London, England · November 6, 1940

On this date in 1940 a German Heinkel 111 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 guid­ance beams from occu­pied Europe that inter­sected, or crossed each other, at the point where bombs should be dropped on Brit­ish tar­gets. (The cross-beam develop­ment had its in­cep­tion in blind landings at air­ports in bad weather or at night.) Ger­man bombers would fly into one beam, or set of radio waves, and “ride” it until the radio opera­tor started hearing the tones from the second beam on a second receiver. When the steady “on course” sound was heard from the second receiver, the bombardier dropped his payload.

Knicke­bein 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 a much higher fre­quency 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

Knickebein radio transmitter locations in Europe Four-beam X-Geraet system for locating target

Left: Map showing Knickebein radio trans­mitters whose inter­secting beams were broad­cast from Schleswig-Hol­stein near the Danish border, at Kleve near the Dutch border, and at 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 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