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 (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 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 a controller device control the actions of a remote device. (Full article...)
Gerry Adams, Sinn Féin president (seen here in 2001) – one of the people affected by the restrictions enacted in 1988.
From October 1988 to September 1994 the British government banned broadcasts of the voices of representatives from Sinn Féin and several Irish republican and loyalist groups on television and radio in the United Kingdom (UK). The restrictions, announced by the Home Secretary, Douglas Hurd, on 19 October 1988, covered eleven organisations based in Northern Ireland. The ban followed a heightened period of violence in the course of the Troubles (1960s to 1990s), and reflected the UK government's belief in a need to prevent Sinn Féin from using the media for political advantage.
Broadcasters quickly found ways around the ban, chiefly by using actors to dub the voices of banned speakers. The legislation did not apply during election campaigns and under certain other circumstances. The restrictions caused difficulties for British journalists who spoke out against censorship imposed by various other countries, such as by Iraq and India. Ireland had its own similar legislation that banned anyone with links to paramilitary groups from the airwaves, but this restriction lapsed in January 1994. This increased pressure on the British government to abandon its policy; John Major lifted the broadcast ban on 16 September 1994, a fortnight after the first Provisional Irish Republican Army ceasefire (declared on 31 August 1994). (Full article...)
Image 2Oliver Lodge's 1894 lectures on Hertz demonstrated how to transmit and detect radio waves (from History of radio)
Image 3In the 1920s, the United States government publication, "Construction and Operation of a Simple Homemade Radio Receiving Outfit", showed how almost any person handy with simple tools could a build an effective crystal radio receiver. (from History of radio)
Image 5Diagram of the electric fields (E) and magnetic fields (H) of radio waves emitted by a monopole radio transmitting antenna (small dark vertical line in the center). The E and H fields are perpendicular, as implied by the phase diagram in the lower right. (from Radio wave)
Image 11Animation of a half-wave dipoleantenna radiating radio waves, showing the electric field lines. The antenna in the center is two vertical metal rods connected to a radio transmitter (not shown). The transmitter applies an alternating electric current to the rods, which charges them alternately positive (+) and negative (−). Loops of electric field leave the antenna and travel away at the speed of light; these are the radio waves. In this animation the action is shown slowed down tremendously. (from Radio wave)
Image 12Animated diagram of a half-wave dipole antenna receiving a radio wave. The antenna consists of two metal rods connected to a receiver R. The electric field (E, green arrows) of the incoming wave pushes the electrons in the rods back and forth, charging the ends alternately positive (+) and negative (−). Since the length of the antenna is one half the wavelength of the wave, the oscillating field induces standing waves of voltage (V, represented by red band) and current in the rods. The oscillating currents (black arrows) flow down the transmission line and through the receiver (represented by the resistance R). (from Radio wave)
Image 13Reginald Fessenden (around 1906) (from History of radio)
Image 14Heinrich Rudolf Hertz (1856–1894) proved the existence of electromagnetic radiation (from History of radio)
Image 15Around 1920, radio broadcasting started to get popular. The Brox Sisters, a popular singing group, gathered around the radio at the time. (from History of radio)
Image 17Advertisement placed on November 5, 1919, Nieuwe Rotterdamsche Courant announcing PCGG's debut broadcast scheduled for the next evening (from Radio broadcasting)
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