WONDERFUL INFORMATION
Compiler: C.K.Mohamed/Tellicherry

 RADAR -Souvenir of 20th Century

RADAR

Radar RAdio Detection And ranging, the use of radio waves for detecting and locating distant objects. The apparatus might be described as a radio searchlight, for it sends out a beam of radio waves, and detects objects on which the beam falls by the waves they reflect back. This idea is nearly as old as the discovery of radio. It was first suggested in 1904 by a German engineer, who took out several patents for it under the name of 'obstacle detector and navigational aid'. It was, however, not then known how to produce a narrow, concentrated beam of radio waves, but when very short- wave radio beams were discovered G. Marconi revived the idea, and strongly urged its development in 1922. In the same year the first primitive radar apparatus was made in America. When used successfully to detect a wooden ship in the Potomac river, its military potential was realised. In 1925 Gregory Breit and Merle Tuve in the course of research on the upper atmosphere at the Carnegie Institute in Washington discovered how to find the height of the ionosphere by sending out a train of very short pulses of radio waves, and measuring the time it took for them to be reflected back to Earth, This was called 'pulse ranging' and marks the invention .of modern radar. By 1930 secret research was being undertaken in America, Britain, Germany, and other countries. The principal British pioneer was Robert A. Watson-Watt. In 1935 he was able to determine the position of an aeroplane at a distance of 50 miles, and by 1938 he had established a series of 'radiolocators' round the south-east coast of England. A second chain completed an efficient radar 'curtain' in time to meet the German air attacks in 1940, and put Britain in the forefront of radar development. In its simplest form a radar installation is a combination of a beam radio transmitter and a sensitive receiver. The beam is transmitted in high-power pulses of very short duration, the reflected 'echoes' being picked up in the intervals between the pulses. The distance of the object reflecting the echo can readily be calculated from the time taken for the echo to arrive, but extremely precise measurement is required because radio waves travel with the velocity of light. For example, an echo received after one millionth of a second indicates that the signal has travelled a distance of 330 yards on the double journey, so that the distance of the target is 165 yards. This means that an error of a millionth of a second on any occasion means an error of 16S yards in the ranging.

Radar is used by the military for the detection of aircraft, ships, and missiles, for the automatic aiming of anti-aircraft guns, for spotting targets from aircraft at night or in fog, and as an aid to navigation. Its principal civil uses are in navigation and the control of traffic at seaports and airports. It is considerably more sensitive than the human eye, and radio waves are not impeded by cloud, fog, or darkness. It is capable of spotting from an aircraft comparatively small targets at distances of up to about 200 miles, but is inferior to the human eye in the identification and recognition of the shape of the target.

A radar transmitter, in general, consists of a special electron valve (magnetron) capable of generating electrical oscillations of high power (from a few kW to more than I MW) at very high frequencies (from 100 MHz to 25,000 MHz), and of special circuits which establish the duration of the pulses and their repetition frequency (p.r.f.). The duration of each pulse varies between 0.1 and 10 microseconds, according to the purpose for which the radar is used. The p.r.f. varies be- tween 200 and 10,000 pulses per second. The energy generated by the magnetron is fed to the aerial which radiates it into space in the form of a highly directional beam of radio waves. Until recently almost all aerials had the shape of a paraboloid and were rotated by an electric motor so that the beam scanned all the space surrounding the radar. The development of low cost electronic microcircuits in the early 1970s has led to the increasing use of aerials made up of many hundreds, or even thousands, of tiny dipole aerials, each under the control of an individual microcircuit. Radar beams produced by such aerials can be steered almost instantaneously. Moreover, the beam shape can be changed at will to suit different operational requirements, and several beams of the same or different shape can be used at the same time. The receiving portion of the radar consists of detecting equipment and an oscilloscope, which provides a visible indication of the echoes coming back from the targets. The same aerial may be used for both the transmission and the reception, suitable devices ensuring that the receiving circuits are active only during the intervals between the transmitted pulses. Various methods are used for the visual display on the oscilloscope screen of the targets picked up.

Some types of radar show only moving targets, automatically cancelling the echoes from fixed objects, and these generally radiate a continuous wave instead of short pulses. They make it possible to measure the radial velocity of the moving target. Other types are designed to track a moving target, always keeping the radiated beam aimed at it. In this case the radar can also be made to control a gun or missile. An important accessory of radar in the Second World War was the IFF (/dentification Friend or Foe) system. This equipment installed on British aircraft was essentially a radar transmitter which, on reception of the wave emitted by the primary radar, emitted a secondary radio signal which made it possible to discriminate on the screen of the primary radar between friendly and enemy aircraft. This system is used in aerial and marine navigation for the identification of radio beacons (transponder beacons) and for identifying. civil airliners. In this latter application the secondary radar also transmits a coded signal giving the height of the aircraft and other information useful to air traffic controllers on the ground. Still another development is the PPI (Plan Position indicator), in which the beam rotates and very rapidly scans a large circular area. All the reflecting objects in this area are visible simultaneously on the screen as spots of light, so that their distribution and shapes can be seen as on a map.

Reaping machine (Agricultural)

McCormick, Cyrus Hall [makawr'mik] an American (1809-1884), who invented the first reliable reaping machine. Cyrus H. McCormick was Son of a Virginian farmer who was interested in the mechanization of farming and devised several unsatisfactory agricultural machines, Cyrus H. McCormick set himself to succeed where his father had failed. In 1831, at the age of 22, he produced the first reliable reaping machine and patented it in 1834. McCormick's brothers Leander and William then became managers of his expanding business. The machine was pulled instead of being pushed, and the cutting was done by a knife and cutter-bar. In 1847 he established a factory in Chicago to produce it on a large scale. He introduced it to Europe at the 1851 Exhibition in London. After a difficult initial period the business began to expand; in 1902 it became the International Harvester Company, of which his son (also Cyrus Hall) was president, and it is now one of the biggest concerns in the United States.