Radiophysics (also modern writing radio physics ) is a branch of physics focused on the theoretical and experimental study of certain kinds of radiation , its emission, propagation and interaction with matter.
92-522: The term is used in the following major meanings: Among the main applications of radiophysics are radio communications, radiolocation , radio astronomy and radiology. This physics -related article is a stub . You can help Misplaced Pages by expanding it . Radio Radio is the technology of communicating using radio waves . Radio waves are electromagnetic waves of frequency between 3 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called
184-471: A directional antenna transmits radio waves in a beam in a particular direction, or receives waves from only one direction. Radio waves travel at the speed of light in vacuum and at slightly lower velocity in air. The other types of electromagnetic waves besides radio waves, infrared , visible light , ultraviolet , X-rays and gamma rays , can also carry information and be used for communication. The wide use of radio waves for telecommunication
276-418: 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 generate
368-492: A radar screen . Doppler radar can measure a moving object's velocity, by measuring the change in frequency of the return radio waves due to the Doppler effect . 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
460-632: A transmitter connected to an antenna which radiates oscillating electrical energy, often characterized as a wave . They can be received by other antennas connected to a radio receiver ; this is the fundamental principle of radio communication. In addition to communication, radio is used for radar , radio navigation , remote control , remote sensing , and other applications. In radio communication , used in radio and television broadcasting , cell phones, two-way radios , wireless networking , and satellite communication , among numerous other uses, radio waves are used to carry information across space from
552-474: A " push to talk " button on their radio which switches off the receiver and switches on the transmitter. Or the radio link may be full duplex , a bidirectional link using two radio channels so both people can talk at the same time, as in a cell phone. One way, unidirectional radio transmission is called simplex . This is radio communication between a spacecraft and an Earth-based ground station, or another spacecraft. Communication with spacecraft involves
644-606: 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 22 June 1907 Electrical World about
736-613: A commemoration of the event. The Photophone Centenary commemoration had first been proposed by electronics researcher and writer Forrest M. Mims , who suggested it to Dr. Melville Bell Grosvenor , the inventor's grandson, during a visit to his office at the National Geographic Society. The historic grouping later observed the centennial of the photophone's first successful laboratory transmission by using Mims hand-made demonstration photophone, which functioned similar to Bell and Tainter's model. Mims also built and provided
828-412: A controller device control the actions of a remote device. The existence of radio waves was first proven by German physicist Heinrich Hertz on 11 November 1886. In the mid-1890s, building on techniques physicists were using to study electromagnetic waves, Italian physicist Guglielmo Marconi developed the first apparatus for long-distance radio communication, sending a wireless Morse Code message to
920-403: A deposit of lampblack, produced a tone that Bell described as "painfully loud" to an ear pressed close to the device. In its ultimate electronic form, the photophone receiver used a simple selenium cell photodetector at the focus of a parabolic mirror. The cell's electrical resistance (between about 100 and 300 ohms ) varied inversely with the light falling upon it, i.e., its resistance
1012-421: 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
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#17328584727641104-545: A government license, such as the general radiotelephone operator license in the US, obtained by taking a test demonstrating adequate technical and legal knowledge of safe radio operation. 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
1196-572: A large economic cost, but it can also be life-threatening (for example, in the case of interference with emergency communications or air traffic control ). To prevent interference between different users, the emission of radio waves is strictly regulated by national laws, coordinated by an international body, the International Telecommunication Union (ITU), which allocates bands in the radio spectrum for different uses. Radio transmitters must be licensed by governments, under
1288-595: A metal conductor called an antenna . As they travel farther from the transmitting antenna, radio waves spread out so their signal strength ( intensity in watts per square meter) decreases (see Inverse-square law ), so radio transmissions can only be received within a limited range of the transmitter, the distance depending on the transmitter power, the antenna radiation pattern , receiver sensitivity, background noise level, and presence of obstructions between transmitter and receiver . An omnidirectional antenna transmits or receives radio waves in all directions, while
1380-542: A meter to see the effects of light acting on selenium connected in a circuit to a battery. However Bell reasoned that by adding a telephone receiver to the same circuit he would be able to hear what Sabine could only see. As Bell's former associate, Thomas Watson , was fully occupied as the superintendent of manufacturing for the nascent Bell Telephone Company back in Boston, Massachusetts, Bell hired Charles Sumner Tainter , an instrument maker who had previously been assigned to
1472-427: A 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,
1564-443: A novelty, and radio was decades away from commercialization. The social resistance to the photophone's futuristic form of communications could be seen in an August 1880 New York Times commentary: The ordinary man ... will find a little difficulty in comprehending how sunbeams are to be used. Does Prof. Bell intend to connect Boston and Cambridge ... with a line of sunbeams hung on telegraph posts , and, if so, what diameter are
1656-453: A pair of modern hand-held battery-powered LED transceivers connected by 100 yards (91 m) of optical fiber . The Bell Labs' Richard Gundlach and the Smithsonian's Elliot Sivowitch used the device at the commemoration to demonstrate one of the photophone's modern-day descendants. The National Geographic Society also mounted a special educational exhibit in its Explorer's Hall, highlighting
1748-470: A primitive spark-gap transmitter . Experiments by Hertz and physicists Jagadish Chandra Bose , Oliver Lodge , Lord Rayleigh , and Augusto Righi , among others, showed that radio waves like light demonstrated reflection, refraction , diffraction , polarization , standing waves , and traveled at the same speed as light, confirming that both light and radio waves were electromagnetic waves, differing only in frequency. In 1895, Guglielmo Marconi developed
1840-420: A public audience. Analog audio is the earliest form of radio broadcast. AM broadcasting began around 1920. FM broadcasting was introduced in the late 1930s with improved fidelity . A broadcast radio receiver is called a radio . Most radios can receive both AM and FM. Television broadcasting is the transmission of moving images by radio, which consist of sequences of still images, which are displayed on
1932-459: 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 ). 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
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#17328584727642024-489: A receiver that is typically colocated with the transmitter. In radio navigation systems such as GPS and VOR , a mobile navigation instrument receives radio signals from multiple navigational radio beacons whose position is known, and by precisely measuring the arrival time of the radio waves the receiver can calculate its position on Earth. In wireless radio remote control devices like drones , garage door openers , and keyless entry systems , radio signals transmitted from
2116-521: A recipient over a kilometer away in 1895, and the first transatlantic signal on 12 December 1901. The first commercial radio broadcast was transmitted on 2 November 1920, when the live returns of the Harding-Cox 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, coordinated by
2208-644: A reference to the radiotelegraph and radiotelegraphy . The use of radio as a standalone word dates back to at least 30 December 1904, when instructions issued by the British Post Office for transmitting telegrams specified that "The word 'Radio'... is sent in the Service Instructions." This practice was universally adopted, and the word "radio" introduced internationally, by the 1906 Berlin Radiotelegraphic Convention, which included
2300-403: A reporter shortly before his death that the photophone was "the greatest invention [I have] ever made, greater than the telephone". The photophone was a precursor to the fiber-optic communication systems that achieved worldwide popular usage starting in the 1980s. The master patent for the photophone ( U.S. patent 235,199 Apparatus for Signalling and Communicating, called Photophone )
2392-446: 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,
2484-783: A shadow and I have even perceived by ear the passage of a cloud across the sun's disk. You are the grandfather of the Photophone and I want to share my delight at my success. Bell transferred the photophone's intellectual property rights to the American Bell Telephone Company in May 1880. While Bell had hoped his new photophone could be used by ships at sea and to also displace the plethora of telephone lines that were blooming along busy city boulevards, his design failed to protect its transmissions from outdoor interferences such as clouds, fog, rain, snow and such, that could easily disrupt
2576-441: 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
2668-416: 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. 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
2760-400: A transmitter to a receiver, by modulating the radio signal (impressing an information signal on the radio wave by varying some aspect of the wave) in the transmitter. In radar, used to locate and track objects like aircraft, ships, spacecraft and missiles, a beam of radio waves emitted by a radar transmitter reflects off the target object, and the reflected waves reveal the object's location to
2852-652: 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 on the state of the device back to the control station. Uncrewed 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
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2944-435: A variety of license classes depending on use, and are restricted to certain frequencies and power levels. In some classes, such as radio and television broadcasting stations, the transmitter is given a unique identifier consisting of a string of letters and numbers called a call sign , which must be used in all transmissions. In order to adjust, maintain, or internally repair radiotelephone transmitters, individuals must hold
3036-514: A wireless voice telephone message from the roof of the Franklin School to the window of Bell's laboratory, some 213 meters (about 700 ft.) away. Bell believed the photophone was his most important invention . Of the 18 patents granted in Bell's name alone, and the 12 he shared with his collaborators, four were for the photophone, which Bell referred to as his "greatest achievement", telling
3128-413: 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. This voltage is applied to the radio receiver , which amplifies
3220-703: 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: 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. 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
3312-549: Is a telecommunications device that allows transmission of speech on a beam of light . It was invented jointly by Alexander Graham Bell and his assistant Charles Sumner Tainter on February 19, 1880, at Bell's laboratory at 1325 L Street in Washington, D.C. Both were later to become full associates in the Volta Laboratory Association , created and financed by Bell. On June 3, 1880, Bell's assistant transmitted
3404-445: 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
3496-433: Is an audio transceiver , a receiver and transmitter in the same device, used for bidirectional person-to-person voice communication with other users with similar radios. An older term for this mode of communication is radiotelephony . The radio link may be half-duplex , as in a walkie-talkie , using a single radio channel in which only one radio can transmit at a time, so different users take turns talking, pressing
3588-410: Is called "tuning". The oscillating radio signal from the desired station causes the tuned circuit to resonate , oscillate in sympathy, and it passes the signal on to the rest of the receiver. Radio signals at other frequencies are blocked by the tuned circuit and not passed on. A modulated radio wave, carrying an information signal, occupies a range of frequencies . The information ( modulation ) in
3680-427: Is called an uplink , while a link that transmits data from the spacecraft to the ground is called a downlink. Radar is a radiolocation method used to locate and track aircraft, spacecraft, missiles, ships, vehicles, and also to map weather patterns and terrain. A radar set consists of a transmitter and receiver. The transmitter emits a narrow beam of radio waves which is swept around the surrounding space. When
3772-553: 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. A two-way radio
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3864-417: Is mainly due to their desirable propagation properties stemming from their longer wavelength. 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. The modulation signal may be an audio signal representing sound from
3956-524: Is the one-way transmission of information from a transmitter to receivers belonging to a public audience. Since the radio waves become weaker with distance, a broadcasting station can only be received within a limited distance of its transmitter. Systems that broadcast from satellites can generally be received over an entire country or continent. Older terrestrial radio and television are paid for by commercial advertising or governments. In subscription systems like satellite television and satellite radio
4048-476: 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. ELF 3 Hz/100 Mm 30 Hz/10 Mm SLF 30 Hz/10 Mm 300 Hz/1 Mm ULF 300 Hz/1 Mm 3 kHz/100 km Photophone The photophone
4140-564: The German Navy , which were further adapted to increase their range to 11 kilometres (6.8 mi) using voice-modulated ship searchlights . British Admiralty research during WWI resulted in the development of a vibrating mirror modulator in 1916. More sensitive molybdenite receiver cells, which also had greater sensitivity to infra-red radiation, replaced the older selenium cells in 1917. The United States and German governments also worked on technical improvements to Bell's system. By 1935
4232-591: The International Telecommunication Union (ITU), which allocates frequency bands in the radio spectrum for various uses. The word radio is derived from the Latin word radius , meaning "spoke of a wheel, beam of light, ray". It was first applied to communications in 1881 when, at the suggestion of French scientist Ernest Mercadier [ fr ] , Alexander Graham Bell adopted radiophone (meaning "radiated sound") as an alternate name for his photophone optical transmission system. Following Hertz's discovery of
4324-465: The U.S. 1874 Transit of Venus Commission , for his new 'L' Street laboratory in Washington , at the rate of $ 15 per week. On February 19, 1880, the pair had managed to make a functional photophone in their new laboratory by attaching a set of metallic gratings to a diaphragm, with a beam of light being interrupted by the gratings movement in response to spoken sounds. When the modulated light beam fell upon their selenium receiver Bell, on his headphones,
4416-497: The ionosphere without refraction , and at microwave frequencies the high-gain antennas needed to focus the radio energy into a narrow beam pointed at the receiver are small and take up a minimum of space in a satellite. Portions of the UHF , L , C , S , k u and k a band are allocated for space communication. A radio link that transmits data from the Earth's surface to a spacecraft
4508-400: The radio spectrum into 12 bands, each beginning at a wavelength which is a power of ten (10 ) metres, with corresponding frequency of 3 times a power of ten, and each covering a decade of frequency or wavelength. Each of these bands has a traditional name: It can be seen that the bandwidth , the range of frequencies, contained in each band is not equal but increases exponentially as
4600-531: The 1920s with the introduction of broadcasting. Electromagnetic waves were predicted by James Clerk Maxwell in his 1873 theory of electromagnetism , now called Maxwell's equations , who proposed that a coupled oscillating electric field and magnetic field could travel through space as a wave, and proposed that light consisted of electromagnetic waves of short wavelength . On 11 November 1886, German physicist Heinrich Hertz , attempting to confirm Maxwell's theory, first observed radio waves he generated using
4692-474: 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 . Radio broadcasting means transmission of audio (sound) to radio receivers belonging to
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#17328584727644784-562: The Franklin School commemorating the first formal trial On March 3, 1947, the centenary of Alexander Graham Bell 's birth, the Telephone Pioneers of America dedicated a historical marker on the side of one of the buildings, the Franklin School , which Bell and Sumner Tainter used for their first formal trial involving a considerable distance. Tainter had originally stood on the roof of the school building and transmitted to Bell at
4876-668: The German Carl Zeiss Company had started producing infra-red photophones for the German Army 's tank battalions, employing tungsten lamps with infra-red filters which were modulated by vibrating mirrors or prisms. These also used receivers which employed lead sulfide detector cells and amplifiers, boosting their range to 14 kilometres (8.7 mi) under optimal conditions. The Japanese and Italian armies also attempted similar development of lightwave telecommunications before 1945. Several military laboratories, including those in
4968-455: The Photophone that are undreamed of just now. Although Bell Telephone researchers made several modest incremental improvements on Bell and Tainter's design, Marconi's radio transmissions started to far surpass the maximum range of the photophone as early as 1897 and further development of the photophone was largely arrested until German-Austrian experiments began at the turn of the 20th century. The German physicist Ernst Ruhmer believed that
5060-638: The Sun's radiant energy in multiple bands including the invisible infrared band . Bell used the name for a while but it should not be confused with the later invention " radiophone " which used radio waves . While honeymooning in Europe with his bride Mabel Hubbard , Bell likely read of the newly discovered property of selenium having a variable resistance when acted upon by light, in a paper by Robert Sabine as published in Nature on 25 April 1878. In his experiments, Sabine used
5152-399: The U.S. by Edison . The transmitter in their latter experiments had sunlight reflected off the surface of a very thin mirror positioned at the end of a speaking tube; as words were spoken they cause the mirror to oscillate between convex and concave, altering the amount of light reflected from its surface to the receiver. Tainter, who was on the roof of the Franklin School , spoke to Bell, who
5244-580: 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. Below are some of the most important uses of radio, organized by function. Broadcasting
5336-707: The United States, continued R&D efforts on the photophone into the 1950s, experimenting with high-pressure vapour and mercury arc lamps of between 500 and 2,000 watts power. FROM THE TOP FLOOR OF THIS BUILDING WAS SENT ON JUNE 3, 1880 OVER A BEAM OF LIGHT TO 1325 'L' STREET THE FIRST WIRELESS TELEPHONE MESSAGE IN THE HISTORY OF THE WORLD. THE APPARATUS USED IN SENDING THE MESSAGE WAS THE PHOTOPHONE INVENTED BY ALEXANDER GRAHAM BELL INVENTOR OF THE TELEPHONE THIS PLAQUE WAS PLACED HERE BY ALEXANDER GRAHAM BELL CHAPTER TELEPHONE PIONEERS OF AMERICA MARCH 3, 1947 THE CENTENNIAL OF DR. BELL'S BIRTH Marker on
5428-405: The air simultaneously without interfering with each other because each transmitter's radio waves oscillate at a different rate, in other words, each transmitter has a different frequency , measured in hertz (Hz), kilohertz (kHz), megahertz (MHz) or gigahertz (GHz). The receiving antenna typically picks up the radio signals of many transmitters. The receiver uses tuned circuits to select
5520-427: The audio-frequency variations in air pressure—the sound waves—which acted upon the mirror. In its initial form, the photophone receiver was also non-electronic, using the photoacoustic effect . Bell found that many substances could be used as direct light-to-sound transducers. Lampblack proved to be outstanding. Using a fully modulated beam of sunlight as a test signal, one experimental receiver design, employing only
5612-414: The beam strikes a target object, radio waves are reflected back to the receiver. The direction of the beam reveals the object's location. Since radio waves travel at a constant speed close to the speed of light , by measuring the brief time delay between the outgoing pulse and the received "echo", the range to the target can be calculated. The targets are often displayed graphically on a map display called
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#17328584727645704-668: The continuous waves which were needed for audio modulation , so radio was used for person-to-person commercial, diplomatic and military text messaging. Starting around 1908 industrial countries built worldwide networks of powerful transoceanic transmitters to exchange telegram traffic between continents and communicate with their colonies and naval fleets. During World War I the development of continuous wave radio transmitters, rectifying electrolytic, and crystal radio receiver detectors enabled amplitude modulation (AM) radiotelephony to be achieved by Reginald Fessenden and others, allowing audio to be transmitted. On 2 November 1920,
5796-466: The customer pays a monthly fee. In these systems, the radio signal is encrypted and can only be decrypted by the receiver, which is controlled by the company and can be deactivated if the customer does not pay. 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
5888-476: The day and at night. He continued his experiments around Berlin through 1904, in conjunction with the German Navy, which supplied high-powered searchlights for use in the transmissions. The German Siemens & Halske Company boosted the photophone's range by utilizing current-modulated carbon arc lamps which provided a useful range of approximately 8 kilometres (5.0 mi). They produced units commercially for
5980-494: The enemy. Bell pondered the photophone's possible scientific use in the spectral analysis of artificial light sources, stars and sunspots . He later also speculated on its possible future applications, though he did not anticipate either the laser or fiber-optic telecommunications : Can Imagination picture what the future of this invention is to be!.... We may talk by light to any visible distance without any conduction wire.... In general science, discoveries will be make by
6072-570: The existence of radio waves in 1886, the term Hertzian waves was initially used for this radiation. The first practical radio communication systems, developed by Marconi in 1894–1895, 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
6164-413: The first commercial radio broadcast was transmitted by Westinghouse Electric and Manufacturing Company in Pittsburgh, under the call sign KDKA featuring live coverage of the Harding-Cox presidential election . Radio waves are radiated by electric charges undergoing acceleration . They are generated artificially by time-varying electric currents , consisting of electrons flowing back and forth in
6256-580: The first radio communication system, using a spark-gap transmitter to send Morse code over long distances. By December 1901, he had transmitted 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". During radio's first two decades, called the radiotelegraphy era, the primitive radio transmitters could only transmit pulses of radio waves, not
6348-615: The frequency increases; each band contains ten times the bandwidth of the preceding band. The term "tremendously low frequency" (TLF) has been used for wavelengths from 1–3 Hz (300,000–100,000 km), though the term has not been defined by the ITU. The airwaves are a resource shared by many users. Two radio transmitters in the same area that attempt to transmit on the same frequency will interfere with each other, causing garbled reception, so neither transmission may be received clearly. Interference with radio transmissions can not only have
6440-505: The increased sensitivity of his improved selenium cells, combined with the superior receiving capabilities of professor H. T. Simon's "speaking arc", would make the photophone practical over longer signalling distances. Ruhmer carried out a series of experimental transmissions along the Havel river and on Lake Wannsee from 1901 to 1902. He reported achieving sending distances under good conditions of 15 kilometers (9 miles), with equal success during
6532-402: The longest transmission distances of any radio links, up to billions of kilometers for interplanetary spacecraft . In order to receive the weak signals from distant spacecraft, satellite ground stations use large parabolic "dish" antennas up to 25 metres (82 ft) in diameter and extremely sensitive receivers. High frequencies in the microwave band are used, since microwaves pass through
6624-466: The need for legal restrictions warned that "Radio chaos will certainly be the result until such stringent regulation is enforced." 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
6716-471: The photophone the world's earliest known voice wireless telephone system, at least 19 years ahead of the first spoken radio wave transmissions. Before Bell and Tainter had concluded their research in order to move on to the development of the Graphophone , they had devised some 50 different methods of modulating and demodulating light beams for optical telephony. The telephone itself was still something of
6808-505: 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. 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
6900-451: 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,
6992-409: The radio signal desired out of all the signals picked up by the antenna and reject the others. A tuned circuit (also called resonant circuit or tank circuit) acts like a resonator , similar to a tuning fork . It has a natural resonant frequency at which it oscillates. The resonant frequency of the receiver's tuned circuit is adjusted by the user to the frequency of the desired radio station; this
7084-451: 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. A slow transition from analog to digital radio transmission technologies began in the late 1990s. Part of the reason for this is that digital modulation can often transmit more information (a greater data rate) in
7176-456: The radio waves that carry the information through the air. The modulation signal is used to modulate the carrier, varying some aspect of the carrier wave, impressing the information in the modulation signal onto the carrier. Different radio systems use different modulation methods: 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
7268-481: 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 . The ITU arbitrarily divides
7360-426: The simplest form of apparatus for producing the effect consists of a plane mirror of flexible material against the back of which the speaker's voice is directed. Under the action of the voice the mirror becomes alternately convex and concave and thus alternately scatters and condenses the light. The brightness of a reflected beam of light, as observed from the location of the receiver, therefore varied in accordance with
7452-439: The sunbeams to be ....[and] will it be necessary to insulate them against the weather ... until (the public) sees a man going through the streets with a coil of No. 12 sunbeams on his shoulder, and suspending them from pole to pole, there will be a general feeling that there is something about Professor Bell's photophone which places a tremendous strain on human credulity. However at the time of their February 1880 breakthrough, Bell
7544-552: The transmission of light. Factors such as the weather and the lack of light inhibited the use of Bell's invention. Not long after its invention laboratories within the Bell System continued to improve the photophone in the hope that it could supplement or replace expensive conventional telephone lines . Its earliest non-experimental use came with military communication systems during World War I and II, its key advantage being that its light-based transmissions could not be intercepted by
7636-431: 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 . Radiolocation is a generic term covering a variety of techniques that use radio waves to find the location of objects, or for navigation. Radio remote control is the use of electronic control signals sent by radio waves from
7728-510: 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. The radio waves from many transmitters pass through
7820-580: The window of his laboratory. The marker did not acknowledge Tainter's scientific and engineering contributions. On February 19, 1980, exactly 100 years to the day after Bell and Tainter's first photophone transmission in their laboratory, staff from the Smithsonian Institution , the National Geographic Society and AT&T's Bell Labs gathered at the location of Bell's former 1325 'L' Street Volta Laboratory in Washington, D.C. for
7912-536: Was a need for a more precise term referring exclusively to electromagnetic radiation. The French physicist Édouard Branly , who in 1890 developed the radio wave detecting coherer , called it in French a radio-conducteur . 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
8004-470: Was able to clearly hear Tainter singing Auld Lang Syne . In an April 1, 1880, Washington, D.C. , experiment, Bell and Tainter communicated some 79 metres (259 ft) along an alleyway to the laboratory's rear window. Then a few months later on June 21 they succeeded in communicating clearly over a distance of some 213 meters (about 700 ft.), using plain sunlight as their light source, practical electrical lighting having only just been introduced to
8096-696: Was converted back into variations of air pressure—sound—by the earphone. In his speech to the American Association for the Advancement of Science in August 1880, Bell gave credit for the first demonstration of speech transmission by light to Mr. A.C. Brown of London in the Fall of 1878. Because the device used radiant energy , the French scientist Ernest Mercadier suggested that the invention should not be named 'photophone', but 'radiophone', as its mirrors reflected
8188-401: Was higher when dimly lit, lower when brightly lit. The selenium cell took the place of a carbon microphone—also a variable-resistance device—in the circuit of what was otherwise essentially an ordinary telephone, consisting of a battery, an electromagnetic earphone, and the variable resistance, all connected in series. The selenium modulated the current flowing through the circuit, and the current
8280-399: Was immensely proud of the achievement, to the point that he wanted to name his new second daughter "Photophone", which was subtly discouraged by his wife Mabel Bell (they instead chose "Marian", with "Daisy" as her nickname ). He wrote somewhat enthusiastically: I have heard articulate speech by sunlight! I have heard a ray of the sun laugh and cough and sing! ...I have been able to hear
8372-414: Was in his laboratory listening and who signaled back to Tainter by waving his hat vigorously from the window, as had been requested. The receiver was a parabolic mirror with selenium cells at its focal point. Conducted from the roof of the Franklin School to Bell's laboratory at 1325 'L' Street, this was the world's first formal wireless telephone communication (away from their laboratory), thus making
8464-454: Was issued in December 1880, many decades before its principles came to have practical applications. The photophone was similar to a contemporary telephone, except that it used modulated light as a means of wireless transmission while the telephone relied on modulated electricity carried over a conductive wire circuit . Bell's own description of the light modulator: We have found that
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