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). An electronic device called a transmitter, connected to an antenna, creates the waves and sends them out. Other antennas connected to a radio receiver can pick them up, and this is the basic idea behind radio communication. Radio is also used for radar, radio navigation, remote control, remote sensing, and other applications.

Radio communication is used in radio and television broadcasting, cell phones, two-way radios, wireless networking, and satellite communication, among many other uses. Radio waves carry information across space from a transmitter to a receiver by modulating the signal, which means changing some part of the wave so that it carries the information.

Radar is used to find and follow objects such as aircraft, ships, spacecraft, and missiles. A beam of radio waves from a radar transmitter bounces off the target, and the reflected waves show a receiver, 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 positions. By precisely 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 remote device.

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 built on techniques physicists were using to study electromagnetic waves and developed the first equipment for long-distance radio communication. In 1895 he sent a wireless Morse Code message to a recipient over a kilometer away, and on 12 December 1901 he sent the first transatlantic signal. The first commercial radio broadcast was transmitted on 2 November 1920, when the live returns of the 1920 United States presidential election were broadcast by Westinghouse Electric and Manufacturing Company in Pittsburgh, under the call sign KDKA.

The emission of radio waves is regulated by law. The International Telecommunication Union (ITU) coordinates this and allocates frequency bands in the radio spectrum for various 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, meaning "spoke of a wheel, beam of light, ray." In 1881, French scientist Ernest Mercadier suggested the word for communication. Alexander Graham Bell then used radiophone (meaning "radiated sound") as another name for his photophone system. After Hertz discovered radio waves in 1886, the radiation was first called Hertzian waves. Marconi built the first practical radio systems in 1894–1895, which sent telegraph signals by radio waves, so radio was first called wireless telegraphy. Until about 1910, that name also covered other experimental ways of 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 developed 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 on its own was used by 30 December 1904, when British Post Office instructions for sending telegrams included it. The 1906 Berlin Radiotelegraphic Convention made the word "radio" standard around the world. The change from wireless to radio was slow and uneven in English-speaking countries. Lee de Forest helped make the new word popular in the United States, and the United States Navy began promoting radio in 1912. The general public began 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 his 1873 theory of electromagnetism, now called Maxwell's equations. He proposed that an electric field and a magnetic field could travel through space together as a wave, and that light was made of electromagnetic waves with a short wavelength. On 11 November 1886, German physicist Heinrich Hertz first observed radio waves that he made using a simple spark-gap transmitter, while 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 behave like light, with reflection, refraction, diffraction, polarization, and standing waves. Radio waves also travel at the same speed as light. The only difference is their frequency. In 1895, Guglielmo Marconi built the first radio communication system, which 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 radio's first two decades, called the radiotelegraphy era, transmitters could only send short pulses of radio waves. They could not send the continuous waves needed to carry sound. Radio was therefore used to send text messages between people for business, diplomacy, and the military. Starting around 1908, industrial countries built networks of powerful transmitters that 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 made it possible for Reginald Fessenden and others to achieve radiotelephony using amplitude modulation (AM), which 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 given off by electric charges that accelerate. People make them artificially using time-varying electric currents, which are electrons moving back and forth in a metal conductor called an antenna. As radio waves travel farther from the antenna, they spread out, so their signal strength (intensity in watts per square meter) gets weaker (see Inverse-square law). Because of this, radio signals can only be received within a limited range. That range depends on the transmitter's power, the antenna's radiation pattern, the receiver's sensitivity, the background noise, and obstructions between transmitter and receiver. An omnidirectional antenna sends or receives radio waves in all directions, while a directional antenna sends them in one beam or receives them from one direction.

Radio waves travel at the speed of light in a vacuum and at a slightly lower speed in air. Other electromagnetic waves, such as infrared, visible light, ultraviolet, X-rays and gamma rays, can also carry information and be used for communication. Radio waves are widely used for telecommunication mainly because of their useful propagation properties, which come from their longer wavelength. They can pass through the atmosphere in any weather and through foliage, and at longer wavelengths through most building materials. Through diffraction, longer wavelengths can bend around obstructions. Unlike other electromagnetic waves, they tend to be scattered rather than absorbed by objects larger than their wavelength.

Communication systems

In radio communication, information is carried across 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 images from a video camera, or a digital signals made of bits from a computer. The modulation signal goes to a radio transmitter, where 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 strength changes; frequency modulation (FM), where frequency changes; and frequency-shift keying (FSK), used in wireless digital devices, where frequency shifts between values. Orthogonal frequency-division multiplexing (OFDM) is a family of digital modulation methods used in Wi-Fi networks, cellphones, digital television broadcasting, and digital audio broadcasting (DAB). It sends many bits at once using as little radio spectrum bandwidth as possible, and it resists fading better than AM or FM. The modulated carrier is amplified and sent to a transmitting antenna, which radiates radio waves. At the receiver, the waves create a tiny voltage in the receiving antenna. The radio receiver amplifies this weak signal, demodulates it to recover the original information, and a transducer turns it into sound waves through a loudspeaker, into images on a display, or into data for a computer.

Many radio waves pass through the air at once without mixing, because each one uses a different frequency, 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 has a resonant frequency, and tuning sets it to the station's frequency so that the desired signal is passed on 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, 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 there is a push to use less of it. Because of growing demand, it has become crowded, which has led to new ideas such as trunked radio systems, spread spectrum transmission, frequency reuse, dynamic spectrum management, and cognitive radio. A slow move from analog to digital radio began in the late 1990s, partly because digital modulation can send more information in a given bandwidth than analog modulation, and it has greater noise immunity. Digital signal processors also offer more power and flexibility.

The ITU divides the radio spectrum into 12 bands, each starting 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

Radio waves are shared by many users. When two transmitters nearby 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, and the International Telecommunication Union (ITU) coordinates them, allocating 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, including 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 them use ISM bands, which are reserved for unlicensed use, but they generally must be type-approved before they are sold. Radio jamming is the deliberate sending of radio signals to interfere with other signals. During wartime, militaries use jamming against enemies' tactical radio communication. Some totalitarian countries use jamming to stop their citizens from listening to foreign radio stations. US federal law prohibits the nonmilitary operation or sale of jamming devices.

Applications

Main article: Applications of radio

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

Radio has many practical uses. These include broadcasting, voice communication, data communication, radar, radiolocation, and remote control.