A telephone, or phone, is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be heard directly. A telephone converts sound and most efficiently the human voice, into electronic signals that are transmitted via cables and other communication channels to another telephone which reproduces the sound to the receiving user. In 1876, Scottish emigrant Alexander Graham Bell was the first to be granted a United States patent for a device that produced intelligible replication of the human voice; this instrument was further developed by many others. The telephone was the first device in history that enabled people to talk directly with each other across large distances. Telephones became indispensable to businesses and households and are today some of the most used small appliances; the essential elements of a telephone are a microphone to speak into and an earphone which reproduces the voice in a distant location. In addition, most telephones contain a ringer to announce an incoming telephone call, a dial or keypad to enter a telephone number when initiating a call to another telephone.
The receiver and transmitter are built into a handset, held up to the ear and mouth during conversation. The dial may be located either on a base unit to which the handset is connected; the transmitter converts the sound waves to electrical signals which are sent through a telephone network to the receiving telephone, which converts the signals into audible sound in the receiver or sometimes a loudspeaker. Telephones are duplex devices; the first telephones were directly connected to each other from one customer's office or residence to another customer's location. Being impractical beyond just a few customers, these systems were replaced by manually operated centrally located switchboards; these exchanges were soon connected together forming an automated, worldwide public switched telephone network. For greater mobility, various radio systems were developed for transmission between mobile stations on ships and automobiles in the mid-20th century. Hand-held mobile phones were introduced for personal service starting in 1973.
In decades their analog cellular system evolved into digital networks with greater capability and lower cost. Convergence has given most modern cell phones capabilities far beyond simple voice conversation, they may be able to record spoken messages and receive text messages and display photographs or video, play music or games, surf the Internet, do road navigation or immerse the user in virtual reality. Since 1999, the trend for mobile phones is smartphones that integrate all mobile communication and computing needs. A traditional landline telephone system known as plain old telephone service carries both control and audio signals on the same twisted pair of insulated wires, the telephone line; the control and signaling equipment consists of three components, the ringer, the hookswitch, a dial. The ringer, or beeper, light or other device, alerts the user to incoming calls; the hookswitch signals to the central office that the user has picked up the handset to either answer a call or initiate a call.
A dial, if present, is used by the subscriber to transmit a telephone number to the central office when initiating a call. Until the 1960s dials used exclusively the rotary technology, replaced by dual-tone multi-frequency signaling with pushbutton telephones. A major expense of wire-line telephone service is the outside wire plant. Telephones transmit both the outgoing speech signals on a single pair of wires. A twisted pair line rejects electromagnetic interference and crosstalk better than a single wire or an untwisted pair; the strong outgoing speech signal from the microphone does not overpower the weaker incoming speaker signal with sidetone because a hybrid coil and other components compensate the imbalance. The junction box arrests lightning and adjusts the line's resistance to maximize the signal power for the line length. Telephones have similar adjustments for inside line lengths; the line voltages are negative compared to earth. Negative voltage attracts positive metal ions toward the wires.
The landline telephone contains a switchhook and an alerting device a ringer, that remains connected to the phone line whenever the phone is "on hook", other components which are connected when the phone is "off hook". The off-hook components include a transmitter, a receiver, other circuits for dialing and amplification. A calling party wishing to speak to another party will pick up the telephone's handset, thereby operating a lever which closes the switchhook, which powers the telephone by connecting the transmitter and related audio components to the line; the off-hook circuitry has a low resistance which causes a direct current, which comes down the line from the telephone exchange. The exchange detects this current, attaches a digit receiver circuit to the line, sends a dial tone to indicate readiness. On a modern push-button telephone, the caller presses the number keys to send the telephone number of the called party; the keys control a tone generator circuit. A rotary-dial telephone uses pulse
Television, sometimes shortened to tele or telly, is a telecommunication medium used for transmitting moving images in monochrome, or in color, in two or three dimensions and sound. The term can refer to a television set, a television program, or the medium of television transmission. Television is a mass medium for advertising and news. Television became available in crude experimental forms in the late 1920s, but it would still be several years before the new technology would be marketed to consumers. After World War II, an improved form of black-and-white TV broadcasting became popular in the United States and Britain, television sets became commonplace in homes and institutions. During the 1950s, television was the primary medium for influencing public opinion. In the mid-1960s, color broadcasting was introduced in most other developed countries; the availability of multiple types of archival storage media such as Betamax, VHS tape, local disks, DVDs, flash drives, high-definition Blu-ray Discs, cloud digital video recorders has enabled viewers to watch pre-recorded material—such as movies—at home on their own time schedule.
For many reasons the convenience of remote retrieval, the storage of television and video programming now occurs on the cloud. At the end of the first decade of the 2000s, digital television transmissions increased in popularity. Another development was the move from standard-definition television to high-definition television, which provides a resolution, higher. HDTV may be transmitted in various formats: 1080p, 720p. Since 2010, with the invention of smart television, Internet television has increased the availability of television programs and movies via the Internet through streaming video services such as Netflix, Amazon Video, iPlayer and Hulu. In 2013, 79 % of the world's households owned; the replacement of early bulky, high-voltage cathode ray tube screen displays with compact, energy-efficient, flat-panel alternative technologies such as LCDs, OLED displays, plasma displays was a hardware revolution that began with computer monitors in the late 1990s. Most TV sets sold in the 2000s were flat-panel LEDs.
Major manufacturers announced the discontinuation of CRT, DLP, fluorescent-backlit LCDs by the mid-2010s. In the near future, LEDs are expected to be replaced by OLEDs. Major manufacturers have announced that they will produce smart TVs in the mid-2010s. Smart TVs with integrated Internet and Web 2.0 functions became the dominant form of television by the late 2010s. Television signals were distributed only as terrestrial television using high-powered radio-frequency transmitters to broadcast the signal to individual television receivers. Alternatively television signals are distributed by coaxial cable or optical fiber, satellite systems and, since the 2000s via the Internet; until the early 2000s, these were transmitted as analog signals, but a transition to digital television is expected to be completed worldwide by the late 2010s. A standard television set is composed of multiple internal electronic circuits, including a tuner for receiving and decoding broadcast signals. A visual display device which lacks a tuner is called a video monitor rather than a television.
The word television comes from Ancient Greek τῆλε, meaning'far', Latin visio, meaning'sight'. The first documented usage of the term dates back to 1900, when the Russian scientist Constantin Perskyi used it in a paper that he presented in French at the 1st International Congress of Electricity, which ran from 18 to 25 August 1900 during the International World Fair in Paris; the Anglicised version of the term is first attested in 1907, when it was still "...a theoretical system to transmit moving images over telegraph or telephone wires". It was "...formed in English or borrowed from French télévision." In the 19th century and early 20th century, other "...proposals for the name of a then-hypothetical technology for sending pictures over distance were telephote and televista." The abbreviation "TV" is from 1948. The use of the term to mean "a television set" dates from 1941; the use of the term to mean "television as a medium" dates from 1927. The slang term "telly" is more common in the UK; the slang term "the tube" or the "boob tube" derives from the bulky cathode ray tube used on most TVs until the advent of flat-screen TVs.
Another slang term for the TV is "idiot box". In the 1940s and throughout the 1950s, during the early rapid growth of television programming and television-set ownership in the United States, another slang term became used in that period and continues to be used today to distinguish productions created for broadcast on television from films developed for presentation in movie theaters; the "small screen", as both a compound adjective and noun, became specific references to television, while the "big screen" was used to identify productions made for theatrical release. Facsimile transmission systems for still photographs pioneered methods of mechanical scanning of images in the early 19th century. Alexander Bain introduced the facsimile machine between 1843 and 1846. Frederick Bakewell demonstrated a working laboratory version in 1851. Willoughby Smith discovered the photoconductivity of the element selenium in 1873; as a 23-year-old German university student, Paul Julius Gottlieb Nipkow proposed and patented the Nipkow disk in 1884.
This was a spinning disk with a spiral pattern of holes in it, so each hole scanned a line of the image. Although he never built a working model
Telecommunications in Angola
Telecommunications in Angola include telephone, radio and the Internet. The government controls all broadcast media with a nationwide reach. In 2001, toward the end of Angolan Civil War, the government began adopting regulations to liberalize the telecom industry; this enabled private investments to revitalize the country’s telecommunications infrastructure, damaged by the decades-long conflict. By 2012, Angola had one of the largest mobile telecom markets in sub-Saharan Africa and Internet access was growing steadily; the Ministry of Post and Telecommunications oversees the telecommunications sector, regulated by the Angolan National Institute of Telecommunication. 29 satellite earth stations. SAT-3/WASC fiber optic submarine cable provides connectivity to Europe and Asia. AngoSat 1, Angola's first communication satellite, built by RSC Enegria with a credit from Rosoboronexport, is scheduled to launch in 2017. Angola Telecom is one of twelve companies participating in the West Africa Cable System consortium, a submarine communications cable running along the west coast of Africa and on to Portugal and the United Kingdom.
The landing station for the older Sat3 cable, located at Cacuaco in Luanda, is operated by Angola Telecom. Angola Cables is an operator of fiber optic telecommunication systems formed in 2009 by the major Angolan telecommunication companies, Angola Telecom, Unitel, MSTelcom and Mundo Startel. On 23 March 2012 Angola Cables signed an agreement to participate in the construction of the South Atlantic Cable System of about 6000 km length linking Fortaleza in Brazil with the Angolan capital Luanda; this cable is planned to be operational from the 2014 world football championship in Brazil. ADONES consists of 1,800 kilometers of fiber-optic submarine cable linking eight Angolan coastal cities. About 70 percent of Angolans live close to the sea. Other planned fibre optic cables to Angola include SAex and ACE. 303,200 fixed lines, 116th in the world, two lines per 100 persons. 13 million mobile cellular lines, 65 lines per 100 persons.) International country code: 244. Angola Telecom, the state-owned telecom, held a monopoly for fixed-line telephone service until 2005.
Demand outstripped capacity, prices were high, services poor. Telecom Namibia, through an Angolan company, became the first private licensed operator in Angola's fixed-line telephone network. By 2010, the number of fixed-line providers had expanded to five. A owned, mobile-cellular service provider began operations in 2001. HF radiotelephone is used extensively for military links. 21 AM, 6 FM, 7 shortwave radio broadcast stations 630,000 radios The state-owned Radio Nacional de Angola broadcasts on 5 stations. A half dozen private radio stations broadcast locally. 6 television broadcast stations 150,000 televisions The state-owned Televisão Pública de Angola provides terrestrial TV service on two channels and a third TPA channel is available via cable and satellite. TV subscription services are available. Internet hosts: 20,703 hosts, 116th in the world. Internet users: 3,058,195 users, 78th in the world. Fixed broadband: 27,987 subscriptions, 124th in the world. Mobile broadband: 5.000.000 subscriptions.
2015. Top level domain name:.ao. First introduced in 1996, the Internet reached a penetration rate of 16.9 percent in 2012, up from just over 3 percent in 2007, according to the International Telecommunications Union. Fixed-line broadband subscriptions, remain low with a penetration rate of only 0.2 percent in 2012, are concentrated in the capital city, due to the country’s high poverty rate and poor infrastructure in rural areas. Mobile Internet access is higher at 1.5% and access to mobile phones is much higher with a penetration rate of 49% in 2012. In June 2012, Unitel launched a project in partnership with the education ministry and Huawei to provide free Internet access for secondary school students in both public and private schools across the country’s 18 provinces. Known as “E-Net,” the project aims to benefit over 18,000 students with computers supplied by Huawei and Internet access provided by Unitel. Citizens have taken to the Internet as a platform for political debate, to express discontent with the country’s current state of affairs, to launch digital activism initiatives.
Similar to many other African countries, Angolan youth have embraced social media tools and used them to fuel protest movements across the country. The positive impact of digital media tools in Angola was noticeable during the August 2012 parliamentary elections when the Internet was used in innovative ways to advance electoral transparency. For example, citizens were able to report electoral irregularities in real time, while the National Electoral Commission used the Internet and iPads to scan voter registration cards. Internet access in Angola is provided by private ISP's. Telecommunication companies: Angola Telecom, the state-owned telecommunications provider Itelnet MS Telcom, Sonangol owned provider, main focus on oil and gas sector Startel Internet Service Providers: ACS CmcAngola covers majority of luanda and other areas with VSAT technologies ITA - Internet Technologies Angola owned with a focus on corporate services Multitel, corporate focused ISP, subsidiary of Angola Telecom MVcomm NetOne, residential WiMAX services TSOLNETWORKS - Corporate Internet Se
Polytechnic University of Turin
The Polytechnic University of Turin is a partly-public engineering university based in Turin, Italy. Established in 1859, it is Italy’s oldest technical university; the university offers several courses in the fields of Engineering and Industrial Design. The Regio Politecnico di Torino was established in 1906; the present-day institution was preceded by the Scuola di Applicazione per gli Ingegneri and the Museo Industriale Italiano founded in 1862 by the Ministry of Agriculture and Industry. The Technical School for Engineers was part of the University, which led to technical studies being accepted as part of higher education. In those times Italy was about to begin a new industrial era, which the Industry Museum was to address more directly thanks to famous scholars and researchers dealing with new subjects such as electrotechnics and building science; the new school was concerned with the needs of the Italian society and its development perspectives. Like other well known Polytechnic Schools in the first years of the 20th century the Regio Politecnico di Torino had several goals and began contacting the European academic world and the Italian industry.
Aeronautics began as a subject. Students from all over Italy came to Turin and found in the new laboratories built for the study of various subjects ranging from chemistry to architecture in a positive and helpful atmosphere. During November 1958 a large complex of buildings located in Corso Duca degli Abruzzi was inaugurated in order to expand the volume and the facilities offered by the historical headquarters of Valentino Castle, given in 1859 to the Technical School for Engineers. In the 1990s, new teaching campuses were opened in Alessandria, Biella and Mondovì. Campuses of the Politecnico di Torino draw inspiration from the structure of Anglo-Saxon ones, with multipurpose buildings for teaching and applied research and services to the students in Turin, a regional network of technological centers, dedicated to research activities, technological transfer, specialist education and services to the region; the historical and representative base of the Politecnico is in the town, on the River Po: the Castle of Valentino, a House of Savoy of the 17th Century.
It is the main teaching campus for Architecture and has an area of 23.000 sq. m. The big complex in corso Duca degli Abruzzi – with 122.000 sq. m. the main campus of Engineering – was opened in 1958 and it is completed by the Cittadella Politecnica: a modern complex of 170,000 sq. m. adjoining to the main building, including areas dedicated to students, research activities, technological transfer and services. The newest campus is the Design and Sustainable Mobility Citadel, in an area adjoining to the manufacturing establishment of Mirafiori, FIAT manufacturing facility, remodeled as well as the Lingotto building, which hosts the Master School. STUDENTS 32.000 students 30% women 42% students from outside Piedmont 16.5% international students 4,900 first year students 12% first year international students 400 Specialization Master students 633 PhD students PROGRAMS 28 Degree programs 32 MS Degree programs 18 Courses in English 6 I level Specialization Masters 27 II level Specialization Masters 24 PhD programs 6 Advanced training programs 1 Specialization programGRADUATES 5,371 graduates in 2012 2,802 first cycle level graduates 2,569 second cycle level graduates Employment rate of second cycle students one year after graduation: 74.5% 42% have permanent contracts Research activities, in particular, are structured in four macro-areas: Industrial Engineering.
DEPARTEMENTS The two souls characterizing Departments are research and teaching. Departments indeed carry out duties of coordination, promotion of research and management of the teaching activity, following the recent reform of the University system. DAD – Department of Architecture and Design DAUIN – Department of Control and Computer Engineering DENERG – Department of Energy DET – Department of Electronics and Telecommunications DIATI – Department of Environment and Infrastructure Engineering DIGEP – Department of Management and Production Engineering DIMEAS – Department of Mechanical and Aerospace Engineering DISAT – Department of Applied Science and Technology DISEG – Department of Structural and Building Engineering DISMA – Department of Mathematical Sciences DIST – Interuniversity Department of Regional and Urban Studies and Planning Politecnico di Torino has agreement with Turin Polytechnic University in Tashkent Uzbekistan. There Politecnico di Torino prepare students in the field of Mechanical Engineering, Computer science and Civil engineering.
The main courses offered are architecture, architectural engineering, industrial design, aerospace engineering, automotive engineering, biomedical engineering, chemical engineering, civil engineering, computer engineering, electrical engineering, electronic engineering, environmental engineering, energy engineering, engineering physics, material engineering, mechanical engineering, mining engineering, nuclear engineering, nanotechnology
Broadcasting is the distribution of audio or video content to a dispersed audience via any electronic mass communications medium, but one using the electromagnetic spectrum, in a one-to-many model. Broadcasting began with AM radio, which came into popular use around 1920 with the spread of vacuum tube radio transmitters and receivers. Before this, all forms of electronic communication were one-to-one, with the message intended for a single recipient; the term broadcasting evolved from its use as the agricultural method of sowing seeds in a field by casting them broadly about. It was adopted for describing the widespread distribution of information by printed materials or by telegraph. Examples applying it to "one-to-many" radio transmissions of an individual station to multiple listeners appeared as early as 1898. Over the air broadcasting is associated with radio and television, though in recent years, both radio and television transmissions have begun to be distributed by cable; the receiving parties may include the general public or a small subset.
The field of broadcasting includes both government-managed services such as public radio, community radio and public television, private commercial radio and commercial television. The U. S. Code of Federal Regulations, title 47, part 97 defines "broadcasting" as "transmissions intended for reception by the general public, either direct or relayed". Private or two-way telecommunications transmissions do not qualify under this definition. For example and citizens band radio operators are not allowed to broadcast; as defined, "transmitting" and "broadcasting" are not the same. Transmission of radio and television programs from a radio or television station to home receivers by radio waves is referred to as "over the air" or terrestrial broadcasting and in most countries requires a broadcasting license. Transmissions using a wire or cable, like cable television, are considered broadcasts but do not require a license. In the 2000s, transmissions of television and radio programs via streaming digital technology have been referred to as broadcasting as well.
The earliest broadcasting consisted of sending telegraph signals over the airwaves, using Morse code, a system developed in the 1830s by Samuel F. B. Morse, physicist Joseph Henry and Alfred Vail, they developed an electrical telegraph system which sent pulses of electric current along wires which controlled an electromagnet, located at the receiving end of the telegraph system. A code was needed to transmit natural language using only these pulses, the silence between them. Morse therefore developed the forerunner to modern International Morse code; this was important for ship-to-ship and ship-to-shore communication, but it became important for business and general news reporting, as an arena for personal communication by radio amateurs. Audio broadcasting began experimentally in the first decade of the 20th century. By the early 1920s radio broadcasting became a household medium, at first on the AM band and on FM. Television broadcasting started experimentally in the 1920s and became widespread after World War II, using VHF and UHF spectrum.
Satellite broadcasting was initiated in the 1960s and moved into general industry usage in the 1970s, with DBS emerging in the 1980s. All broadcasting was composed of analog signals using analog transmission techniques but in the 2000s, broadcasters have switched to digital signals using digital transmission. In general usage, broadcasting most refers to the transmission of information and entertainment programming from various sources to the general public. Analog audio vs. HD Radio Analog television vs. Digital television WirelessThe world's technological capacity to receive information through one-way broadcast networks more than quadrupled during the two decades from 1986 to 2007, from 432 exabytes of information, to 1.9 zettabytes. This is the information equivalent of 55 newspapers per person per day in 1986, 175 newspapers per person per day by 2007. There have been several methods used for broadcasting electronic media audio and video to the general public: Telephone broadcasting: the earliest form of electronic broadcasting.
Telephone broadcasting began with the advent of Théâtrophone systems, which were telephone-based distribution systems allowing subscribers to listen to live opera and theatre performances over telephone lines, created by French inventor Clément Ader in 1881. Telephone broadcasting grew to include telephone newspaper services for news and entertainment programming which were introduced in the 1890s located in large European cities; these telephone-based subscription services were the first examples of electrical/electronic broadcasting and offered a wide variety of programming. Radio broadcasting. Radio stations can be linked in radio networks to broadcast common radio programs, either in broadcast syndication, simulcast or subchannels. Television broadcasting, experimentally from 1925, commercially from t
Qatar University is a public university in Qatar, located on the northern outskirts of the capital Doha. As of 2014, there are over 16,000 students. Courses are taught in English; the university is the only government university in the country. The university hosts ten colleges – Arts and Sciences and Economics, Engineering, Law and Islamic Studies, College of Health Science, College of Medicine, the latest College of Dental Medicine – with a total of 8000 students at a 13:1 student-teacher ratio. Students entering the university are placed in a “Foundation Program”, which ensures the acquirement of skills such as Math and Computer technology. Many of its academic departments have received or are under evaluation for accreditation from a number of organizations. In addition to undergraduate academics, QU has a research infrastructure including research labs, an ocean vessel, technical equipment and a library including a collection of rare manuscripts; the university serves on behalf of the government and private industry to conduct regional research in areas of the environment and energy technologies.
Qatar University has a student body of 65 % of which are Qatari nationals. About 35% are children of expats. Women make up 70% of the student population, are provided their own set of facilities and classrooms. QU has an alumni body of over 30,000 graduates; the institution was established as the College of Education by a decree from the Emir of Qatar in 1973. The college began with a total of 150 students and was expanded to become the University of Qatar in 1977 with four new colleges: Education, Humanities & Social Sciences, Sharia & Law & Islamic Studies, Science. Three years the College of Engineering was established. By the number of enrolled students was 2,600; this was followed by the establishment of the College of Business & Economics in 1985. The new colleges prompted a large expansion of the university campus, overseen by Aga Khan Award for Architecture recipient Kamal El Kafrawi. By Fall Semester 2005 / 2006, the number of registrants for study at Qatar University had reached 7660 male and female students, equaling 1/6 of the eligible Qatari population.
As of 2011, there are seven colleges: College of Education, College of Arts and Sciences, College of Shariah and Islamic Studies, College of Engineering, College of Law, College of Business & Economics, College of Pharmacy. The new College of Pharmacy was established in 2006, with its first intake of BSc students in 2007. Between 2003 and 2015, the president of the university was Sheikha Abdulla Al-Misnad, she left in 2015. Her replacement was Hassan Rashid Al-Derham, who holds the position, is the university's sixth president. Al-Derham received his doctoral degree from the University of South Wales. In December 2015, the university awarded its first-ever honorary doctorate degree to Turkish president Recep Tayyip Erdoğan. In 2019, the College of Dental Medicine was founded, the tenth college for Qatar University; the college has a competitive 25 seats in its first class and is a six-year program leading to a Doctor of Dental Medicine. The Qatar University'Reform Project' evaluated and restructured the university administration and direction to enhance the quality of instruction and place emphasis on research.
The reform was initiated in 2003, led by His Highness Sheikh Tamim Bin Hamad Al-Thani, QU President Dr. Sheikha Al Misnad, the newly established Office of Institutional Research and Planning, it focused on three principles. While the university had operated as a government entity, the reformed institution would be an autonomous body governed by a board of regents who reported to the Emir; this change allowed the University to manage its own finances, stated objectives and vision, personnel and decentralization within the university granted similar financial and personnel control to colleges and programs. Academically, the reform resulted in the establishment of offices such as the Student Learning Support Center and Student Counseling Center. Construction was undertaken to ensure accessibility of university facilities by handicapped persons. A newsletter, began publication in the universityThe reform changed the title of the university from “University of Qatar” to “Qatar University” with a new slogan.
A new strategy was put in place "from reform to transformation" covering the years 2018-2022. President of Qatar University Dr. Hassan Al-Derham: "we have developed a new five-year strategy, which will map out the routes to achieving continuing performance excellence and build on our achievements in our key areas of Education, Research and Engagement." This new strategy takes into consideration the Qatar National Vision 2030. The university contributes to the process of “Qatarization”, which places an emphasis on the hiring and support of national citizens. While western nations may have trouble implementing such a system due to equality legislation, Qatari nationals only account for 1/4 of the country's population, this movement is deemed necessary to maintain cultural and national identity. Qatari leaders have recognized the vulnerability of oil and natural gases as a long-term economic model for a smaller area such as Qatar; the university has directed funding towards contributions to interna
Radio is the technology of signalling or communicating using radio waves. Radio waves are electromagnetic waves of frequency between 300 gigahertz, they are generated by an electronic device called a transmitter connected to an antenna which radiates the waves, received by a radio receiver connected to another antenna. Radio is widely used in modern technology, in radio communication, 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 a transmitter to a receiver, by modulating the radio signal in the transmitter. In radar, used to locate and track objects like aircraft, ships and missiles, a beam of radio waves emitted by a radar transmitter reflects off the target object, the reflected waves reveal the object's location. In radio navigation systems such as GPS and VOR, a mobile receiver receives radio signals from navigational radio beacons whose position is known, by measuring the arrival time of the radio waves the receiver can calculate its position on Earth.
In wireless remote control devices like drones, garage door openers, keyless entry systems, radio signals transmitted from a controller device control the actions of a remote device. Applications of radio waves which do not involve transmitting the waves significant distances, such as RF heating used in industrial processes and microwave ovens, medical uses such as diathermy and MRI machines, are not called radio; the noun radio is 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 1895-6 by Italian Guglielmo Marconi, 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 Telecommunications Union, which allocates frequency bands in the radio spectrum for different uses. 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 a metal conductor called an antenna. In transmission, a transmitter generates an alternating current of radio frequency, 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. As they travel further from the transmitting antenna, radio waves spread out so their signal strength decreases, so radio transmissions can only be received within a limited range of the transmitter, the distance depending on the transmitter power, antenna radiation pattern, receiver sensitivity, noise level, presence of obstructions between transmitter and receiver. An omnidirectional antenna transmits or receives radio waves in all directions, while a directional antenna or high gain antenna transmits radio waves in a beam in a particular direction, or receives waves from only one direction.
Radio waves travel through a vacuum at the speed of light, in air at 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. 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 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 - in an AM transmitter, the amplitude of the radio carrier wave is varied by the modulation signal. FM - in an FM transmitter, the frequency of the radio carrier wave is varied by the modulation signal. FSK - 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 - a family of complicated digital modulation methods widely used in high bandwidth systems such as WiFi networks, digital television broadcasting, digital audio broadcasting to transmit digital data using a minimum of radio spectrum bandwidth. OFDM has higher spectral efficiency and more resistance to fading than AM or FM. Multiple radio carrier waves spaced in frequency are transmitted within the radio channel, with each carrier modulated with bits from the incoming bitstream