The Battle of the Beams was a period early in the
Second World War
World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...
when bombers of the German Air Force (''
Luftwaffe
The Luftwaffe () was the aerial warfare, aerial-warfare branch of the before and during World War II. German Empire, Germany's military air arms during World War I, the of the Imperial German Army, Imperial Army and the of the Imperial Ge ...
'') used a number of increasingly accurate systems of
radio navigation
Radio navigation or radionavigation is the application of radio waves to geolocalization, determine a position of an object on the Earth, either the vessel or an obstruction. Like radiolocation, it is a type of Radiodetermination-satellite servi ...
for night bombing in the
United Kingdom
The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Northwestern Europe, off the coast of European mainland, the continental mainland. It comprises England, Scotlan ...
. British scientific intelligence at the
Air Ministry
The Air Ministry was a department of the Government of the United Kingdom with the responsibility of managing the affairs of the Royal Air Force and civil aviation that existed from 1918 to 1964. It was under the political authority of the ...
fought back with a variety of their own increasingly effective means, involving
jamming and deception signals. The period ended when the
Wehrmacht
The ''Wehrmacht'' (, ) were the unified armed forces of Nazi Germany from 1935 to 1945. It consisted of the German Army (1935–1945), ''Heer'' (army), the ''Kriegsmarine'' (navy) and the ''Luftwaffe'' (air force). The designation "''Wehrmac ...
moved their forces to the East in May 1941, in preparation for the
attack on the Soviet Union.
The idea of
beam radio navigation was developed during the 1930s, initially as a
blind landing aid. The basic concept is to produce two directional radio signals that are aimed slightly to the left and right of a
runway
In aviation, a runway is an elongated, rectangular surface designed for the landing and takeoff of an aircraft. Runways may be a human-made surface (often asphalt concrete, asphalt, concrete, or a mixture of both) or a natural surface (sod, ...
's midline. Radio operators in the aircraft listen for these signals and determine which of the two beams they are flying in. This is normally accomplished by sending
Morse code
Morse code is a telecommunications method which Character encoding, encodes Written language, text characters as standardized sequences of two different signal durations, called ''dots'' and ''dashes'', or ''dits'' and ''dahs''. Morse code i ...
signals into the two beams, to identify right and left.
For bombing, the ''Luftwaffe'' built huge versions of the antennas to provide much greater accuracy at long range, named ''Knickebein'' and ''X-Gerät''. These were used during the early stages of the
Blitz with great effect, in one case laying a strip of bombs down the centerline of a factory deep in England. Tipped off about the system's operation by pre-war
military intelligence
Military intelligence is a military discipline that uses information collection and analysis List of intelligence gathering disciplines, approaches to provide guidance and direction to assist Commanding officer, commanders in decision making pr ...
, the British responded by sending their own Morse code signals so that the aircraft crew believed they were always properly centred in the beam while they flew wildly off course. The Germans became convinced the British had somehow learned to bend radio signals.
When the problem became widespread, the Germans introduced a new system that worked on different principles, the ''Y-Gerät''. Having guessed 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 abandoned the entire concept of radio navigation over the UK, concluding the British would continue to successfully jam it.
Background

Before the
start of the war on 1 September 1939,
''Lufthansa'' and the German aircraft industry
invested heavily in the development of commercial aviation, and in systems and methods that would improve safety and reliability. Considerable effort went into
blind-landing aids which allowed aircraft to approach an airport at night or in foul weather. The primary system developed for this 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 centre
dipole
In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways:
* An electric dipole moment, electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple ...
, which had a slightly longer
reflector element on either side set slightly back. A switch rapidly alternated the opened midpoint connection of each reflector in turn, sending the beam slightly to the left and then slightly to the right of the centreline of the runway. The beams widened as they spread from the antennas, 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 aircraft approaching the airport would tune one of their radios to the Lorenz frequency. If the crew was on the left side of the centreline, they 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", they would know that they had to veer to the right to fly down the centreline. If the crew was on the right side of the centerline, they would hear a series of long tones followed by short pauses, meaning the aircraft was on the "dash" side of the antenna. Hearing the "dashes", they would know that they had to veer to the left to fly down the centreline. In the centre, the radio would receive both signals, where the dots filled in the gaps in the dashes and produced 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 aircraft 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.
Night bombing
During the early 1930s, the concept of a
night bombing strategic campaign began to become paramount in military aviation circles. This was due to the ever-increasing performance of
bombers
A bomber is a military combat aircraft that utilizes
air-to-ground weaponry to drop bombs, launch torpedoes, or deploy air-launched cruise missiles.
There are two major classifications of bomber: strategic and tactical. Strategic bombing is ...
, which were beginning to have the capability to strike across Europe with useful bomb loads. These aircraft were slow and lumbering, easy prey for
interceptors, but this threat could be essentially eliminated by flying at night. A bomber, painted black, could be spotted only at very short ranges. As the bomber's altitude and speed increased, the threat from ground-based defences was greatly reduced. Planners believed that "
the bomber will always get through".
The problem with night bombing is that the same limitations in visibility meant the bomb crew would have a difficult time finding their targets, especially a
blacked-out target at night. Only the largest targets, cities, could be attacked with any probability of success.
To support this mission, the RAF invested heavily in navigation training, equipping their aircraft with various devices, including an
astrodome
The NRG Astrodome, formerly and also known as the Houston Astrodome or simply the Astrodome, was the world's first multi-purpose, domed sports stadium, located in Houston, Texas, United States. It seated around 50,000 fans, with a record atte ...
for taking a
star fix and giving the navigator room to do his calculations in an illuminated workspace. This system was put to 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 from their intended targets.
The Luftwaffe continued to research accurate night bombing against smaller targets. Not depending on celestial navigation, they invested their efforts in
radio navigation
Radio navigation or radionavigation is the application of radio waves to geolocalization, determine a position of an object on the Earth, either the vessel or an obstruction. Like radiolocation, it is a type of Radiodetermination-satellite servi ...
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.
German systems
''Knickebein''

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". The word is also the name of a magical raven in
Germanic mythology
Germanic mythology consists of the body of myths native to the Germanic peoples, including Norse mythology, Anglo-Saxon paganism#Mythology, Anglo-Saxon mythology, and Continental Germanic mythology. It was a key element of Germanic paganism.
O ...
. 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.

The first of these new ''Knickebein'' transmitters was set up in 1939 on
Stollberg hill in
Nordfriesland
Nordfriesland (; ; Low German: Noordfreesland), also known as North Frisia, is the northernmost Districts of Germany, district of Germany, part of the state of Schleswig-Holstein. It includes almost all of traditional North Frisia (with the e ...
near the border with
Denmark
Denmark is a Nordic countries, Nordic country in Northern Europe. It is the metropole and most populous constituent of the Kingdom of Denmark,, . also known as the Danish Realm, a constitutionally unitary state that includes the Autonomous a ...
; 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. 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.
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.
The search for the beams
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
The Royal Aircraft Establishment (RAE) was a British research establishment, known by several different names during its history, that eventually came under the aegis of the Ministry of Defence (United Kingdom), UK Ministry of Defence (MoD), bef ...
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
Sir Winston Leonard Spencer Churchill (30 November 1874 – 24 January 1965) was a British statesman, military officer, and writer who was Prime Minister of the United Kingdom from 1940 to 1945 (Winston Churchill in the Second World War, ...
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
Amateur radio, also known as ham radio, is the use of the radio frequency radio spectrum, spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radiosport, contesting, and emer ...
receiver 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
Rolls-Royce (always hyphenated) may refer to:
* Rolls-Royce Limited, a British manufacturer of cars and later aero engines, founded in 1906, now defunct
Automobiles
* Rolls-Royce Motor Cars, the current car manufacturing company incorporated in ...
engine factory at
Derby
Derby ( ) is a City status in the United Kingdom, city and Unitary authorities of England, unitary authority area on the River Derwent, Derbyshire, River Derwent in Derbyshire, England. Derbyshire is named after Derby, which was its original co ...
, 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.
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
Aerial reconnaissance is reconnaissance for a military or Strategy, strategic purpose that is conducted using reconnaissance aircraft. The role of reconnaissance can fulfil a variety of requirements including Artillery observer, artillery spott ...
returned photographs of the RAF bombing raids, showing that they were rarely, if ever, anywhere near their targets.
Countermeasure
Efforts to block the ''Knickebein'' headache were codenamed "Aspirin". Initially, modified medical
diathermy sets transmitted interference. Later, local radio transmitters broadcast an extra "dot signal" at low power on nights where raids were expected. The German practice of turning on the beams long before the bombers reached the target area aided the British efforts. Avro Ansons fitted with receivers would be flown around the country to find the beams' location to be 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" and no 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 equisignal or Germany again. Some Luftwaffe bombers even landed at RAF bases, believing they were back at base.
''X-Gerät''
As good as ''Knickebein'' was, it was never intended to be used in the long-range role. Plendl had been working for some time to produce a much more accurate version of the same basic concept, which was eventually delivered as ''X-Gerät'' (X-Apparatus). ''X-Gerät'' used a series of beams to locate the target, each beam named after a river. The main beam, ''Weser'', was similar in concept to the one used in ''Knickebein'' but operated at a much higher frequency. Due to the nature of radio propagation, this allowed its two beams to be pointed much more accurately than ''Knickebein'' from a similarly sized antenna; the equi-signal area was only about wide at a distance of from the antenna. The beams were so narrow that bombers could not find them without help, 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
Cherbourg is a former Communes of France, commune and Subprefectures in France, subprefecture located at the northern end of the Cotentin peninsula in the northwestern French departments of France, department of Manche. It was merged into the com ...
in France.
The "cross" signal in ''X-Gerät'' used a series of three very narrow single beams, ''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 ''Weser'' before the release point. The ''Oder'' beam intersected ''Weser'' before the release point, or before ''Elbe''. ''Rhine'' did not require the same precision and was approximately before the release point. The beams' width added a small error to the intersection coordinates, on the order of tens to hundreds of meters.
As the bomber followed the ''Weser'' beam and reached ''Rhine'', the radio operator heard a brief signal and set up his equipment. This consisted of a special stop-clock with two hands. When the ''Oder'' signal was received the clock automatically started and the two hands simultaneously swept up from zero. When the ''Elbe'' signal was received, one hand stopped and the other reversed, sweeping back towards zero. The stopped hand indicated an accurate measurement of travel time from ''Oder'' to ''Elbe''. Since the ''Oder'' to ''Elbe'' distance equalled the ''Elbe'' to 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.
''X-Gerät'' operated at a much higher frequency than ''Knickebein'' (around 60 MHz) and thus required the use of new radio equipment. There was not enough equipment to fit all bombers, so the experimental unit
''Kampfgruppe'' 100 (''KGr'' 100) was given the task of using their ''X-Gerät'' equipment to guide other aircraft to the target. To do this, ''KGr'' 100 aircraft would attack as a small group first, dropping flares which other aircraft would then see and bomb visually. This is the first use of the
pathfinder concept that the RAF improved to great effect against the Germans some three years later.
The system was first tested on 20 December 1939 when a bomber from KGr 100 flown by
Oberleutnant Hermann Schmidt flew over London at 7,000m (23,000 ft).
[Hooton 1999, p. 199.]
''X-Gerät'' was used effectively in a series of raids known to the Germans as Moonlight Sonata, against
Coventry
Coventry ( or rarely ) is a City status in the United Kingdom, cathedral city and metropolitan borough in the West Midlands (county), West Midlands county, in England, on the River Sherbourne. Coventry had been a large settlement for centurie ...
,
Wolverhampton
Wolverhampton ( ) is a city and metropolitan borough in the West Midlands (county), West Midlands of England. Located around 12 miles (20 km) north of Birmingham, it forms the northwestern part of the West Midlands conurbation, with the towns of ...
and
Birmingham
Birmingham ( ) is a City status in the United Kingdom, city and metropolitan borough in the metropolitan county of West Midlands (county), West Midlands, within the wider West Midlands (region), West Midlands region, in England. It is the Lis ...
. 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 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.
Countermeasure
''X-Gerät'' proved more difficult to stop than ''Knickebein''. Initial defences against the system were deployed in a similar fashion to ''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 aircraft.
The mystery was eventually revealed after an ''X-Gerät''-equipped
Heinkel He 111 crashed on 6 November 1940 on the English coast at
West Bay, Bridport. Although the aircraft sank during the recovery operation, the waterlogged ''X-Gerät'' equipment was recovered. On examination, it was learned that a new instrument was being used that 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. While the jammers were modified accordingly, this came too late for the raid on Coventry on 14 November; but the modified jammers were able to successfully disrupt a raid on Birmingham on 19 November.
''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 after the ''Oder'' beam — much earlier than the expected . Since the final stages of the release were automatic, the clock would reverse prematurely and drop 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 switch the ''X-Gerät'' beams on until as late as possible, making it much more difficult to arrange the "false ''Elbe''" in time.
''Y-Gerät''
As the British slowly gained the upper hand in the Battle of the Beams, they started considering 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 ''Y-Gerät'', which was also sometimes referred to as ''Wotan''. Jones had already concluded the Germans used code names that were too descriptive, so he asked a specialist in the German language and literature at
Bletchley Park
Bletchley Park is an English country house and Bletchley Park estate, estate in Bletchley, Milton Keynes (Buckinghamshire), that became the principal centre of Allies of World War II, Allied World War II cryptography, code-breaking during the S ...
about the word ''Wotan''. The specialist realised ''Wotan'' referred to
Wōden, a one-eyed god, and might refer to a single-beam navigation system. Jones agreed, and knew that a system with one beam would have to include a distance-measurement system. He concluded that it might work on the basis described by the anti-Nazi German mathematician and physicist
Hans Mayer, who while visiting Norway had provided a large amount of information in what is now known as the
Oslo Report.
''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 transponder (
FuG 28a) that received the signal from the beam and immediately re-transmitted it to the ground station. The ground station listened for the return signal and compared the phase of its modulation to the transmitted signal, which accurately determined the transit time of the signal, and hence the distance to the aircraft. 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, instead 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 X-Gerät it was unlikely he would have concluded the system used a single beam.
Countermeasure

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 badly. It operated at 45 MHz, which happened to be the frequency of the powerful, but dormant,
BBC
The British Broadcasting Corporation (BBC) is a British public service broadcaster headquartered at Broadcasting House in London, England. Originally established in 1922 as the British Broadcasting Company, it evolved into its current sta ...
television transmitter at
Alexandra Palace
Alexandra Palace is an entertainment and sports venue in North London, situated between Wood Green and Muswell Hill in the London Borough of Haringey. A listed building, Grade II listed building, it is built on the site of Tottenham Wood and th ...
. All Jones had to do was arrange for the return signal to be received from the aircraft and then sent to Alexandra Palace for re-transmission. 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 enough to spoil the accuracy of the system. Over subsequent nights, the transmitter power was gradually increased.
As ''Y-Gerät''s use went on, the aircrew accused the ground station of sending bad signals and the ground station alleged the aircraft 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 the system was being interfered with, but believed that it suffered several 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, completely lost faith in electronic navigation aids as the British had predicted, and did not deploy any further system against Great Britain, although by this time Hitler's attention was turning towards Eastern Europe.
See also
*
Belfast Blitz
The Belfast Blitz consisted of four German air raids on strategic targets in the city of Belfast in Northern Ireland, in April and May 1941 during World War II, causing high casualties. The first was on the night of 78 April 1941, a small atta ...
*
Chain Home
*
GEE, the early war RAF navigation system for night bombing
*
Kammhuber Line
*
List of World War II electronic warfare equipment
*
Oboe
The oboe ( ) is a type of double-reed woodwind instrument. Oboes are usually made of wood, but may also be made of synthetic materials, such as plastic, resin, or hybrid composites.
The most common type of oboe, the soprano oboe pitched in C, ...
, the December 1941-launched successor to GEE for RAF night bomber raids.
Notes
Footnotes
References
*
*
*
*
* (Expanded from Episode 1 of
The Secret War (TV series) BBC 1977)
*
*
*
External links
Site (in German) listing locations of radio beam transmitters as well as radar early-warning stations (interesting photos and extensive list of references)
{{Use dmy dates, date=August 2019
Alexandra Palace
the Beams
Electronic countermeasures
Electronic warfare
History of telecommunications in Germany
Military history of the United Kingdom during World War II
Radio navigation
Science and technology during World War II
The Blitz
the Beams
the Beams