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Retrospect

Under the surveillance of the waves

Radar was developed in the first half of last century, and became indispensable in times of war and peace

A complex network of radars spread all over the Amazon is the heart of a project that sets out to control the region’s air space. The Amazon Surveillance System (Sivam in the Portuguese acronym), which started to work on an experimental basis in July, depends, to a great extent, on a technology that is indispensable nowadays. Like so many other important creations, radar was not actually invented, but developed over the decades. The mathematical essays of Scottish physicist James Clerk Maxwell, in 1871, and the laboratory experiments on electromagnetic waves, carried out by German physicist Heinrich Hertz between 1885 and 1889, were the outset.

At the beginning of last century, in 1904, Christian Hülsmeyer was awarded patent No.13.170 in Germany, and afterwards, one in England that had the theoretical explanation of radar. The apparatus consisted of a transmitter and a receiver, assembled side by side, “in such a way that the waves emitted by the transmitter could act on the receiver only when reflected by any other metal body, which at sea would probably be another ship”, according to the description of a rare Brazilian specialist on the theme, Stefan Jucewicz, in his book Radar (Editora Asa, 144 pages).

The invention would not work in practice, because the technical conditions for this were only created in the 30s. The first real application of the knowledge of radio waves took place in 1925, when the Carnegie Institute, in the United States, used intermittent emissions of radio frequency to determine the altitude of the layer of ionized gas that surrounds the Earth, measuring the difference in time between transmitting the signal and the return of its echo. In 1929, Hydetsugu Yagi, from Japan, published a study on the emission and transmission of waves, using directional antennas that made it possible to guide the beam.

In 1935, Britain’s Royal Air Force (RAF) set aside £ 10,000 for studies on a Radio Direction Finder (RDF), the code name for the English radar. The American army made the first pulse emitting radar in 1936, while the German navy was working on a radar to determine the range of its artillery.

The contribution that made the equipment functional and precise came from two Englishmen, J.T. Randall and A.H. Bott, in 1940. They built a magnetron valve, capable of generating 10,000 watts at the very high frequency of 3,000 megahertz. The RAF used it to build the first airborne centrimetric radar, which bore the name of H2S (home sweet home) and equipped the target marking bombers. Very effective, the new equipment became one of the Allies’ most important weapons. Directed downwards, the antenna made it possible for the navigator to “see” the ground on a cathode ray tune.

“The H2S was the first radar with scanning, with which it is possible to see ships, aircraft and geographical features as if on a map”, says Jucewicz. When the conflict ended, the RDF came to be called radar – actually, another abbreviation, standing for Radio Detection and Ranging. Since then, the military and civilians have never again dispensed with the equipment in operations of war and in flight protection. One of the perfected versions, Doppler radar, is used today in meteorology and to police the speed of cars on the roads.

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