ELECTRONICS
January 1946

By McGraw-Hill Staff
Page 92 - 97

The Army's first radar countermeasures laboratory
was in the attic of the Marconi Hotel (above)

diagram on the c-r screen, the maximum of which indicates the direction of the radar under observation. This equipment, when observing a point source, plots the polar radiation diagram of the antenna in use. It has found much use in measuring the polar diagram characteristics of developmental antennas.

Chaff Dipoles and Rope

An effective way to confuse enemy radar operators is to simulate targets by dispersing large quantities of reflecting material in the sky. The most efficient material for this purpose, from the stand-point of echo area per unit weight, is aluminum foil cut in strips one half wavelength long at the enemy

While apparently a simple device, chaff presented many interesting technical problems. The primary objective was to obtain as large as possible a reflecting area from a given weight of aluminum foil. This implies thin, pliable foil which tends to bend when thrown into the slip stream of the aircraft. Such bending causes interweaving of the strips in tangled bird's nests which present little area and fall rapidly. Moreover, adjacent strips of foil tend to adhere to one another, preventing rapid dispersal. These problems were solved by embossing the foil and crimping each strip along its length to give it rigidity. Chaff thus manufactured is highly dispersive and falls at the slow rate of 150 feet per minute. The mate-

radar frequency. Such dipole strips, when used by the British, were called window. The American version is known as chaff. Three quarters of the entire wartime production of aluminum foil, some 20,000 tons in all, was devoted to the manufacture of chaff. Allied aircraft dispensed hundreds of packages of foil strips, each containing several thousand dipoles, on every flight over enemy territory. The material was designed to disperse widely and to remain aloft as long as possible, thus providing a radar smoke screen within which following aircraft could avoid detection by gunfire-control radars below.

rial most widely used over Europe was tuned to the region 450 to 600 mc, which covered the operating frequencies of the German Wurzburg fighter-direction and gunfire-control radars.
Since the resonance of the foil strips extends over a band only 8 percent of the center frequency (at 3 db down), it was necessary to provide two lengths in each package, roughly 10 and 11.5 inches long. About 1000 such dipoles, dispersed at an average separation of about four inches, were found to equal the echo area of a heavy bomber. The weight of these 1000 dipoles in the latest version of the

material was only 2 ounces. Each heavy bomber carried with it sufficient chaff to simulate 700 bombers, and dispensed it at regular intervals over areas known to be protected by radar-controlled gunfire ( Large areas of the German countryside thus became littered with aluminum strips, which were used by the natives to decorate Christmas trees.
    On the indicator of a gun-fire control radar, the chaff-dispensing aircraft appears as if it were reproducing itself. As the pulse representing the aircraft moves across the indicator screen, additional pulses appear behind it and remain stationary. As the chaff disperses, the pulses assume an amorphous shape in which succeeding aircraft are nearly invisible. Aircraft outside the cloud of dipoles are not hidden. Aircraft behind (but not within) the cloud are detected by signals which pass through the dipoles.
    The most effective protection against German radar-controlled flak was a combination of electronic jamming by transmitters tuned to the radar frequencies, and chaff dipoles. In addition to adding to the general confusion on enemy indicators, electronic jamming protected the first plane in a flight as well as succeeding planes. This combination reduced the effectiveness of anti-aircraft fire to about 25 percent of normal, which saved the U. S. forces an estimated 450 aircraft and 4500 casualties. The value of these aircraft alone more than equalled the cost of the countermeasures program directed against German flak. During the height of the campaign, 20 billion dipoles were scattered on Germany and France each month.
    In the Pacific, chaff was not used to any great extent because the Japanese radars used many widely different frequencies, which would have required as many different sizes of chaff to combat them. Instead, very long strips of aluminum foil, about one half inch wide and 400 feet long, were dropped, sometimes supported from small parachutes. This device, known as rope, was effective over a very wide range, covering all of the many frequencies employed by the

96 January 1946 - ELECTRONICS

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ELECTRONICS January 1946

ELECTRONICS
January 1946