Battle of the Beams

Knickebein [ edit ]

Map of Knickebein transmitters

For bombing use, the modifications to Lorenz were fairly minor. Much larger antennas were needed to provide the required accuracy. This was achieved by using aerials with many more elements, but it retained the simple switching of two of the reflector elements to alter the beam directions very marginally. The beam angles were so dramatically reduced that it was only a few tens of yards wide over the target. It was the shape of the aerials that gave the system its code name, Knickebein, which means "crooked leg".[4][a] The word is also the name of a magical raven in Germanic mythology.[5] For the required range, transmitted power was increased considerably. The Knickebein receivers were disguised as a standard blind landing receiver system, consisting apparently of the EBL-1 and the EBL-2 receivers.

The beam from a single transmitter would guide the bombers towards the target, but could not tell them when they were over it. To add this ranging feature, a second transmitter similar to the first was set up so its beam crossed the guidance beam at the point where the bombs should be dropped. The aerials could be rotated to make the beams from two transmitters cross over the target. The bombers would fly into the beam of one and ride it until they started hearing the tones from the other (on a second receiver). When the steady "on course" sound was heard from the second beam, they dropped their bombs.

Later, smaller Knickebein antenna

The first of these new Knickebein transmitters was set up in 1939 on Stollberg hill in Nordfriesland near the border with Denmark; at Kleve (Cleves) near the Dutch border, almost the most westerly point in Germany, and at Lörrach near the border with France and Switzerland in south-western Germany.[8] Following the fall of France in June 1940, additional transmitters were installed on the French coast. Stations were also constructed in Norway and the Netherlands.[9]

Knickebein were used in the early stages of the German night-bombing offensive and proved to be fairly effective, but the tactics for using the system in a widespread bombing effort were not yet developed, so much of the early German night bombing offensive was limited to area bombing.

The search for the beams [ edit ] Efforts in Britain to block the Knickebein system required time to implement. British intelligence at the Air Ministry, led by R. V. Jones, became aware of the system when the Royal Aircraft Establishment analysed a downed German bomber's Lorenz system and observed it was far more sensitive than required for a mere landing aid. Secretly recorded transcripts from German prisoner of war pilots indicated this may have been a bomb aiming aid. Winston Churchill had also been given Ultra intelligence from decrypted Enigma messages that mentioned 'bombing beams'. When Jones mentioned the possibility of bombing beams to Churchill, he ordered further investigation. The British codenamed the system Headache. Many in the Air Ministry did not believe that the system was in use. Frederick Lindemann, leading scientific adviser to the government, argued that any such system would not be able to follow the curvature of the Earth, although T. S. Eckersley of the Marconi company had said it could. Eckersley's assertion was eventually demonstrated after Churchill ordered a flight to try to detect the beams. The RAF lacked equipment capable of detecting 30–33 MHz Lorenz signals, so they purchased an American Hallicrafters S-27 amateur radio receiver[14] from a shop in Lisle Street, London. The receiver was fitted into an Avro Anson and operated by a member of the Y Service. The flight was nearly cancelled when Eckersley withdrew his assertion that the beams would bend around the Earth. Jones saved the flight by pointing out that Churchill himself had ordered it, and he would make sure that the Prime Minister would learn who cancelled it. The crew were not told specifics, and were simply ordered to search for radio signals around 30 MHz having Lorenz characteristics and, if they found any, to determine their bearing. The flight took off and eventually flew into the beam from Kleve, on 31.5 MHz. It subsequently located the cross beam from Stollberg (its origin was unknown prior to this flight). The radio operator and navigator were able to plot the path of the beams and discovered that they intersected above the Rolls-Royce engine factory at Derby, at that time the only factory producing the Merlin engine. It was subsequently realised that the argument over whether the beams would bend around the Earth was entirely academic, as the transmitters were more or less in line-of-sight to high altitude bombers. [citation needed] British sceptics started regarding the system as proof that the German pilots were not as good as their own, who they believed could do without such systems. The Butt Report proved this to be wrong: aerial reconnaissance returned photographs of the RAF bombing raids, showing that they were rarely, if ever, anywhere near their targets.