London, England November 6, 1940

On this date in 1940 a two-engine German Heinkel He 111 bomber was shot down and sank in the shal­lows off South­ern England. A water­logged X‑Geraet (“X‑device”) was recovered. The X‑Geraet played a role in the Battle of the Beams, a period early in the war when German bombers were equipped with increasingly accurate systems of radio navigation.

Before the X-Geraet there was the Knicke­bein (“Crooked Leg”), first tried out in 1939, which broad­cast narrow radio guid­ance beams from occu­pied Europe. The inter­section point of the two beams over enemy targets indi­cated where bombs should be dropped. (The limited line-of-sight cross-beam develop­ment had its incep­tion in elec­tron­ically assisted safe landings at air­ports in foul weather or at night.) The way it worked was German bombers would fly into one beam, or set of radio waves, trans­mitted on one fre­quency, con­sisting, say, of a series of Morse code “dots,” and “ride” that beam until the radio opera­tor started hearing the audio tones, con­sisting of Morse code “dashes,” from the second beam on a second receiver opera­ting on a differ­ent fre­quency. When the radio opera­tor effec­tively heard a con­tin­uous tone (the dashes on one fre­quency filling in the spaces between the dots on the other), he signaled the bombardier to drop the bomber’s payload.

Knickebein (officially “X-Leitstrahlbake,” or “Direction Beacon”) was used early in the Luft­waffe’s night-bombing cam­paign and was suc­ceeded by the four-beam X‑Geraet, which was simi­lar in con­cept, but it oper­ated at much a higher fre­quency (60 MHz, not 30 MHz) and was used to greater effect. The British city of Coven­try, with its 14th-century Gothic cathe­dral, was its best-known victim on the night of Novem­ber 14/15, 1940. The Y‑Geraet was an improve­ment over the X‑Geraet in that it used a single narrow beam from the ground station pointed over the target. Day­time bombing could rarely achieve the accu­racy of these night­time bombing raids. By way of example, bombs dropped using the X‑Geraet were placed within 100 yards of the device’s centering beam, good enough to hit a large factory.

The British scientific community fought back with a variety of its own increas­ingly effec­tive counter­measures involving jam­ming (i.e., throwing out power­ful radio noise over a wide range of fre­quencies to disrupt radio trans­missions), “bending” or dis­torting the German navi­ga­tional beams, and producing false signals that tricked the Germans into dropping bombs miles from their intended targets. Three consecu­tive raids on Britain’s second-largest city, Birming­ham, between Novem­ber 19 and 21/22, 1940, less than a week after the success­ful Coventry raid, were disrupted by jamming. British electronic wizards were slowly gaining the upper hand. But the Battle of the Beams and the back-to-back aerial duo Battle of Britain and the Blitz on British cities, especially London which was a large and easy target, really only ended when the Luft­waffe moved its bombers to the East­ern Front in May 1941, in pre­para­tion for Opera­tion Bar­ba­rossa, the attack on the Soviet Union and the harbinger of Nazi Germany’s ruinous end.

Battle of the Beams: Defeating Germany’s Radio Guidance Systems

Battle of the Beams: Knickebein radio transmitter locations in EuropeBattle of the Beams: Four-beam X-Geraet system for locating target

Left: Map showing Knickebein radio trans­mitters whose two inter­secting beams were broad­cast from separate locations, with both beams fixed on a target, in this example Derby in the English Mid­lands: from Schleswig-Hol­stein near the Danish border, from Kleve near Essen and the Dutch border, or from Loerrach (south­west of Maul­burg) in Baden-Wuerttem­berg near the border with France and Switzer­land. Following Nazi victories in Norway, the Nether­lands, and France in April–June 1940, the Germans installed addi­tional Knicke­bein trans­mitters in those coun­tries as well. The Knicke­bein was the pre­de­ces­sor to the more accurate X- and Y‑Geraet systems, which required new, more sophisticated radio equipment.

Right: The X-Geraet used a series of beams to locate the target, each beam named after a German river. The guide beam “Weser” was inter­sected by a series of three very narrow single beams, the “Rhine,” “Oder,” and “Elbe.” The inter­secting beams accu­rately mea­sured dis­tances across the guide beam. The “Oder” and “Elbe” were spaced roughly 5 to 10 kilo­meters (3.1 to 6.2 miles) from the bomb release point along the line of “Weser.” The bombs were auto­ma­tically released on signal from the device. The British were able to defeat the auto­mated sys­tem by trans­mitting a false “Elbe,” so that the bombs dropped prematurely, miles short of their target.

Battle of Britain, Part of the “Why We Fight” Series Produced by Frank Capra for the U.S. War Department