on the IEEE 802.11 standards, these are the most widely used computer networks, used to implement local area networks without cables, linking computers, laptops, cell phones, video game consoles, smart TVs and printers in a home or office together, and to a wireless router connecting them to the Internet with a wire or cable connection. Wireless routers in public places like libraries, hotels and coffee shops create wireless access points (hotspots) to allow the public to access the Internet with portable devices like smartphones, tablets or laptops. Each device exchanges data using a wireless modem (wireless network interface controller), an automated microwave transmitter and receiver with an omnidirectional antenna that works in the background, exchanging data packets with the router. Wi-Fi uses channels in the 2.4 GHz and 5 GHz 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.
(c) Collin Knopp-Schwyn, CC BY 4.0
Neighborhood wireless WAN router on telephone pole
• Wireless WAN (wireless wide area network, WWAN) – a variety of technologies that provide wireless internet access over a wider area than Wi-Fi networks do – from an office building to a campus to a neighborhood, or to an entire city. The most common technologies used are: cellular modems, that exchange computer data by radio with 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.
• Bluetooth – a very short-range wireless interface on a portable wireless device used as a substitute for a wire or cable connection, mainly to exchange files between portable devices and connect cellphones and music players with wireless headphones. In the most widely used mode, transmission power is limited to 1 milliwatt, giving it a very short range of up to 10 m (30 feet). The system uses frequency-hopping spread spectrum transmission, in which successive data packets are transmitted in a pseudorandom order on one of 79 1 MHz Bluetooth channels between 2.4 and 2.83 GHz in the 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.
• Packet radio – a long-distance peer-to-peer wireless ad-hoc network in which 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. Uses the X.25 network protocol. Packet radio systems are used to a limited degree by commercial telecommunications companies and by the amateur radio community.
• Text messaging (texting) – this is a service on cell phones, allowing a user to type a short alphanumeric message and send it to another phone number, and the text is displayed on the recipient's phone screen. It is based on the 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.
Parabolic antennas of microwave relay links on tower in Australia
• Microwave relay – a long-distance high bandwidth point-to-point digital data transmission link consisting of a microwave transmitter connected to a dish antenna that transmits a beam of 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.
• 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. Telemetry is starting to be used in consumer applications such as:
• Automated meter readingelectric power meters, 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.
• 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.
RFID tag from a DVD
• 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.

### Space communication

Satellite Communications Center Dubna in Russia[13]

Communications satellite belonging to Azerbaijan
• Communication satellite – an artificial satellite used as a telecommunications relay to transmit data between widely separated points on Earth. These are used because the microwaves used for telecommunications travel by line of sight and so cannot propagate around the curve of the Earth. As of 1 January 2021, there were 2,224 communications satellites in Earth orbit.[14] Most are in geostationary orbit 22,200 miles (35,700 km) above the equator, so that the satellite appears stationary at the same point in the sky, so the satellite dish antennas of ground stations can be aimed permanently at that spot and do not have to move to track it. In a 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.
• 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.

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

Radar sets mainly use high frequencies in the microwave bands, because these frequencies create strong reflections from objects the size of vehicles and can be focused into narrow beams with compact antennas. Parabolic (dish) antennas are widely used. In most radars the transmitting antenna also serves as the receiving antenna; this is called a monostatic radar. A radar which uses separate transmitting and receiving antennas is called a bistatic radar.

ASR-8 airport surveillance radar antenna. It rotates once every 4.8 seconds. The rectangular antenna on top is the secondary radar.
• Airport surveillance radar – In aviation, radar is the main tool of air traffic control. A rotating dish antenna sweeps a vertical fan-shaped beam of microwaves around the airspace and the radar set shows the location of aircraft as "blips" of light on a display called a radar screen. Airport radar operates at 2.7 – 2.9 GHz in the microwave S band. In large airports the radar image is displayed on multiple screens in an operations room called the TRACON (Terminal Radar Approach Control), where air traffic controllers direct the aircraft by radio to maintain safe aircraft separation.
• 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.
• 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.
• Radar altimeter – a specialized radar on an aircraft that measures the altitude of the aircraft above terrain by bouncing a radio beam off the ground surface and measuring the time for the echo to return.
Rotating marine radar antenna on a ship
• Marine radar – an X band radar on ships used to detect nearby ships and obstructions like bridges. A rotating antenna sweeps a vertical fan-shaped beam of microwaves around the water surface surrounding the craft out to the horizon.
• Weather radar – A Doppler radar which maps weather systems and measures wind speeds by reflection of microwaves from raindrops.
• 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.
• 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.
• 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.
• Collision avoidance system – a short range radar or 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.
• 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.
• Radar speed gun – A handheld Doppler radar used by traffic police to measure the speed of vehicles to determine if they are obeying the local speed limit. When the officer points the gun at a vehicle and presses a trigger, its speed appears on a numeric display. Speed guns use the X band or Ku band.

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

A personal navigation assistant GPS receiver in a car, which can give driving directions to a destination.
• Global Navigation Satellite System (GNSS) or satnav system – A system of satellites which allows geographical location on Earth (latitude, longitude, and altitude/elevation) to be determined to high precision (within a few metres) by small portable navigation instruments, by timing the arrival of radio signals from the satellites. These are the most widely used navigation systems today. The main satellite navigation systems are the US Global Positioning System (GPS), Russia's GLONASS, China's BeiDou Navigation Satellite System (BDS) and the European Union's Galileo.
• 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.
• Non-directional beacon (NDB) – Legacy fixed radio beacons used before the VOR system that transmit a simple signal in all directions for aircraft or ships to use for radio direction finding. Aircraft use automatic direction finder (ADF) receivers which use a directional antenna to determine the bearing to the beacon. By taking bearings on two beacons they can determine their position. NDBs use frequencies between 190 and 1750 kHz in the LF and MF bands which propagate beyond the horizon as ground waves or skywaves much farther than VOR beacons. They transmit a callsign consisting of one to 3 Morse code letters as an identifier.
EPIRB emergency locator beacon on a ship
• Emergency locator beacon – a portable battery powered radio transmitter used in emergencies to locate airplanes, vessels, and persons in distress and in need of immediate rescue. Various types of emergency locator beacons are carried by aircraft, ships, vehicles, hikers and cross-country skiers. In the event of an emergency, such as the aircraft crashing, the ship sinking, or a hiker becoming lost, the transmitter is deployed and begins to transmit a continuous radio signal, which is used by search and rescue teams to quickly find the emergency and render aid. The latest generation Emergency Position Indicating Rescue Beacons (EPIRBs) contain a GPS receiver, and broadcast to rescue teams their exact location within 20 meters.
• Cospas-Sarsat – an international humanitarian consortium of governmental and private agencies which acts as a dispatcher for search and rescue operations. It operates a network of about 47 satellites carrying radio receivers, which detect distress signals from emergency locator beacons anywhere on Earth transmitting on the international Cospas distress frequency of 406 MHz. The satellites calculate the geographic location of the beacon within 2 km by measuring the Doppler frequency shift of the radio waves due to the relative motion of the transmitter and the satellite, and quickly transmit the information to the appropriate local first responder organizations, which perform the search and rescue.
Wildlife officer tracking radio-tagged mountain lion
• Radio direction finding (RDF) – this is a general technique, used since the early 1900s, of using specialized radio receivers with directional antennas (RDF receivers) to determine the exact 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.
• Animal migration tracking – a widely used technique in wildlife biology, conservation biology, and wildlife management in which small battery-powered radio transmitters are attached to wild animals so their movements can be tracked with a directional RDF receiver. Sometimes the transmitter is implanted in the animal. The VHF band is typically used since antennas in this band are fairly compact. The receiver has a directional antenna (typically a small Yagi) which is rotated until the received signal is strongest; at this point the antenna is pointing in the direction of the animal. Sophisticated systems used in recent years use satellites to track the animal, or geolocation tags with GPS receivers which record and transmit a log of the animal's location.

### Remote control

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

Radio remote control is the use of electronic control signals sent by radio waves from a transmitter to control the actions of a device at a remote location. Remote control systems may also include telemetry channels in the other direction, used to transmit real-time information of the state of the device back to the control station. Unmanned spacecraft are an example of remote controlled machines, controlled by commands transmitted by satellite ground stations. Most handheld remote controls used to control consumer electronics products like televisions or DVD players actually operate by 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. Examples of radio remote control:

• Unmanned aerial vehicle (UAV, drone) – A drone is an aircraft without an onboard pilot, flown by remote control by a pilot in another location, usually in a piloting station on the ground. They are used by the military for reconnaissance and ground attack, and more recently by the civilian world for news reporting and aerial photography. The pilot uses aircraft controls like a joystick or steering wheel, which create control signals which are transmitted to the drone by radio to control the flight surfaces and engine. A telemetry system transmits back a video image from a camera in the drone to allow the pilot to see where the aircraft is going, and data from a GPS receiver giving the real-time position of the aircraft. UAVs have sophisticated onboard automatic pilot systems that maintain stable flight and only require manual control to change directions.
Remote keyless entry fob for a car
• Keyless entry system – a short-range handheld battery powered 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.
• 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 when he drives up in his car, and close it after he leaves. 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.
• Radio-controlled models – a popular hobby is playing with radio-controlled model boats, cars, airplanes, and helicopters (quadcopters) which are controlled by radio signals from a handheld console with a joystick. Most recent transmitters use the 2.4 GHz ISM band with multiple control channels modulated with PWM, PCM or FSK.
• 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.

### 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.[15]

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.

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.[16]

### Scientific research

• 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.

## 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, Alexander Graham Bell adopted "radiophone" (meaning "radiated sound") as an alternate name for his photophone optical transmission system.[17][18] However, this invention would not be widely adopted.

Following Heinrich Hertz's discovery of the existence of radio waves in 1886, a variety of terms were initially used for this radiation, including "Hertzian waves", "electric waves", and "ether waves". The first practical radio communications systems, developed by Guglielmo Marconi in 1894–5, transmitted telegraph signals by radio waves, 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.

The first use of radio- in conjunction with electromagnetic radiation appears to have been by French physicist Édouard Branly, who in 1890 developed the coherer detector, which he called in French a radio-conducteur.[19] 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",[20] The French text of both the 1903 and 1906 Berlin Radiotelegraphic Conventions includes the phrases "radiotélégraphique" and "radiotélégrammes".

The use of "radio" as a standalone word dates back to at least December 30, 1904, when instructions issued by the British Post Office for transmitting telegrams specified that "The word 'Radio'... is sent in the Service Instructions".[21] 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'".

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 June 22, 1907 Electrical World about the need for legal restrictions warned that "Radio chaos will certainly be the result until such stringent regulation is enforced".[22] 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. (the word broadcasting originated with the agricultural term meaning roughly "scattering seeds widely".)[23] British Commonwealth countries continued to commonly use the term "wireless" until the mid-20th century, though the magazine of the British Broadcasting Corporation in the UK has been called Radio Times since its founding in the early 1920s.

In recent years "wireless" has gained renewed popularity as a more general term for devices communicating using electromagnetic radiation, either radio waves or light, due to the rapid growth of short-range computer networking, e.g., wireless local area networks Wi-Fi, and Bluetooth, as well as cell phones, to distinguish these uses from traditional "radio" communication, such as broadcasting.

## References

1. ^ "Radio". Oxford Living Dictionaries. Oxford University Press. 2019. Retrieved 26 February 2019.
2. ^ "Definition of radio". Encyclopedia. PCMagazine website, Ziff-Davis. 2018. Retrieved 26 February 2019.
3. ^ a b c d Ellingson, Steven W. (2016). Radio Systems Engineering. Cambridge University Press. pp. 1–4. ISBN 978-1316785164.
4. ^ Kraus, John D. (1988). Antennas, 2nd Ed. Tata-McGraw Hill. p. 50. ISBN 0074632191.
5. ^ Serway, Raymond; Faughn, Jerry; Vuille, Chris (2008). College Physics, 8th Ed. Cengage Learning. p. 714. ISBN 978-0495386933.
6. ^ Balanis, Constantine A. (2005). Antenna theory: Analysis and Design, 3rd Ed. John Wiley and Sons. pp. 10. ISBN 9781118585733.
7. ^ a b c d Ellingson, Steven W. (2016). Radio Systems Engineering. Cambridge University Press. pp. 16–17. ISBN 978-1316785164.
8. ^ a b c Brain, Marshall (2000-12-07). "How Radio Works". HowStuffWorks.com. Retrieved 2009-09-11.
9. ^ a b c d "Spectrum 101" (PDF). US National Aeronautics and Space Administration (NASA). February 2016. Retrieved 2 December 2019. `{{cite journal}}`: Cite journal requires `|journal=` (help), p. 6
10. ^ "Radio Regulations, 2016 Edition" (PDF). International Telecommunication Union. 3 November 2016. Retrieved 9 November 2019. `{{cite journal}}`: Cite journal requires `|journal=` (help) Article 2, Section 1, p.27
11. ^ Baker, William (2020). "DAB vs FM: The differences between analog and digital radio". Radio Fidelity online magazine. Retrieved 14 September 2020.
12. ^ a b Brain, Marshall; Jeff Tyson & Julia Layton (2018). "How Cell Phones Work". How Stuff Works. InfoSpace Holdings LLC. Retrieved 31 December 2018.
13. ^ "Ground infrastructure". Russian Satellite Communications Company.
14. ^ "UCS Satellite Database". Union of Concerned Scientists. 1 January 2021. Retrieved 21 May 2021.
15. ^ "What jamming of a wireless security system is and how to resist it | Ajax Systems Blog". Ajax Systems. Retrieved 2020-01-18.
16. ^ "Jammer Enforcement". Federal Communications Commission. 2011-03-03. Retrieved 2020-01-18.
17. ^ "radio | Origin and meaning of radio by Online Etymology Dictionary". www.etymonline.com. Retrieved 2020-05-24.
18. ^ "Production of Sound by Radiant Energy" by Alexander Graham Bell, Popular Science Monthly, July 1881, pages 329–330: "[W]e have named the apparatus for the production and reproduction of sound in this way the "photophone", because an ordinary beam of light contains the rays which are operative. To avoid in future any misunderstandings upon this point, we have decided to adopt the term "radiophone", proposed by M. Mercadier, as a general term signifying the production of sound by any form of radiant energy..."
19. ^ "The Genesis of Wireless Telegraphy" by A. Frederick Collins, Electrical World and Engineer, May 10, 1902, page 811.
20. ^ "Wireless Telegraphy", The Practical Engineer, February 25, 1898, page 174. "Dr. O. J. Lodge, who preceded Marconi in making experiments in what may be called "ray" telegraphy or radiotelegraphy by a year or two, has devised a new method of sending and receiving the messages. The reader will understand that in the radiotelegraph electric waves forming the signals of the message starting from the sending instrument and travel in all directions like rays of light from a lamp, only they are invisible."
21. ^ "Wireless Telegraphy", The Electrical Review (London), January 20, 1905, page 108, quoting from the British Post Office's December 30, 1904 Post Office Circular.
22. ^ "Interference with Wireless Messages", Electrical World, June 22, 1907, page 1270.
23. ^ "broadcast | Origin and meaning of broadcast by Online Etymology Dictionary". www.etymonline.com. Retrieved 2020-05-24.

Telecom-icon.svg
Icon for telecommunications
EPIRB (1).jpg
Author/Creator: Marwan Mohamad, Licence: CC BY-SA 4.0
Emergency Position-Indicating Radio Beacon (EPIRB) adalah beacon 406 Mhz untuk pelayaran merupakan elemen dari Global Maritime Distress Safety System (GMDSS) yang memancarkan isyarat marabahaya jika kapal dalam keadaan darurat.
Automobile remote keyless entry transmitter.jpg
Author/Creator: Chetvorno, Licence: CC0
Keyless entry remote control fob for a Subaru Forrester. About 1.1 inch wide. Pressing the lefthand button unlocks the driver's door. Pressing the button again unlocks all the doors. Pressing the righthand button locks the doors. Between the two buttons is a green LED light that blinks when a button is pushed, to indicate that the transmitter is working.
Zivile Satellitentelefone.jpg
Zivil nutzbare Satellitentelefone im Größenvergleich. Von links nach rechts: Inmarsat "IsatPhone Pro", Iridium 9555, Thuraya XT
Cell-Tower.jpg
Author/Creator: The original uploader was J.smith at English Wikipedia., Licence: CC BY-SA 2.5
The top of a cell tower. Photo taken June 16, 2007.
Paris-PorteMolitor-GPS.jpg
Author/Creator: Als33120, Licence: CC BY-SA 4.0
Paris, Porte Molitor au GPS
Tracking Mountain Lions.jpg
January 2013 - US Fish and Wildlife Service employee Mary Jo Hill attempting to locate two radio collared mountain lions siblings (a male and a female) with radio direction finding equipment. They had been collared in December 2012 and we knew they were traveling with one more male sibling who was not collared. Using their locations we were able to track and collar the last male sibling. The animals have small radio transmitters on collars around their necks that emit a low-powered radio signal in the VHF band. The employee holds a RDF radio receiver with which she can hear the signal. She turns the Yagi antenna she is holding until the signal is strongest in her earphones; then the antenna is pointed at the animal. Credit: Carmen Luna / USFWS
The antenna system of an Airport Surveillance Radar-9 with primary radar (parabolic antenna) and secondary radar (rectangular antenna, top). The secondary radar interrogates the transponder on the aircraft, which sends back a coded signal indentifying the craft and giving its altitude.
RFID Chip 001.JPG
Author/Creator: No machine-readable author provided. Maschinenjunge assumed (based on copyright claims)., Licence: CC BY-SA 3.0

: RFID Chip mit Resten eines Barcode Aufkleber, Sicherungsetikett auf einer DVD eines Elektrogeschäftes

•  : RFID in a form of a sticker with bar code on the opposite side, security chip for a DVD
• Source: own work
• Date: March 11, 2008
• Author: Maschinenjunge
• Licence: GFDL, cc-by 3.0

Md4-200.jpg
md4-200 im Flug
Ein UKW-Funkgerät der Marke "Sailor", VHF-Radio-Telephone, Typ RT144B
Amfm3-en-de.gif
Author/Creator: Berserkerus, Licence: CC BY-SA 2.5
Animated diagram representing the difference between radio waves modulated by amplitude and by frequency.
KWNR Continental 816R-5B SN 247.jpg
Author/Creator: Garrett A. Wollman, Licence: CC BY-SA 2.5
A Continental type 816R-5B 35-kW FM transmitter, serial number 247. This unit belongs to KWNR (95.5 Las Vegas) and is located at the Black Mountain shared transmitter facility in Henderson. Site access courtesy of Clear Channel Communications and Tree Lee.
The power amplifier stage of the transmitter is located in the three cabinets at left. From the Varian Associates logo next to the nameplate, one can deduce that this unit was made between 1985 and 1990. At the bottom of the center amplifier cabinet is a grounding probe, which is used by technicians anytime the case is opened to touch the high voltage terminals inside to discharge any residual voltages left on capacitors prior to performing maintenance. The second cabinet from right contains (from the top) a Moseley remote control system, a Moseley analog studio-transmitter link receivers, an Audemat-Aztec RDS encoder, another STL receiver, some equipment I can't identify, a Continental type 802 exciter (FM signal generator), forward and reflected RF power meters, and another unknown device. The rightmost cabinet contains a Moseley digital studio-transmitter link, a Bird Electronics digital power meter, an Eventide digital delay (set for eight seconds), and three Omnia audio processors.
USI router.jpg
(c) Collin Knopp-Schwyn, CC BY 4.0
A wireless internet router, part of Minneapolis, Minnesota's broadband wireless internet network run by U.S. Internet.
Signal processing system.png
Author/Creator: Brews ohare, Licence: CC BY-SA 3.0
Steps in a signal communications system
HA0478-006 (6011470974).jpg
Author/Creator: We have moved! Please visit /highwaysengland, Licence: CC BY 2.0
Smartphone showing map app
Author/Creator: Chetvorno, Licence: CC0
The frequency spectrum of a typical AM or FM modulated radio signal, as it would be seen on a spectrum analyzer. The vertical axis is signal strength or voltage, the horizontal axis is frequency. It consists of a strong carrier wave (C) at the frequency fC of the radio transmitter. The information (modulation) carried by the signal is contained in two narrow frequency bands above and below the carrier frequency, called sidebands (SB). The sidebands are symmetrical about the carrier. In an audio radio broadcast the sidebands carry the audio signal that represents the sound waves. The portions of the sidebands close to the carrier contain the low audio frequencies, while the portions further from the carrier contain the high audio frequencies. Therefore the sidebands tend to fall off farther from the carrier, and also close to the carrier at points which represent bass frequencies below 20 Hz, the lower human hearing limit.
Author/Creator: Ildar Sagdejev (Specious), Licence: CC BY-SA 4.0
A mast radiator antenna for an AM radio station transmitter in Chapel Hill, North Carolina. The radio frequency current from the transmitter is applied directly to the steel lattice tower, which acts as a monopole antenna element. The tower is supported at its base by a massive porcelain insulator, which insulates it from the ground. The high voltage on the tower can deliver a dangerous shock to anyone that touches it, so it is surrounded by a fence to keep people out. The white shed is called a helix house and contains an antenna tuner matching network to match the impedance of the antenna to the transmitter to allow power to be transferred efficiently to the antenna. The mast is supported by guy wires (faint), which have strain insulators in them to prevent the high voltage on the tower from being conducted to ground.
Superturnstile Tx Muehlacker.JPG
Author/Creator: Hans-Peter Scholz, Birkenfeld (Enzkreis), Germany, Licence: CC BY-SA 2.5
Superturnstile antenna on the Muehlacker television transmitter, Germany. Also called a "batwing antenna", this is a common type of television transmitting antenna. Opposing pairs of "wings" function as a dipole antenna, with each pair of crossed dipoles fed 90° out of phase to create an omnidirectional radiation pattern. Each radiator unit consisting of two pairs of batwings is called a "bay". The vertical collinear stack of batwing "bays" increases the gain of the antenna, concentrating the radiation in the horizontal direction so little of the power is radiated into the sky or down into the ground and wasted. The advantage of the "batwing" design is that it has a wide bandwidth so one antenna design can cover a large part of the VHF television band, and can be used to transmit multiple channels if desired.
US Navy 120208-N-TU894-022 Air-Traffic Controller 2nd Class Gregory Clemmons stands the departure position watch as Air-Traffic Controller 3rd Clas.jpg
ATLANTIC OCEAN (Feb. 8, 2012) Air-Traffic Controller 2nd Class Gregory Clemmons stands the departure position watch as Air-Traffic Controller 3rd Class Marcus T. Tucker sets up on-screen maps in the carrier air traffic control center aboard the aircraft carrier USS George H.W. Bush (CVN 77). George H.W. Bush is in the Atlantic Ocean conducting carrier qualifications. (U.S. Navy photo by Mass Communication Specialist 3rd Class Kasey Krall/Released)
Sony KDL-40V2500 20061107.jpg
Author/Creator: Nate Grigg, Licence: CC BY 2.0
Sony Bravia KDL-40V2500.
ЦКС Дубна ГПКС -6.jpg
Author/Creator: Vsatinet, Licence: CC BY-SA 4.0
Satellite Communications Center "Dubna" owned by Russian Satellite Communications Company
Parabolic antennas.JPG
Author/Creator: Miguel Ferrando, Licence: CC BY 2.5
A communications tower in Australia, supporting a variety of parabolic antennas used for point-to-point microwave communication links. Some of the parabolic dishes are covered by convex white plastic radomes to protect against rain.
Author/Creator:
, Licence: CC BY-SA 2.5 au
A broadcasting antenna for an FM radio station on a communications tower on Willans Hill, Wagga Wagga, New South Wales, Australia. These are called crossed dipole antennas, probably the most common type used for FM broadcasting. They consist of two V-shaped dipole elements facing each other; the one on the right driven by a coaxial cable, the one on the left a parasitic element. They radiate a circularly polarized (Cpol) radio signal. Four crossed-dipole "bays", fed in phase, are stacked vertically to increase the horizontal gain, so less of the signal power is radiated into the sky or down toward the ground and wasted.

Alterations to original image: cropped out other parts of tower, increased brightness some.
Faster communications aid Marine, civilian first responders 130827-M-KX456-009.jpg
Tina Bryant, a firefighter with the Marine Corps Base Quantico, Va., Fire Department, demonstrates how her unit uses the enterprise-land mobile radio Aug. 27. Quantico completed its upgrade to the first-responder radio system, part of a Marine Corps-wide E-LMR project, in February 2013. (U.S. Marine Corps photo by Cpl. Antwaun Jefferson)
Unit: Marine Corps Systems Command
DVIDS Tags: Communications; Department of Defense; Command; Fort Hood; Okinawa; Control; first responders; Pacific; 911; Sept. 11; terrorist attacks; interoperability; Fire Department; acquisition; Provost Marshall Office; California; Bridgeport; Quantico; Marine Corps Base; Marine Corps Systems Command; Lester; Marine Air-Ground Task Force; Department of the Navy; MC3; Mountain Warfare Training Center; SYSCOM; U.S. Forest Service; MCSC; Tengan; MARCORSYSCOM; Force Protection Systems; enterprise-land mobile radio; E-LMR; wireless radio; Fort Buckner; Ieshima; Project 25; digital radio; continental United States
Rotating marine radar - rotating waveguide antenna.gif
Author/Creator: Amada44, Licence: CC BY-SA 3.0
Animated GIF showing a rotating waveguide antenna of a typical X band marine radar on a ship. It radiates a narrow vertical fan-shaped beam of microwaves perpendicular to the antenna bar, along the water surface. The rotating antenna scans the water surface surrounding the ship out to the horizon. The microwaves reflect off any ships present and some return to the antenna, causing a "blip" to be displayed on the radar screen.
Wireless network.jpg
Author/Creator: Clawed, Licence: CC-BY-SA-3.0
A laptop computer, and home wireless router (background), which connects it to the Internet. The laptop has a WiFi wireless modem which transmits data to and receives data from the router, which passes the data on to the Internet through a cable connection provided by an Internet service provider (ISP). The Wifi modem has an automated radio transmitter which transmits packets of digital data by microwaves to a receiver in the router, and the router has a transmitter which transmits data packets back to the laptop. The router may also connect the laptop to other devices on the home network, such as a printer or another computer. The laptop's modem is an add-on card, visible as the turquoise device protruding from the left side.
SuperDISH121.jpg
A en:DISH Network SuperDISH 121 mounted on a house.
D-VOR PEK.JPG
Author/Creator: unknown, Licence: CC BY-SA 2.0
MQ-1 Predator unmanned aircraft.jpg
US Air Force MQ-1 Predator unmanned aerial vehicle. U.S. Air Force photo by Lt. Col. Leslie Pratt.
Radio towers on Sandia Peak - closeup.jpg
Author/Creator: Maveric149 (Daniel Mayer), Licence: CC BY-SA 3.0
A variety of radio antennas on Sandia peak, near Albuquerque, New Mexico. Radio transmitting antennas are often located on the highest geographical point in the area, such as mountain peaks, to give them the maximum transmission range, resulting in antenna farms like this. The vertical rods are mainly whip antennas, often base station antennas used by dispatchers for land mobile radio systems for fire, police, ambulance, taxi, and delivery services. The multiple identical antennas strung in a vertical line along towers are VHF collinear array broadcasting antennas for FM and television stations. The large cylindrical antennas are shrouded parabolic dishes used for point-to-point microwave relay links for internet and telephone companies.