The 'Battle of the Beams' was an electronic conflict waged between the Germans and the British in an early period of World War II when German bombers used a number of increasingly accurate radio navigation systems for night bombing of targets in the UK (1940/May 1941).

In the UK, British scientific intelligence at the Air Ministry fought back with a variety of increasingly effective means involving jamming and deception signals. The period ended when the Germans switched the ficus of their air effort to the east in May 1941 in preparation for the 'Barbarossa' invasion of the USSR.

The concept of 'beam'-based navigation had been developed during the 1930s, initially as a blind landing aid. The basic concept was to generate and transmit two directional radio signals aligned slightly to the left and right of a runway’s centreline. The radio operator in the approaching aeroplane listened for these signals and could thus determine in which of the two beams the aeroplane was flying. This was normally accomplished by sending Morse code signals into the two beams to identify left and right.

For bombing, the Luftwaffe built much enlarged and powerful versions of the antennae to provide considerably enhanced accuracy at long range. These were named Knickebein and X-Gerät, and were used during the early stages of 'The Blitz' over the UK with great effect. Tipped off about the system’s operation by pre-war military intelligence, the British responded by sending their own Morse code signals so that the aircraft believed they were always properly centred in the beam while in fact they were flying radically off course, and the Germans became convinced the British had somehow learned to 'bend' radio signals.

When the problem became widespread, the Germans introduced a new system, the Y-Gerät, which worked on different principles. After guessing the nature of this system from a passing mention, the British had already deployed countermeasures that rendered the system useless almost as soon as it was used. The Germans eventually gave up on the entire concept of radio navigation over the UK, concluding the British would continue to jam it.

Before the outbreak of war in September 1939, the Lufthansa national airline and the German aircraft industry had invested heavily in the development of commercial aviation, and in systems and methods to improve safety and reliability. Considerable effort went into blind-landing aids which allowed aircraft to approach and then to land at an airport at night or in bad weather. The primary system for this role was the Lorenz system, developed by Johannes Plendl, which was in the process of being widely deployed on large civilian and military aircraft.

The Lorenz system worked by feeding a special three-element antenna system with a modulated radio signal. The signal was fed to the central dipole, which had a slightly longer reflector element on either side and set slightly to the rear. A switch rapidly alternated the opened mid-point connection of each reflector in turn, sending the beam slightly to the left and then slightly to the right of the runway centreline. The beams widened as they spread from the antenna, so there was an area directly off the runway approach where the two signals overlapped. The switch was timed so it spent longer on the right side of the antenna than the left.

An aeroplane approaching the airport would tune one of its radio equipments to the Lorenz frequency. If the the aeroplane was on the left side of the centreline, the crew would hear a series of short tones followed by long pauses, meaning the aircraft was on the 'dot' side of the antenna: hearing the 'dots', the crew would know that it had to move to the right in order to fly down the centreline. If the aeroplane was on the right side of the centreline, the crew would hear a series of long tones followed by short pauses, meaning the aeroplane was on the 'dash' side of the antenna; hearing the 'dashes', the crew would know that it had to move to the left in order to fly down the centreline. On the centreline, the radio would receive both signals, with the dots filling the gaps in the dashes and thus producing a continual signal, the so-called 'equisignal'. Flying in the known direction of the runway and keeping the 'equisignal' on the radio, Lorenz-equipped crews could guide an aeroplane down a straight line with a relatively high degree of accuracy, so much so that pilots could then find the runway visually except in the worst conditions.

During the early 1930s, the concept of a night bombing strategic campaign began to become paramount in military aviation circles. This resulted from the ever-increasing performance of bombers, which were beginning to have the capability to strike across Europe with significant bomb loads. These bombers were slow and lumbering, and thus easy prey for interceptors, but this was a threat which could be essentially eliminated by flying at night when the bomber, painted black, could be spotted only at very short ranges. As the bombers' altitude and speed capabilities increased, the threat from ground-based defences was greatly reduced. Simply put, planners believed that 'the bomber will always get through'.

The problem with night bombing is that the same limitations in visibility meant the bomber crew would have difficulty in finding its target, especially a blacked-out target, at night. Only the largest targets, such as major cities, could be attacked with any probability of success.

To support this mission, the Royal Air Force invested very heavily in navigation training, equipping their aircraft with various devices, including an astrodome for taking a star fix and giving the navigator room to do his calculations in an illuminated workspace. This system was put into use as soon as the war began and was initially regarded as successful. In reality, the early bombing effort was a complete failure, with the majority of bombs landing miles away from their intended targets.

The Luftwaffe did not take so fatalistic a view of air warfare, and continued to research accurate night bombing against smaller targets. Not depending on celestial navigation, they invested their efforts in radio navigation systems. The Luftwaffe concentrated on developing a bombing direction system based on the Lorenz concept through the 1930s, as it made night navigation relatively easy by simply listening for signals on a radio set, and the necessary radios were already being installed on many aircraft.

The Lorenz system had a range of about 30 miles (48 km), enough for blind-landing but wholly inadequate for bombing raids over the UK. This was a limitation which could be addressed by using more powerful transmitters and highly sensitive receivers. In addition, the Lorenz beams were deliberately set wide enough apart that they could easily be detected at some distance from the runway centreline, but this meant their accuracy at long ranges was fairly limited. This was not a problem for blind landing, where the distance covered by the fan-shaped beams decreased as the aircraft approached the transmitters, but for use in the bombing role this would be reversed, and the system would have maximum inaccuracy over the target.

The modifications to the Lorenz system for bombing purposes were fairly minor. Much larger antennae were needed to provide the required accuracy. This was achieved by using antennae with many more elements, but the concept nonetheless 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 antennae which gave the system its codename 'Knickebein' (crooked leg), although the word is also the name of a magical raven in Germanic mythology. 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 EBL-2 receivers.

The beam from a single transmitter would guide the bombers toward its 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 were to be dropped. The antennae could be moved in azimuth to make the beams from two transmitters cross over the target. The bombers would fly into the beam of one transmitter and ride it until they started hearing the tones of the other on a second receiver. When the steady 'on course' sound was heard from the second beam, the bombers dropped their bombs.

The first of the new Knickebein transmitters was established in 1939 on Stollberg hill in Nordfriesland near the border with Denmark, at Kleve near the Dutch border and thus on almost the most westerly point in Germany, and at Lörrach near the border with France and Switzerland in south-western Germany. After the defeat of France in June 1940, additional transmitters were installed on the French northern coast. Stations were also constructed in Norway and the Netherlands.

Knickebein was 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.

It took some time for British efforts to block the Knickebein system to begin. British intelligence at the Air Ministry, led by Dr 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 might be a bomb-aiming aid. Prime Minister 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', but many in the Air Ministry did not believe that the system was in use. Professor Frederick Lindemann, the 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 it bought a US Hallicrafters S-27 amateur radio receiver from a shop in London. The receiver was installed in an Avro Anson twin-engined trainer and operated by a member of the Y Service signals intelligence service. The flight was nearly cancelled when Eckersley withdrew his assertion that the beams would bend round the Earth, but Jones saved the flight by pointing out that Churchill himself had ordered it, and that he would make sure that the prime minister would get to know who had cancelled it.

The crew was informed of no specifics, and was simply ordered to search for radio signals around 30-MHz having Lorenz characteristics and, if it found any, to determine their bearing. The flight took off and eventually flew into the 31.5-MHz beam from Kleve. The crew subsequently located the cross beam from Stollberg, whose origin had been unknown before 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 strategically vital Merlin aero engine. It was subsequently realised that the argument over whether the beams would bend round the earth was entirely academic, as the transmitters were more or less in line-of-sight to high-altitude bombers.

British sceptics began to regard the system as proof that German pilots were not as good as their own whom, they believed, could do without such aids. The Butt Report of August 1941 later proved this to be wrong after air reconnaissance returned photographs of RAF bombing raids showing that these were rarely, if ever, anywhere near their targets.

Efforts to block the Knickebein system were codenamed 'Aspirin'. Initially, modified medical diathermy sets transmitted interference, but later local radio transmitters broadcast an extra 'dot' signal at low power on the nights in which raids were expected. The German practice of turning on the beams long before the bombers reached the target area considerably aided the British efforts: Anson aircraft fitted with receivers were flown around the country in an attempt to capture the beams' location, and a successful capture was then reported to nearby broadcasters.

The low-power 'dot' signal was initially transmitted essentially at random, so German navigators would hear two dots. This meant there were many 'equisignal' areas, and no easy way to distinguish them except by comparing them with a known location. The British transmitters were later modified to send their dots at the same time as the German transmitters, making it impossible to tell which signal was which. In this case the navigators would receive the 'equisignal' over a wide area, and navigation along the bomb line became impossible, with the aircraft drifting into the 'dash' area without any way to correct for it.

Thus the beam was seemingly 'bent' away from the target. Eventually, the beams could be inclined by a controlled amount which enabled the British to fool the Germans into dropping their bombs where they wanted them. A side effect was that as the German crews had been trained to navigate solely by the beams, many crews failed to find either the true equi-signal or Germany again. Some Luftwaffe bombers even landed at RAF bases, believing they were back in German or German-controlled territory.

As good as it was, Knickebein was never intended for use in the long-range role. Plendl had been working for some time to produce a considerably more accurate version of the same basic concept, which was eventually delivered as the X-Gerät (X-apparatus) and used a series of beams, each named after a river, to locate the target. The main beam, Weser, was similar in concept to that used in Knickebein but operated at a much higher frequency. As a result of the nature of radio propagation, this allowed its two beams to be pointed much more accurately than by Knickebein from a similarly-sized antenna: the 'equisignal' area was only about 100 yards (91 m) wide at a range of 200 miles (320 km) from the antenna. The beams were so narrow that bombers could not find them on their own, so a low-power wide-beam version of Knickebein was set up at the same station to act as a guide. The main Weser antenna was set up just to the west of Cherbourg in France.

The 'cross' signal for X-Gerät used a series of three very narrow single beams named Rhine, Oder and Elbe. They were carefully aimed to define a precise bomb release trajectory: first, a bomb-release point along Weser was determined by calculating the range or distance the bombs would travel between release and impact, and picking a point at that range to target; the Elbe beam intersected the Weser beam 3.1 miles (5 km) before the release point; and the Oder beam intersected the Weser beam 6.2 miles (10 km) before the release point, or 3.1 miles (5 km) before the Elbe beam. The Rhine beam did not require the same precision and was some 18.5 miles (30 km) before the release point. The beams' width added a small error, in the order of 10s to 100s of metres, to the intersection co-ordinates.

As the bomber followed the Weser beam and reached the Rhine beam, the radio operator heard a brief signal and set up his equipment, which took the form of a special stop-clock with two hands. When the signal of the Oder beam was received, the clock started automatically and the two hands simultaneously swept up from zero. When the signal of the Elbe beam was received, one hand stopped and the other reversed, sweeping back towards zero. The stopped hand indicated an accurate measurement of travel time from the Oder beam to the Elbe beam. Since the Oder to Elbe distance equalled that of the Elbe to the release-point distance, a bomber flying at constant speed arrived at the release point as the moving hand reached zero, when the bombs were automatically released.

The X-Gerät operated on a much higher frequency of about 60 MHz than the Knickebein, and thus required the use of new radio equipment, which was in short supply and could therefore not be installed in all bombers, so instead the Kampfgruppe 100 experimental unit was given the task of using its X-Gerät equipment to guide other aircraft to the target. To do this, KGr 100 aircraft attacked as an initial small group, dropping flares which other aircraft would then see and bomb visually. This is the first use of the pathfinder concept that the RAF would perfect to great effect against the Germans some three years later.

The X-Gerät system was first tested on 20 December 1939 when a bomber of KGr 100 flown by Oberleutnant Hermann Schmidt flew over London at 22,965 ft (7000 m).

The X-Gerät system was then used effectively in a series of raids known to the Germans as 'Mondscheinsonate' against Coventry, Wolverhampton and Birmingham. In the raid on Birmingham only KGr 100 was used and British post-raid analysis showed that the vast majority of the bombs dropped were placed within 100 yards (91 m) of the mid-line of the Weser beam, spread along it for a few hundred yards. This was the sort of accuracy that even daytime bombing could rarely achieve. The raid on Coventry with full support from other units dropping on their flares, almost destroyed the city centre.

The X-Gerät system proved more difficult to stop than the Knickebein system. Initial defences against the system were deployed in a similar fashion to those against Knickebein in an attempt to disrupt the Coventry raid, but proved to be a failure. Although Jones had correctly guessed the beam layout (and acknowledges it was only a guess), the modulation frequency had been measured incorrectly as 1,500 Hz but was in fact 2,000 Hz. At the time it was believed that this would not make any difference, as the tones were close enough that an operator would have a hard time distinguishing them in a noisy aeroplane.

The mystery was eventually revealed after an X-Gerät-equipped Heinkel He 111 twin-engined crashed on 6 November 1940 on the English coast at West Bay, Bridport. Although the aeroplane sank during the recovery operation, its waterlogged X-Gerät equipment was recovered and, on examination, revealed that it incorporated a new instrument which automatically decoded the dots and dashes and moved a pointer on a display in the cockpit in front of the pilot. This device was fitted with a very sharp filter which was sensitive only at 2,000 Hz, and not to the early British 1,500 Hz counter-signals. The jammers were modified accordingly, but this came too late to prevent the raid on Coventry on 14 November; but the modified jammers were able to disrupt a raid on Birmingham on 19 November.

The X-Gerät was eventually defeated in another manner, by way of a 'false Elbe' which was set up to cross the Weser guide beam at a mere 0.6 mile (1 km) after the Oder beam, thus much earlier than the expected 3.1 miles (5 m). As the final stages of the release were automatic, the clock reversed prematurely and the system dropped the bombs kilometres short of the target. Setting up this false beam proved very difficult as the Germans, learning from their mistakes with Knickebein, did not activate the X-Gerät beams until a time as late as possible, making it much more difficult to arrange the 'false Elbe' in sufficient time to play its part.

As they slowly gained the upper hand in the 'Battle of the Beams', the British started to consider what the next German system would entail. Since Germany’s current approaches had been rendered useless, an entirely new system would have to be developed. Jones believed that if they could defeat this system quickly, the Germans might give up on the entire concept.

British monitors soon started receiving intelligence from Enigma decrypts referring to a new device known as the Y-Gerät, which was also sometimes known as Wotan. Jones had already concluded the Germans used codenames which were too descriptive, so he asked a specialist in the German language and literature at Bletchley Park about the word 'Wotan'. The specialist realised that 'Wotan' referred to Woden, a one-eyed god, and might therefore be a single-beam navigation system. Jones agreed, and knew that a system with one beam would have to include a distance-measurement element. He concluded that it might work on the basis described by the anti-Nazi German mathematician and physicist Hans Mayer who, on a visit to Norway, had passed a large quantity of information in what is now known as the Oslo Report.

The Y-Gerät used a single narrow beam pointed over the target, similar to earlier beam systems, transmitting a modulated radio signal. The system used a FuG 28a transponder which received the signal from the beam and immediately retransmitted it back to the ground station. The ground station listened for the return signal and compared the phase of its modulation to the transmitted signal. This is an accurate way of measuring the transit time of the signal, and thus the distance to the aeroplane. Coupled with the direction of the beam, adjusted for a maximum return signal, the bomber’s position could be established with considerable accuracy. The bombers did not have to track the beam as the ground controllers could calculate it and then give radio instructions to the pilot to correct the flight path.

Jones later learned that his guess on the operating principle based on the name Wotan was entirely by luck. Later documents showed that the original X-Gerät was known as Wotan I, and the Y-Gerät as Wotan II. Had he known the name was also associated with the X-Gerät it was unlikely he would have concluded the system used a single beam.

The British were ready for this system even before it was used. By chance, the Germans had chosen the operating frequency of the Wotan system very badly, for it operated on 45 MHz, which just happened to be the frequency of the powerful-but-dormant BBC television transmitter at Alexandra Palace in north London. All Jones had to do was arrange for the return signal to be received from the aeroplane and then sent to Alexandra Palace for retransmission. The combination of the two signals modified the phase shift, and thus the apparent transit delay. Initially, the signal was re-transmitted at low power, not powerful enough for the Germans to realise what was happening, but sufficient to spoil the accuracy of the system. Over subsequent nights, the transmitter power was gradually increased.

As the Y-Gerät's use continued, German aircrews accused the ground station of sending bad signals and the ground station alleged the aeroplane’s equipment had loose connections. The whole scheme appealed to Jones as he was a natural practical joker, and remarked that he was able to play one of the largest practical jokes with virtually any national resource that he required. The gradually increasing power conditioned the Germans such they did not realise that anyone was interfering with the system, but believed that it suffered from a number of inherent defects. Eventually, as the power was increased enough, the whole Y-Gerät system started to ring with all the feedback.

The Luftwaffe, finally realising that the British had been deploying countermeasures from the very first day that the system was used operationally, totally lost faith in electronic navigation aids, as the British had predicted, and did not deploy any other systems against the UK, although by this time the focus of Adolf Hitler and the German high command was turning once more toward eastern Europe.