Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies as high as 300 gigahertz to as low as 30 hertz. At 300 GHz, the corresponding wavelength is 1 mm, at 30 Hz is 10,000 km. Like all other electromagnetic waves, radio waves travel at the speed of light in vacuum, they are generated by electric charges undergoing acceleration, such as time varying electric currents. Occurring radio waves are emitted by lightning and astronomical objects. Radio waves are generated artificially by transmitters and received by radio receivers, using antennas. Radio waves are widely used in modern technology for fixed and mobile radio communication, broadcasting and radio navigation systems, communications satellites, wireless computer networks and many other applications. Different frequencies of radio waves have different propagation characteristics in the Earth's atmosphere. To prevent interference between different users, the artificial generation and use of radio waves is regulated by law, coordinated by an international body called the International Telecommunications Union, which defines radio waves as "electromagnetic waves of frequencies arbitrarily lower than 3 000 GHz, propagated in space without artificial guide".
The radio spectrum is divided into a number of radio bands on the basis of frequency, allocated to different uses. Radio waves were first predicted by mathematical work done in 1867 by British mathematical physicist James Clerk Maxwell, his mathematical theory, now called Maxwell's equations, predicted that a coupled electric and magnetic field could travel through space as an "electromagnetic wave". Maxwell proposed that light consisted of electromagnetic waves of short wavelength. In 1887, German physicist Heinrich Hertz demonstrated the reality of Maxwell's electromagnetic waves by experimentally generating radio waves in his laboratory, showing that they exhibited the same wave properties as light: standing waves, refraction and polarization. Italian inventor Guglielmo Marconi developed the first practical radio transmitters and receivers around 1894-1895, he received the 1909 Nobel Prize in physics for his radio work. Radio communication began to be used commercially around 1900; the modern term "radio wave" replaced the original name "Hertzian wave" around 1912.
Radio waves are radiated by electric charges. They are produced artificially by time-varying electric currents, consisting of electrons flowing back and forth in a specially-shaped metal conductor called an antenna. An electronic device called a radio transmitter applies oscillating electric current to the antenna, the antenna radiates the power as radio waves, they are received by another antenna attached to a radio receiver. When they strike the receiving antenna they push the electrons in the metal back and forth, creating tiny oscillating currents which are detected by the receiver. Radio waves in a vacuum travel at the speed of light; when passing through a material medium, they are slowed according to that object's permeability and permittivity. Air is thin enough that in the Earth's atmosphere radio waves travel close to the speed of light; the wavelength is the distance from one peak of the wave's electric field to the next, is inversely proportional to the frequency of the wave. The distance a radio wave travels in one second, in a vacuum, is 299,792,458 meters, the wavelength of a 1 hertz radio signal.
A 1 megahertz radio signal has a wavelength of 299.8 meters. Radio waves are more used for communication than other electromagnetic waves because of their desirable propagation properties, stemming from their large wavelength. Radio waves have the ability to pass through the atmosphere and most building materials, by diffraction can bend around obstructions, unlike other electromagnetic waves they tend to be scattered rather than absorbed by objects larger than their wavelength; the study of radio propagation, how radio waves move in free space and over the surface of the Earth, is vitally important in the design of practical radio systems. Radio waves passing through different environments experience reflection, polarization and absorption. Different frequencies experience different combinations of these phenomena in the Earth's atmosphere, making certain radio bands more useful for specific purposes than others. Practical radio systems use three different techniques of radio propagation to communicate: Line of sight: This refers to radio waves that travel in a straight line from the transmitting antenna to the receiving antenna.
It does not require a cleared sight path. This is the only method of propagation possible at frequencies above 30 MHz. On the surface of the Earth, line of sight propagation is limited by the visual horizon to about 64 km; this is the method used by FM, television broadcasting and radar. By using dish antennas to transmit beams of microwaves, point-to-point microwave relay links transmit telephone and television signals over long distances up to the visual horizon. Ground stations can communicate with satellites and spacecraft billions of miles from Earth
Jacopo di Michele called Jacopo Gera, Iacopo di Michele, or Gera da Pisa is a 14th-century painter, active in Pisa and elsewhere in Tuscany, in a Gothic style. His activity is documented from 1361-1395, he is the brother of Getto di JacopoA list of attributed works includes: Flagellation of Christ, with Saints and Donors, private owner Rome Madonna and Child, Sanctuary of Montenero, Livorno. St Agatha, Galleria Regionale della Sicilia Palazzo Abatellis, Palermo. Enthroned Madonna and Child with St Francis and Anthony Abbot, Museo Nazionale di San Matteo, Pisa Enthroned Madonna and Child, Pieve dei SS. Giovanni ed Ermolao, Calci. Tryptich: St Anne and Jesus Child. Crucifixion, Church of Santa Maria Novella in Montopoli in Val d'Arno. Mystical Marriage of St Catherine of Alexandria with St Lucy and Museo Civico, Volterra
The Dunne D.5 was an experimental aircraft built in the United Kingdom in 1910. Designed by J. W. Dunne and built by Short Brothers at Leysdown, A tailless swept-wing biplane, the D.5 was the first aircraft built by his company, the Blair Atholl Aeroplane Syndicate Ltd. Like its military predecessors it was driven by twin pusher propellers, but it had a more powerful engine; the D.5 first flew in the summer of 1910, Dunne having long ago dreamed of this flight. The D.5 was certified as the first fixed-wing aircraft to achieve stability in flight, one of the official witnesses being Orville Wright. On leaving the Army Balloon Factory at Farnborough in 1909, J. W. Dunne set up a company, the Blair Atholl Aeroplane Syndicate Ltd. to continue developing his aircraft, none of which had yet flown under power. The D.5 was their first aircraft and it followed the basic design of the earlier D.1B and D.4 Army machines in having a tailless, swept biplane wing with pronounced wash-out. Like the others it was driven by twin pusher propellers.
However it differed in having a streamlined central nacelle or fuselage housing the pilot and engine, the engine itself was a more powerful Green four-cylinder inline type. The controls were unusual. Elevons at the wing tips provided all the control forces and were operated by two levers on either side of the cockpit. There was no rudder, with turning being coordinated by the aerodynamic design of the swept and washed-out wings. Endplate fins were fitted to the wings. Construction of the main airframe was contracted to Short Brothers. Following construction at Leysdown, the D.5 was taken to Eastchurch, the new site of the Aero Club and the Syndicate. Early trials were not encouraging, with the machine in its original form proving too heavy; the D.5 first flew, piloted by Dunne himself, in the summer of 1910. Dunne taxied to the top of a rise in the ground which lay downwind, turned the machine and took off downhill and into the wind. Dunne recalled in his book An Experiment with Time that, as a child, he had experienced this flight in a dream.
The D.5 proved to be aerodynamically stable in flight. Two demonstration flights were made for the Royal Aero Club in December 1910 witnessed by the visiting Orville Wright and by Griffith Brewer. During the second flight, Dunne took his hands off the controls for an extended period, while he wrote a note on a piece of paper provided for him by Brewer; this note was the first documentary evidence of an aircraft's performance written in flight by the pilot himself. The D.5 was subsequently certified as the first fixed-wing aircraft to achieve stable flight. General characteristics Crew: One pilot Length: 20 ft 5 in Wingspan: 46 ft 0 in Height: 11 ft 6 in Wing area: 527 ft2 Gross weight: 1,550 lb Powerplant: 1 × Green, 60 hp Performance Maximum speed: 45 mph Notes BibliographyTaylor, Michael J. H.. Jane's Encyclopedia of Aviation. London: Studio Editions. P. 347. ISBN 0-7106-0710-5. Munson, Kenneth. Pioneer Aircraft 1903-14. London: Macmillan. P. 44