Radio

Radio is the technology of communicating using radio waves. Radio waves are electromagnetic waves with a frequency between 3 hertz (Hz) and 300 gigahertz (GHz). A electronic device called a transmitter creates the waves and sends them out through an antenna. Other antennas connected to a radio receiver can pick up the waves. This is the basic idea behind radio communication. Radio is also used for radar, radio navigation, remote control, remote sensing, and other jobs.

Radio communication is used in radio and television broadcasting, cell phones, two-way radios, wireless networking, and satellite communication, and in many other ways. Radio waves carry information through space from a transmitter to a receiver. To do this, the signal is modulated, which means some part of the wave is changed so that it carries the information.

Radar is used to find and track objects such as aircraft, ships, spacecraft, and missiles. A beam of radio waves from a radar transmitter bounces off the object. The reflected waves show a receiver, which is usually placed with the transmitter, where the object is. Radio navigation systems such as GPS and VOR work by having a navigation instrument receive signals from several navigational radio beacons at known places. By measuring how long the waves take to arrive, the receiver can work out its position on Earth. Wireless radio remote control devices, like drones, garage door openers, and keyless entry systems, use signals from a controller to operate a device from a distance.

The existence of radio waves was first proven by German physicist Heinrich Hertz on 11 November 1886. In the mid-1890s, Italian physicist Guglielmo Marconi used ideas that other physicists had been using to study electromagnetic waves. He developed the first equipment for long-distance radio communication. In 1895 he sent a wireless Morse Code message to a person more than a kilometer away. On 12 December 1901 he sent the first signal across the Atlantic Ocean. The first commercial radio broadcast was sent on 2 November 1920. Its live news was about the 1920 United States presidential election. Westinghouse Electric and Manufacturing Company broadcast it from Pittsburgh under the call sign KDKA.

The sending of radio waves is controlled by law. The International Telecommunication Union (ITU) coordinates this work and assigns frequency bands in the radio spectrum for different uses. This article covers the general technology. For the audio application, see Radio broadcasting. For the device commonly known as a "radio", see Radio receiver. For other uses, see Radio (disambiguation).

Etymology

The word radio comes from the Latin word radius, which means "spoke of a wheel, beam of light, ray." In 1881, French scientist Ernest Mercadier suggested the word for communication. Alexander Graham Bell later used radiophone (meaning "radiated sound") as another name for his photophone system. After Hertz found radio waves in 1886, they were first called Hertzian waves. Marconi built the first useful radio systems in 1894–1895. They sent telegraph signals through radio waves, so radio was first called wireless telegraphy. Until about 1910, this name also covered other experiments for sending signals without wires, such as electrostatic induction, electromagnetic induction, and aquatic and earth conduction. A more exact name was needed for radio waves alone.

The French physicist Édouard Branly, who built the coherer in 1890, called it radio-conducteur in French. The prefix radio- was later used to build many other words, especially in Europe. The word radio by itself was used by 30 December 1904, when British Post Office instructions for sending telegrams included it. The 1906 Berlin Radiotelegraphic Convention made "radio" the standard word around the world. In English-speaking countries, the change from wireless to radio was slow and uneven. Lee de Forest helped make the new word popular in the United States, and the United States Navy began using radio in 1912. The general public started to prefer the word in the 1920s, when broadcasting began.

History

Main article: History of radio

Further information: Invention of radio, Timeline of radio, and History of broadcasting

Electromagnetic waves were predicted by James Clerk Maxwell in 1873. His theory of electromagnetism is now called Maxwell's equations. He said that an electric field and a magnetic field could travel through space together as a wave. He also said that light is made of electromagnetic waves with a short wavelength.

On 11 November 1886, German physicist Heinrich Hertz first saw radio waves. He made them with a simple spark-gap transmitter while he was trying to test Maxwell's theory. Hertz and other scientists, including Jagadish Chandra Bose, Oliver Lodge, Lord Rayleigh, and Augusto Righi, showed that radio waves act like light. They can reflect, bend (refraction), spread out (diffraction), be polarized, and form standing waves. Radio waves travel at the same speed as light. The only difference is their frequency.

In 1895, Guglielmo Marconi built the first radio communication system. It sent Morse code over long distances. By December 1901, he had sent signals across the Atlantic Ocean. Marconi and Karl Ferdinand Braun shared the 1909 Nobel Prize in Physics "for their contributions to the development of wireless telegraphy."

In the first two decades of radio, called the radiotelegraphy era, transmitters could only send short pulses of radio waves. They could not send the continuous waves needed to carry sound. So radio was mostly used to send text messages for business, diplomacy, and the military. Starting around 1908, industrial countries built powerful networks of transmitters. These could send telegram messages across oceans, linking continents, their colonies, and their navy fleets.

During World War I, new continuous wave transmitters, rectifying electrolytic detectors, and crystal radio receiver detectors were developed. These helped Reginald Fessenden and others achieve radiotelephony using amplitude modulation (AM). This allowed audio to be sent by radio.

On 2 November 1920, Westinghouse Electric and Manufacturing Company broadcast the first commercial radio program in Pittsburgh, under the call sign KDKA. The broadcast included live coverage of the 1920 United States presidential election.

Principles

Radio waves are sent out by electric charges that speed up or change direction. People make radio waves on purpose using electric currents that change over time. These currents are electrons moving back and forth in a metal wire called an antenna.

As radio waves travel away from the antenna, they spread out. This makes the signal strength (intensity in watts per square meter) get weaker. This happens because of the Inverse-square law. So radio signals can only be received within a limited distance. That distance depends on how strong the sender is, the antenna's radiation pattern, how well the receiver can pick up signals, the background noise, and obstructions between transmitter and receiver. An omnidirectional antenna sends or receives radio waves in all directions. A directional antenna sends them in one direction or receives them from one direction.

Radio waves travel at the speed of light in a vacuum, and a little slower in air. Other electromagnetic waves, such as infrared, visible light, ultraviolet, X-rays, and gamma rays, can also carry information. Radio waves are used a lot for communication because of their helpful propagation properties. These come from their long wavelength. Radio waves can pass through the air in any weather and through leaves and plants. At longer wavelengths, they can also pass through most building materials. Through diffraction, longer waves can bend around things that block them. Unlike other electromagnetic waves, radio waves tend to bounce off objects that are bigger than their wavelength instead of being soaked up by them.

Communication systems

Radio communication sends information through space using radio waves. At the sending end, a transducer changes the information into a changing electrical signal called the modulation signal. This can be an audio signal for sound from a microphone, a video signal for moving pictures from a video camera, or digital signals made of bits from a computer. The modulation signal goes to a radio transmitter. There, an electronic oscillator makes an alternating current at a radio frequency. This is called the carrier wave, because it carries the information through the air. The modulation signal changes part of the carrier wave to add the information to it. Common methods include amplitude modulation (AM), where the strength changes; frequency modulation (FM), where the frequency changes; and frequency-shift keying (FSK), used in wireless digital devices, where the frequency shifts between set values. Orthogonal frequency-division multiplexing (OFDM) is a group of digital modulation methods. It is used in Wi-Fi networks, cellphones, digital television broadcasting, and digital audio broadcasting (DAB). It sends many bits at once and uses as little radio spectrum space as possible. It also resists fading better than AM or FM. The modulated carrier is amplified and sent to a transmitting antenna, which sends out radio waves. At the receiver, the waves create a tiny voltage in the receiving antenna. The radio receiver amplifies this weak signal and demodulates it to recover the original information. A transducer then turns it into sound waves through a loudspeaker, into images on a display, or into data for a computer.

Many radio waves can pass through the air at once without mixing, because each one uses a different frequency. Frequency is measured in hertz (Hz), kilohertz (kHz), megahertz (MHz), or gigahertz (GHz). A receiver uses tuned circuits, which act like a resonator, similar to a tuning fork. Each one has a resonant frequency. Tuning sets it to the station's frequency, so the wanted signal gets through while others are blocked. A modulated wave fills a range of frequencies, with the information mostly in sidebands just above and below the carrier. The width of that range is its bandwidth. Bandwidth is a measure of information-carrying capacity for a given signal-to-noise ratio. The bandwidth needed depends on the data rate and the spectral efficiency of the method used. The radio spectrum is limited and is treated as an economic good with a cost, so people try to use less of it. Because demand keeps growing, it has become crowded. This has led to new ideas such as trunked radio systems, spread spectrum transmission, frequency reuse, dynamic spectrum management, and cognitive radio. Starting in the late 1990s, radio slowly moved from analog to digital radio. Digital methods can send more information in the same bandwidth than analog modulation, and they have greater noise immunity. Digital signal processors also offer more power and flexibility.

The ITU divides the radio spectrum into 12 bands. Each band starts at a wavelength that is a power of ten (10ⁿ) metres. Each band covers a range of frequency or wavelength equal to a factor of ten, and each band holds ten times the bandwidth of the band before it.

Regulation

Many users share radio waves. When two nearby transmitters use the same frequency, they interfere with each other, and messages can become garbled or unclear. This can be costly and even dangerous, especially when it affects emergency communications or air traffic control. To prevent this, national laws strictly control radio waves. The International Telecommunication Union (ITU) coordinates them and assigns bands in the radio spectrum for different uses. Governments license radio transmitters and limit their frequencies and power levels. Some stations must use a call sign, a unique set of letters and numbers, in every transmission. People who adjust, maintain, or repair transmitters need a government license, such as the general radiotelephone operator license in the US.

Some low-power, short-range devices can be used without a license. These include cordless phones, wireless devices, walkie-talkies, citizens band radios, wireless microphones, garage door openers, and baby monitors. In the US, these devices follow Part 15 of the Federal Communications Commission (FCC) rules. Many of them use ISM bands, which are set aside for unlicensed use. However, they generally must be type-approved before they are sold. Radio jamming is when someone deliberately sends radio signals to interfere with other signals. During wartime, militaries use jamming against enemies' radio communication. Some totalitarian countries use jamming to stop their citizens from listening to foreign radio stations. US federal law does not allow people to operate or sell jamming devices outside of the military.

Applications

Main article: Applications of radio

See also: Radio spectrum § Applications, and Radio receiver § Applications

Radio is used in many ways. People use it to send programs to listeners, talk to each other, and send information. It is also used for radar, for finding where things are, and for controlling machines from far away.