On the afternoon of September 7, 1940, the people of London heard a sound they had been dreading for years: the ominous drone of aero engines. High above, the sky filled with waves of German bombers, and a torrent of high explosives and incendiaries began to rain down on the British capital. The bombardment continued through the night, and by the following morning the East End and the London Docks were ablaze and 1,000 Londoners lay dead and hundreds more severely injured. The Blitz had begun. Over the next 76 days, London and other southern British cities like Bristol, Portsmouth, Birmingham, Liverpool, and Sheffield would suffer near-continuous aerial bombardment, losing more than two million houses and over 100,000 civilian casualties. Throughout the bombing campaign, the Royal Air Force’s Fighter Command fought valiantly to beat back the invaders, soon forcing the Luftwaffe to abandon daylight bombing in favour of night raids. Yet despite the blackouts enforced across the country, German bomber crews always seemed to find their targets and bomb them with uncanny accuracy. Through brilliant detective work, British Intelligence soon discovered the reason: the Germans had a secret weapon, a series of invisible radio beams that could guide their bombers directly to their targets even in pitch darkness. The British effort to uncover the secrets of these beams and develop the means of jamming them was one of the great unsung technical achievements of the war, and proved decisive in turning the tide of the Blitz. This is the forgotten story of the Battle of the Beams.

Our story begins in November 1939, two months after Nazi Germany launched their invasion of Poland and ignited the Second World War. On November 2, an anonymous source – later identified as German mathematician and physicist Hans Meyer – mailed two letters to the British Embassy in Oslo, Norway, which immediately passed them along to MI6, British Foreign Intelligence Service. The Oslo Report, as it became known, contained a bonanza of information on German weapons then under development, including radar, remote controlled bombs, long-range rockets, and acoustic torpedoes. And among the technologies summarized in the report was one that would soon become of utmost interest to British military planners: a single-beam radio apparatus that could accurately measure a bomber aircraft’s heading and range, allowing it to be guided onto its target. One of the greatest intelligence leaks in military history, the Oslo Report gave the Allies an invaluable scoop on many of the advanced German weapons they would later face on the battlefield.

Fast-forward to early 1940. The eight-month period between Britain and France’s declaration of war on September 3, 1939 and Germany’s invasion of France and the Low Countries on May 10, 1940 saw relatively little military action on either side, being sarcastically referred to as the “Phoney War” or “Sitzkrieg.” However, the German Luftwaffe did conduct a number of small-scale reconnaissance and bombing raids over the British Isles during this period, and in March 1940 the Royal Air Force shot down a Heinkel He-111 bomber of Kampfgruppe or KG 26 near London. Inside the wreckage, British military intelligence found a small scrap of paper belonging to the aircraft’s navigator, which bore the mysterious note:

“Navigational aid: Radio Beacons working on Beacon Plan A. Additionally from 0600 hours Beacon Dühnen. Light Beacon after dark. Radio Beacon Knickebein from 0600 hours on 315º.”

A few days later another He-111 from KG 26 was shot down, aboard which was discovered a diary containing the following entry:

“5.3.40. Two thirds of Staffel on leave. In afternoon tidied about Knickebein, collapsible boats, etc.”

The word Knickebein baffled intelligence analysts. Literally translating as “crooked leg”, it was also the name of a legendary raven in Germanic mythology who had the ability to see over great distances. In search of answers, they turned to Squadron Leader Samuel Felkin, the chief Air Intelligence interrogator based at Trent Park in Cockfosters, North London. A grand 18th century country manor, at the outbreak of war Trent Park was commandeered by the Air Ministry as a centre for interrogating captured German airmen. In addition to formal interrogation, the “Cockfosters Cage” – as it became known – also made use of microphones hidden throughout the estate to eavesdrop on private conversations between unsuspecting prisoners. During one interrogation, Squadron Leader Felkin asked a German bomber navigator about the mysterious “Knickebein.” Though initially evasive, the prisoner eventually revealed that it was a radio navigation system “like X-Gerät”, but used a shortwave signal whose beam was less than a kilometre wide over the target.

This information was duly passed to Professor Reginald Victor (or R.V.) Jones, a physicist who in 1939 had been appointed Assistant Director of Scientific Intelligence at the Royal Aircraft Establishment, Farnborough. Throughout the war, Jones would play a vital role in teasing out the secrets of advanced German military technology, including radar and the long-range V-weapons. In a May 23, 1940 report titled Indication of New German Weapons to be Used Against England, Jones wrote:

“It is possible that they have developed a system of intersecting beams, so that they can locate a target such as London with sufficient accuracy for…indiscriminate bombing. No information is available concerning the wavelength to be employed, but the accuracy expected by the Germans is something like a half mile over London fro the western frontier of Germany. Efforts are still being made to determine the probable wavelengths so that counter measures can be employed.”

Soon, the need to learn more about this system would become extremely urgent, for on May 10, 1940 the Germans launched Fall Gelb, the invasion of France and the Low Countries. In only six weeks, the British Expeditionary Force was pushed back to the coast and evacuated from the beaches of Dunkirk, while a defeated France capitulated and signed an armistice with the Germans – and for more on this unnecessarily humiliating process, please check out our previous video Yet Another of Hitler’s Ultimate Dick Moves. With France now conquered, the Germans turned their attention to subduing the British Isles.

In the tiniest of silver linings, the lightning invasion of France provided British Military Intelligence with valuable information on the mysterious Knickebein system. But this intelligence came not from crashed German aircraft or the interrogation of Prisoners of War, but a far more powerful and closely-guarded source: the cracking of the German Enigma cipher, codenamed ULTRA. The invasion of France moved so rapidly that forward units of the Luftwaffe were forced to communicate with each other via radio. These signals were in turn picked up by radio listening stations of the British “Y” Service and passed to the ULTRA codebreakers at the Government Code and Cypher School at Bletchley Park, Buckinghamshire for decipherment. On June 5, 1940, the Y service intercepted a signal sent from a German bomber flying over England to the Chief Signals Officer of the Luftwaffe’s Fliegerkorps IV, which when deciphered read:

“Knickebein, Kleve is established at a point 53º24’ North and 1º west.”

Analysts immediately recognized Kleve as the German spelling of Cleves, a town in the Lower Rhine region of Germany and famously the birthplace of Anne of Cleves, the fourth wife of English King Henry VIII. The town made perfect sense as a site for a radio navigation beacon, being the westernmost point in Germany and thus the closest to the British Isles. Meanwhile, the coordinates in the message were traced to a point near Retford, Nottinghamshire, where the Great North Road or A1 highway intersected the 1º West meridian. Amusingly, the Air Ministry originally assumed that the message indicated that a spy had set up a secret radio navigation beacon in the area, and dispatched troops and police officers to find and arrest him. However, when the message reached R.V. Jones, he interpreted to mean that the transmitting bomber had picked up the navigation beam from Cleves at that location. Indeed, other intelligence revealed that aircraft of Fliegerkorps IV’s squadrons – specifically KG4 and KG37 – had been operating over England on June 5. Tracing the line from Cleves to Retford, Jones further deduced that the beam had been intended to guide the bombers to Sheffield, but had been misaligned.

Armed with this information, on June 12 Jones met with Group Captain Lyster F. Blandy, Deputy Director of Signals Intelligence; and Professor Frederick Lindemann, Jones’s old tutor at Oxford and official Scientific Advisor to Prime Minister Winston Churchill. To Jones’s dismay, Lindemann immediately dismissed the feasibility of a long-range navigational beam, arguing that at the high frequencies indicated by captured Luftwaffe airmen, radio signals would be blocked by the curvature of the earth and would be unable to reach England from Germany.

Undaunted, Jones unearthed a paper by one T.L. Eckersley, a radio propagation expert from the Marconi Company and a consultant for the Y Service, which indicated that a VHF signal transmitted from Cleves should indeed be detectable over England. And there was more good news. Back at the Cockfosters Cage, a conversation between a group of German airmen about Knickebein was secretly recorded in which one prisoner boasted that the British would “never find” the mysterious system. As luck would have it, on October 28, 1939 – just two months after the outbreak of war – a Heinkel He-111 of KG 26 was shot down and crash-landed near Edinburgh, Scotland, and its radio equipment captured intact and thoroughly examined. Jones contacted the man who had examined the equipment, one Squadron Leader Cox-Walker (and yes, that was his real name!), and asked him if there was anything unusual about it. At first, Cox-Walker said no: the bomber was equipped with a standard FuG10 short-wave transceiver and an EBL2 Lorenz Blind Landing Set. But then, after a moment, he exclaimed:

“Wait a minute – yes, you know we were surprised that [the Lorenz set] seems so much more sensitive than they would need for blind landing.”

Jones had his answer. Developed in 1932 by German firm C. Lorenz AG and first used on a large scale by national airline Lufthansa, the Lorenz Blind Landing System was designed to allow pilots to find and align themselves with a runway at night or in bad weather. The system comprised a set of antennas mounted at the end of the runway which transmitted two overlapping Morse Code signal beams: one composed of dots and the other of dashes. The timing of the dots and dashes was arranges such that where the two beams overlapped, the signals merged into a continuous tone, known as the equisignal. To use the system, a pilot located one of the two beams using an onboard radio receiver and adjusted his course laterally until he heard the equisignal in his headphones. He then continued along that course, adjusting left or right if the equisignal gave way to a string of dots or dashes. In this manner he could be guided onto the centre of the runway in even the poorest visibility. Knickebein worked the same way but in reverse, with the aircraft flying away rather than towards the transmitting antennas. However, thanks to the high signal frequencies used, the beams were only a few hundred metres wide by the time they reached London and other southern English cities – accurate enough for strategic bombing.

However, while the Lorenz system could give a bomber the correct heading towards a target, it could not give its range – that is, the proper point at which to drop its bombs. For this reason, a second beam operating at a different frequency was used, the two antennas being arranged so that the beams intersected over the target. As a particularly cooperative Luftwaffe prisoner of war codenamed A231 explained:

“As soon as the aircraft picks up the beam, the pilot flies a level course, seeing that the Turn and Bank Indicators…and the Artificial Horizon all read zero. He then flies along the beam.”

Once close to the target, the navigator switched to the second beam frequency. As soon as the aircraft entered the equisignal of that beam, the bombardier released the bombs. A231 also sketched out the prototype Knickebein antenna he had seen at the Reich Air Ministry or RLM development centre at Rechlin, which was identical to an antenna photographed by the Royal Air Force at Hörnum in northern Germany but which military intelligence had been unable to identify. The name Knickebein or “crooked leg” was derived from the asymmetric shape of the transmitting antenna, though the connection to a far-seeing mythological raven was likely also intentional. As we shall see, this would not be the first time the Germans would pick a far-too-obvious codename to their own detriment.

Once again, Jones presented his findings to Frederick Lindemann who, thoroughly convinced this time, wrote to Winston Churchill on June 13 that:

“There seems to be some reason to suppose that the Germans have some type of radio device with which they hope to find their targets. Whether this is come for of RDF or some other invention, it is vital to investigate and especially to seek to discover what the wavelength is. If we know this, we could devise a means to mislead then…with your approval I will take this up with the Air Ministry and try to stimulate action.”

Churchill, in turn, passed on Lindemann’s recommendation to Secretary of State for Air Sir Archibald Sinclair, adding a note that:

“This seems most intriguing, and I hope you will have it thoroughly examined.”

Air Marshall Philip Joubert was duly appointed to head an official investigation into Knickebein, with the first priority being to determine what frequencies at which the twin beams were transmitted. He wouldn’t have long to wait. On June 18 Air Intelligence received an assortment of papers captured from German aircraft shot down over France before the Dunkirk evacuation, one of which confirmed the location of a second Knickebein transmitter at Bredstedt in Schleswig-Holstein, northern Germany. More valuable intelligence soon fell into British hands – literally. Indeed, use of Knickebein led to many German bomber crews becoming careless, with some even leaving their navigation lights on. This made them highly vulnerable to British night fighters, which, in the absence of effective airborne intercept radar, normally found it almost impossible to locate and shoot down German aircraft in the dark. On June 19, a Heinkel He-111 of KG4 was shot down whose navigator’s log revealed the first beam frequency to be 31.5 megahertz. Two days later a German navigator bailed out of his bomber only to realize that he still had his logbook on him. He valiantly proceeded to shred the book into tiny pieces, but was arrested before he could fully destroy the evidence. The book was painstakingly pieced back together like a jigsaw puzzle, revealing – among other things – that the second beam frequency was 30 megahertz.

With this vital information in hand, all that remained was to actually detect the beams as they crossed over England. To this end, Wing Commander Robert S. Blucke of the Blind Landing Development Flight at Boscombe Down had three Avro Anson training and maritime patrol aircraft fitted with American-made Hallicrafters S27 receivers – the only radios available in the UK that could detect the high-frequency Knickebein beams. Indeed, it is rumoured that an Air Ministry representative bought up the entire stock of Webb’s Radio shop in Soho – on credit. Never designed for use aboard aircraft, the radios had to be extensively modified for the task – for example, being adapted to run off an aircraft’s 28 Volts DC electrical system rather than the typical 250 Volts AC household supply. And while the Ansons were ageing and rather rickety aircraft by this point, they were all that could be spared. This was, after all, the height of the invasion scare, when even de Havilland Tiger Moth training biplanes were being fitted with bomb racks in a desperate hopes of fending off landing German troops.

One of the modified aircraft, its radio manned by a volunteer from the Y Service, made its first flight on June 19, but this was cut short by a faulty generator. The crew tried again the following day, but failed to intercept the beams. And before another flight could be attempted, on June 21 a meeting was called at 10 Downing Street chaired by Prime Minister Churchill and attended by Sir Archibald Sinclair, Secretary of State for Air; Max Aitken, Lord Beaverbrook, the Minister of Aircraft Production; Henry Tizard and Robert Watson-Watt, two of the inventors of radar; and Frederick Lindemann. Amusingly, R.V. Jones discovered the meeting invitation on his desk after the meeting had already begun, and was forced to scramble to Whitehall to present his findings. In Their Finest Hour, the second volume of his book series The Second World War, Winston Churchill recalled:

“Being master and not having to argue too much, once I was convinced about the principles of this queer and deadly game I gave all the necessary orders that very day in June, for the existence of the beams to be assumed and for all counter-measures to receive absolute priority. The slightest reluctance or deviation in carrying out this policy was to be reported to me.”

However, that same afternoon, Jones was pulled into another meeting at the Air Ministry attended by members of the RAF Signals Office and the Air Ministry Research Establishment, including Lindemann and Marconi Company consultant T.L. Eckersley. To Jones’s shock, Lindemann proceeded to restate his previous opinion that VHF signals like those supposedly used by Knickebein could not possibly reach England from Germany due to the earth’s curvature. To his even greater shock, Eckersley backed this view, stating, as another attendee recalled:

“…in his opinion the reception at any distance of this wavelength was quite impossible and that he’d stake his reputation on the statement.”

As Jones later wrote:

“It was an absolute blow from my point of view because if that really were true, all the fuss that I had been the cause of would now be exploded, and what would happen to me for the rest of my life was problematic…I pointed out to Eckersley that I’d used one of his own papers to convince Lindemann; that this showed quite clearly that the beams would go that far. “Oh,” he said, “well you don’t want to believe that, I was showing how far they might go but I don’t really think they would go as far as that.””

Nonetheless, Jones managed to convince the committee to allow the modified Ansons at Boscombe Down to make one final flight in search of the Knickebein beams. That same evening, an Anson crewed by one Flight Lieutenant Bufton and a Corporal Mackie from Y service took off and began zig-zagging across the Wiltshire countryside. Within minutes, they succeeded in locating the two Knickebein beams at 31.5 and 30 megahertz – exactly as Air Intelligence had predicted. Tellingly, the beams crossed over the Rolls Royce Aircraft Engine Plant in Darby, the only establishment producing the legendary Merlin V12 engine that powered the Supermarine Spitfire and Hawker Hurricane fighters that were soon to become vitally important to Britain’s survival. Jones was ecstatic, writing that:

“It is fitting to close the account as far as scientific intelligence is concerned. In the course of ten days the matter has developed from conjecture to certainty…several technical points remain to be cleared up, but their elucidation is only a matter of time…if our good fortunes hold we may yet pull the crooked leg!”

Stubbornly, Eckersley stood his ground, declaring that the navigation beams “…did not extend any great distance within our coastline…are in the nature of freaks” But Jones had made his case, and soon a new unit dubbed 80 Wing commanded by RAF Signals Office Group Captain E.B. Addison was established to develop countermeasures against Knickebein.

This could not have come at a better time, for on June 10, 1940, the Luftwaffe finally launched a concerted bombing campaign against British airfields and aircraft factories in an effort to destroy the Royal Air Force on the ground. The Battle of Britain had begun. Though this campaign is often characterized as a prelude to invasion, in reality the Germans were ill equipped to invade the British Isles, and the bombing was instead intended to destroy Britain’s fighting ability and force the government to the negotiating table. Famously, however, the pilots of RAF Fighter Command, aided by the Chain Home early warning radar network and the highly-efficient Dowding System of command and control, proved more than a match for the German intruders. Furthermore, the Luftwaffe had been developed as a tactical air force for providing close air support to advancing troops, not as a long-range strategic force. Escort fighters like the Messerschmitt Bf-109 thus had only enough fuel to linger 15 minutes over England, whereupon they were forced to cross back over the channel, leaving the bombers sitting ducks for British fighters. Consequently, on September 7 the Germans switched to the strategic or “terror” bombing of civilian centres, a campaign forever seared into the British consciousness as The Blitz. At first the Germans bombed by both day and night, but as in the earlier phase of the battle the unescorted bombers proved horrendously vulnerable to British fighters in daylight, and raids were soon conducted exclusively at night. By this time, further Knickebein transmitters had been constructed near Cherbourg and Dieppe on the coast of newly-occupied France. Being much closer to Britain, these transmitters generated stronger signals to guide the night raiders to their targets.

At first, the British were all but helpless in the face of this nocturnal onslaught. With effective airborne and gun-laying radar still under development, searchlights and anti-aircraft guns were woefully inaccurate and night fighters effectively blind. But there was one hope: jam the Knickebein beams and lead the bombers away from their targets. This was easier said than done, for no radio sets commonly used in the UK at the time could transmit at the high frequencies used by the navigation beams. However, someone in the Air Ministry realized there was a type of device that could: diathermy machines. Still used to this day in physical therapy, diathermy sets use high-voltage, high-frequency electricity to induce heating in deep muscle tissue. Operating at frequencies in excess of 30 megahertz, they were ideal for interfering with the Knickebein beams. In another very British feat of subterfuge, a diathermy expert from Harley Street in London was instructed to don a Flight Lieutenant’s uniform and commandeer as many diathermy sets as he could find from hospitals and private clinics. These were modified into 150 watt VHF radio transmitters and installed across the country, many in rural police stations in towns like Glastonbury and Wimborne Minster.

Unfortunately, the modified diathermy sets proved underpowered and ineffective. However, the Telecommunications Research Establishment in Matravers near Swanage soon succeeded in developing a more powerful jamming transmitter codenamed Aspirin – a reference to the official codename Headache for the navigation beams. Eventually, 28 Aspirin jammers would be installed along the Channel coast. Meanwhile, 80 Wing – now renamed 190 Squadron, flew nightly missions in radio-equipped Ansons and Armstrong-Whitworth Whitley medium bombers to locate the navigation beams, these flights being supplemented by stationary VHF receivers mounted on Chain Home radar towers at Ottercops, Bawdsey, West Beckham, Stanton Wold, and Dover. The beams proved relatively easy to find due to the peculiar German habit of switching on the transmitters long before a raid began. British Intelligence assumed this was done to train the transmitters’ crews, but could not understand why the Germans insisted on aiming the beams over the British Isles rather than the vast area of Europe they now controlled. It was one of many careless mistakes that would eventually cost the Germans dearly.

Once the location and bearing of the Knickebein beams was established, the appropriate Aspirin transmitters would be switched on to lead them off-course. Aspirin worked by transmitting a spoof signal of Morse Code dots, which would overlap and mask the original signal and make it almost impossible for bomber crews to locate the correct equisignal. When used correctly, this countermeasure could be spectacularly effective, as revealed by an Air Ministry report on a Heinkel He-111 bomber of KG55 that was brought down during a night raid on Birmingham:

“This particular aircraft had an individual target, an aircraft factory to the north of Birmingham…they picked up the beam (at 13,000 feet) over the Channel and started to fly along it. After a time the equi-signal became variable and then disappeared altogether, and no matter how they tried they could not pick it up again. Panic then seems to have overtaken the crew of the aircraft. They complained that the electrical apparatus of the aircraft had gone wrong and that neither the compass nor the artificial horizon were functioning properly and that the night was so dark the pilot could not keep the aircraft on a level course…the [radio] operator, however, insisted that his apparatus continued to function, which tends to disprove the notion that that the electrical apparatus had gone out of order. Eventually the observer jettisoned the bombs and then he and the [radio] operator bailed out. The pilot and a gunner were killed when the aircraft crashed.”

But the Germans were quick learners, and soon began switching on their navigation beams just before a raid was launched, giving the British less time to find and jam them. They also sometimes aimed the beams at a dummy target before switching them to the real target, but this often confused bomber crews nearly as much as the British. More experienced bomber pilots could defeat Aspirin by flying along one edge of the beams rather than down the equisignal, but such airmen were few and far in between. As mentioned earlier, the Luftwaffe was designed as a tactical daylight air force, and most pilots had little experience in night instrument flying. Consequently, most bomber crews opted to navigate by what few landmarks they could make out on the dark ground below – particularly the River Thames. They also made extensive use of regular non-directional radio beacons installed back in Germany and France. By determining the bearing of two or more known beacons using the aircraft’s radio direction finding or DF loop, a navigator could accurately fix his position. However, the British were also aware of this method, and devised equally clever countermeasures to defeat it. For example, civilian BBC radio stations were grouped into clusters operating at a common frequency and were shut off whenever an enemy aircraft came within 25 miles. This way, no single station could be used as a navigation beacon. The British also developed an elaborate system whereby radio receivers on the coast picked up German radio beacon signals and relayed them via General Post Office telephone lines to transmitters further inland for re-broadcasting, creating a confusing mess of signals that made it impossible for German navigators to fix their position. This technique, known as “Meaconing”, was impervious to any callsigns, codes, or other identifiers the Germans might try to integrate into the radio signal, as these would just be automatically rebroadcast.

But while clever, there was only so much Asprin and Meaconing could do, and by January 1, 1941 the Blitz had succeeded in killing some 13,000 people, injuring 16,000, and inflicting millions of pounds of property damage in cities including London, Bristol, Southampton, Plymouth, and Liverpool. Meanwhile, British Intelligence learned that the Germans were preparing to deploy a new, more sophisticated radio navigation system – the mysterious “X-Gerät” mentioned by the captured Luftwaffe airman back in early 1940. Slowly, through radio intelligence and Enigma decrypts, they pieced together how the system worked. Unlike Knickebein, X-Gerät used not two but four beams, codenamed after German rivers: Weser, Rhein, Oder, and Elbe, the transmitters located on the Hague Peninsula and in Calais-Boulongue.

Weser was the pilot approach beam and worked similarly to Knickebein, giving bomber pilots their bearing towards the target. However, it operated at a much higher frequency – 65-75 megahertz – allowing the beam to be narrower and thus more accurate. But this made the beam difficult for pilots to find, so a wider Knickebein “coarse” beam was added to guide them towards the “fine” beam equisignal. The three other beams, meanwhile, were transmitted crosswise to the Weser beam at specific intervals: Rhein at 30 kilometres from the target, Oder at 10 kilometres, and Elbe at 5 kilometres, giving the bombardier sequential signals as to when to drop his bombs. This system allowed for a bombing accuracy of as little as 100 metres at 300 kilometres – greater even than was usually achievable with daylight bombing. Unlike Knickebein, however, which used standard blind landing equipment already installed on all Luftwaffe bombers, X-Gerät needed specialized receiver equipment. A special unit, KGr100, was thus formed whose aircraft would use X-Gerät to find the target, which they would then mark using flares and incendiary bombs. Following waves of bombers would then home in on the flares and fires to drop their own payloads. Such “pathfinder” tactics would later be adapted to devastating effect by the Allies in their own strategic bombing campaign against Germany.

X-Gerät greatly worried R.V. Jones and the rest of the Air Ministry, for early intelligence indicated that the “fine” navigation beam was transmitted at a wavelength of only 10 centimetres – far beyond the ability of British radio equipment to jam. The only hope of countering the system was thus to jam the lower-frequency “coarse” beam and make it more difficult for German pilots to locate the narrow “fine” beam. To this end, the British modified a number of gun-laying radar sets into countermeasures code-named “Bromide.” It would later be determined that the fine beam was not, in fact, transmitted at 10 centimetres; indeed achieving this narrow a wavelength required a special piece of equipment called a cavity magnetron, which had just been invented by John Randall and Harry Boot of the University of Birmingham and which would go on to become one of the Allies’ most decisive secret weapons. The Germans, meanwhile, would not develop their own magnetrons until much later in the war, after reverse-engineering examples recovered from downed Allied aircraft. The “fineness” of the main X-Gerät beam, it turned out, was the result of the complex structure of the radio transmission itself. As an aside, while top-secret at the time, cavity magnetrons are now found in nearly every household, being the key component of microwave ovens – but that is a fascinating story for another video.

The Bromide sets were ready by early November 1940, just in time for an anticipated large-scale raid codenamed Moonlight Sonata. Targeted against the historic medieval city of Coventry, the attack began on the evening of November 14. Over the next 10 hours, German aircraft dropped 503 tons of high explosives and over 30,000 incendiary bombs on the city, killing 500 people, severely injuring 1200 more, and demolishing over 60,000 buildings including Coventry Cathedral. One of the most devastating raids of the entire bombing campaign, the “Coventry Blitz” went almost entirely unopposed, with the city’s anti-aircraft batteries only managing to shoot down a single German bomber.

Strangely, the Bromide jamming sets appeared completely ineffective, and it would not be long until their designers found out why. One week earlier on November 6, a Heinkel He-111 of KGr100 crash-landed in the surf at West Bay near Bridport in Dorset. One crew member was killed on impact, but the three others waded ashore and were soon captured, apparently surprised to find their captors English; thanks to Meaconing, the navigator thought they had crash-landed in Spain! The wreck of the aircraft was soon recovered and its X-Gerät equipment examined. Far more sophisticated than Knickebein, the receiver used a visual rather than auditory system to indicate the aircraft’s position relative to the target. This consisted of a clock-like bombing computer with three hands, coloured green, black and red. When the aircraft crossed the Rhein beam, the navigator heard a brief audio signal prompting him to set up the computer. Then, when the aircraft crossed the Oder beam 10 kilometres from the target, the green and black hands began to sweep across the dial. When the aircraft crossed the Elbe beam 5 kilometres from the target, the green hand stopped, the black hand reversed back to the 12 o’clock position, and the red hand began sweeping from the black hand towards the green. As this interval represented 5 kilometres of travel, when the red hand reached the green an aircraft travelling at a constant speed would be over the target and the bombs would be automatically released. Not only was this system far more difficult to fool than a human navigator listening to Morse Code signals over headphones, but the equipment included a very narrow band pass filter that blocked any signals that differed from the genuine Weser beam by more than 50 kilohertz. During the Coventry Blitz, the modulation of the Weser beam was set to 2,000 hertz while the Bromide jammers were set to 1,500 hertz – far outside the filter pass band. The jammers therefore had no effect.

Learning from their mistakes, the British duly modified the jamming frequencies and by early December 1940 17 Bromide transmitters were deployed across the country. At the same time, Whitley bombers of 109 Squadron carried out 10 raids against the X-Gerät transmitters on the French coasts, but due to poor visibility these had little effect. Though the Bromides succeeded in fooling several bombers, the skilled crews of KGr100 never relied exclusively on X-Gerät and used multiple navigation methods to find their targets. The British thus switched to a completely different strategy codenamed Starfish. Starfish or Decoy Fire Sites sites comprised large arrangements of oil burners set up in open countryside which at night would resemble a burning town or factory and trick bombers into dropping their bombs on the wrong target. Even more elaborate were the so-called Q Sites which used arrangements of dummy buildings and aircraft and both stationary and moving lights to simulate airfields, factories, and other installations. This strategy proved remarkably effective, with a contemporary report stating that:

“Starfish have attracted the attention of the enemy on numerous occasions, drawing a considerable number of bombs. On the other hand, there have been some occasions, particularly when visibility conditions were good, when Starfish were ignored. Outstanding successes were achieved at Cardiff on 4 March 1941, at Bristol 16 March and at Portsmouth on 17 April. On the first occasion Home Security Specialists reported that no less than 102 high explosive bombs had been aimed at the Starfish. The total weight of bombs being 25 tons…300 bombs were dropped on the Downside Starfish, Bristol…the largest number of bombs ever collected by a decoy fire fell on the Sinah Common Starfish near Portsmouth. No less than 170 bombs, 26 landmines 20 oil bombs and innumerable incendiary bombs were dropped, representing 95% of the total effort against Portsmouth.”

It appears that the Germans were at least somewhat aware of the Starfish and Q sites, with a common story telling of a Junkers Ju-88 mocking the British by attacking one of the sites with wooden bombs.

In the end, X-Gerät was largely defeated by setting up a “false Elbe” beam just 1 kilometre after the Oder beam. As the bombing computer was automatically triggered by these beams, the premature signal would cause the bombs to be released four kilometres early. However, this tactic was not always effective, as by now the Germans had learned to only switch on the X-Gerät beams at the last possible moment, making it difficult for the British to properly set up the false Elbe beam in time.

But once again, the Germans had another trick up their sleeve, and in early 1941 Y Service listening stations began picking up a new navigation signal transmitted at between 42 and 39 Megahertz. At the same time, Enigma decrypts revealed the installation of a transmitter codenamed Wotan near Cherbourg and Brest in occupied France. Suspecting that the name contained a clue to the transmitter’s function, R.V. Jones contacted Professor Frederick Norman at the codebreaking centre at Bletchley Park:

“He said, “Well, he was the head of the German Gods.” Then he said, “Wait a minute, one eye! One beam!” He shouted down the telephone. “One beam! Can you make a system work with one beam?” And I said I could use one beam in the ordinary war and then for example we could use the ranging system mentioned in the Oslo report. He said “That will be it!” And so we started to look…”

As luck would have it, in November 1940 an Enigma message had been intercepted instructing the Luftwaffe to designate“Target No.1 for Y”. Target No. 1 was known to be a codename for the Royal Armoured Corps depot in Bovington, Dorset, and as the message was sent to only a single transmission station, this confirmed that the new navigation system – known as Y-Gerät – used only one beam. Once again, the Germans’ penchant for clever, far too descriptive codenames had worked against them. It was a mistake which in the postwar period would lead the British to develop a truly bizarre system of weapons naming known as the Rainbow Codes – and for more on this, please check out our previous video Death by Blue Peacock: Britain’s Bizarre and Deadly Cold War “Rainbow Codes.”

The directional function of Y-Gerät worked similarly to Knickebein and X-Gerät, with pilots following a narrow guide beam. Instead of cross-beams, however, the range of an aircraft was determined using a transponder system, with the onboard FuG 28a transceiver receiving a signal from the ground station and immediately re-broadcasting it back. By comparing the modulation phase of the two signals, the transit time and thus the range of the aircraft could be accurately determined using only a single beam. Even better, a pilot did not have to ride the beam the entire way; as the ground station could determine his aircraft’s position at any time, they could radio course corrections to keep him on-target.

Despite all this, however, Y-Gerät had a number of glaring weaknesses. Firstly, being an automatic system it was potentially easier to fool than an experienced human radio operator. Second, the re-transmitted transponder signal could be homed in on by suitably equipped night fighters. Thirdly, only one aircraft could be guided by each ground station at a time, making the whole system much easier to disrupt. And finally, the system operated at 45 Megahertz – an unintentionally poor choice of frequency, as it would turn out. Indeed, of all the German navigation beam systems, Y-Gerät proved the easiest to defeat. The countermeasure the British developed, codenamed Domino, comprised a radio receiver at Swains Lane in Highgate which picked up the Y-Gerät beam and transmitted it via telephone landlines to the BBC television transmitter at Alexandra Palace in North London, where it was boosted and rebroadcast. The site of the first experimental public television broadcasts in the 1930s, the station was shut down on the outbreak of war but reactivated when it was realized it operated at the same 45 Megahertz frequency as Y-Gerät. This jamming scheme worked so well that for weeks the Germans thought there was some inherent technical flaw with Y-Gerät itself. When they finally realized that the British had been jamming their signals from the outset, the Luftwaffe completely lost faith in electronic navigation aids. By this time, however, the German bombing campaign against Britain was coming to an end. The introduction of new fighters like the Bristol Beaufighter equipped with the compact and powerful Airborne Intercept or AI Mk.IV radar finally succeeded in sweeping the intruders from the night sky, while at the same time Adolf Hitler began redeploying his forces east in preparation for Operation Barbarossa, the German invasion of the Soviet Union. After 8 months and 5 days, the Blitz officially came to an end on May 11, 1941. While the so-called ‘Battle of the Beams’ was just one part of a much larger effort to counter the German aerial onslaught, it was nonetheless a vital one, as Jones later recalled:

“This had been our only defence: night fighters, until airborne radar was good enough, were powerless anti-aircraft guns were inaccurate, therefore we could not stop the bombers. Our only hope was to throw them off. Well, we didn’t always succeed but we did certainly on a few number of occasions, with the result that a good many people were alive at the end of the war who otherwise wouldn’t have been and a good deal less vital damage was done…If one thinks of what could have happened if with Knickebein all the German Air Force could have bombed accurately. Well, we saw what happened when things went wrong as at Coventry…That could have happened every night.”

But the Battle of the Beams was to have another, even greater impact on the course of the war. From 1942 onwards, the Royal Air Force and United States Army Air Force carried out a concerted campaign of round-the-clock strategic bombing against Germany and occupied Europe. Pathfinder units, first deployed during the Blitz, played a key role in this campaign, as did electronic navigation aids like GEE, Oboe, and the H2S ground-scanning radar. By the time Germany surrendered on May 7, 1945, the bombardment had left countless German cities and towns in ruins and over 600,000 German citizens dead. By demonstrating the effectiveness of strategic bombing tactics, the Germans had ironically contributed to their own demise. As the commander of RAF Bomber Command, Air Marshall Arthur “Bomber” Harris, famously stated on June 3, 1942:

“The Nazis entered this war under the rather childish delusion that they were going to bomb everybody else and nobody was going to bomb them. At Rotterdam, London, Warsaw, and half a hundred other places, they put that rather naïve theory into operation. They sowed the wind and now they are going to reap the whirlwind…”

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