Michael Dawson Jr., age 13, of Dodge City, Kan., for his question:
HOW WAS RADAR DEVELOPED?
Radar is an electronic system that is used to locate objects beyond the range of vision and to determine their distance by projecting radio waves against time. The first successful radio range fining experiment occured in 1924 when a British physicist named Sir Edward Appleton used radio echoes to determine the height of the ionosphere, an ionized layer of the upper atmosphere that reflects longer radio waves.
The term "radar" is derived from the phrase "radio detection and ranging." This name was used by the United States and its allies during World War II for a variety of devices concerned with radio detection and position finding.
The basic concepts of radar are based on the laws of radio wave reflection which were first demonstrated in 1886 by the German physicist Heinrich Hertz. Another German, an engineer named Christian Hulsmeyer, was the first to propose the application of radio echoes in a detecting device designed to avoid collisions in marine navigation.
By the outbreak of World War II radar devices had been independently developed in the United States, Germany and France. After 1940, British and U.S. physicists and engineers worked in close collaboration on radar research.
The most important advance during World War II was the invention in 1939 of an electron tube, called the resonant cavity magnetron, by the British physicist Henry Boot and biophysicist John Randall. This type of tube is capable of generating high frequency radio pluses with a large amount of power.
Present day radar systems operate efficiently at the highest frequencies that justifiably can be called radio waves. Scientists have even developed radar in the wavelength band of less than one centimeter although this radar has been used more for nuclear research than for communications or navigation.
Radar equipment includes a transmitter, an antenna, a receiver and an indicator. Unlike radio broadcasting, in which a transmitter sends out radio waves and receivers intercept them; radio transmitters and receivers are usually located in the same place.
The transmitter broadcasts a beam of electromagnetic waves by means of an antenna which concentrates the waves into a shaped beam pointing in the desired direction. When these waves strike an object in the path of the beam, some are reflected from the object, forming an echo signal.
The antenna collects the energy contained in the echo signal and delivers it to the receiver. Through an amplification process the radar receiver produces a visual signal on the screen of the indicator, a cathode ray tube that resembles the picture tube of a TV set.
To operate radar successfully, the transmitter must emit a large burst of energy and receive, detect and measure a tiny fraction (about a billionth of a billionth) of the total radio energy, returned in the form of an echo.