Chain Home, or CH for short, was the codename for the ring of coastal
Early Warning radar stations built by the
Royal Air Force
Royal Air Force (RAF) before
and during the Second World War to detect and track aircraft. The
term also referred to the radar equipment itself, until it was given
the official name
Air Ministry Experimental Station Type 1 (AMES Type
1) in 1940.
Chain Home was the first early warning radar network in
the world, and the first military radar system to reach operational
status. Its effect on the outcome of the war made it one of the
most powerful weapons of what is today known as the "Wizard
With the idea of a death ray common in popular culture and reports
suggesting Germany had built some sort of radio-based weapon,
including images of a large radio tower in German newspapers, the
Tizard Committee was formed to consider the possibility, and asked
Robert Watson-Watt to comment. His assistant, Arnold Wilkins,
demonstrated that a death ray was impossible but suggested radio could
be better used for long-range detection. In February 1935, a
demonstration was arranged by placing a receiver near a
transmitter and flying an aircraft around the area; an oscilloscope
connected to the receiver showed a pattern from the aircraft's
reflection. Watt's team quickly built a prototype system of a pulsed
transmitter based on commercial shortwave radio hardware, and on 17
June 1935 it successfully measured the angle and range of an aircraft
that happened to be flying by. Basic development was completed by the
end of the year, with detection ranges on the order of 100 miles
(160 km). Through 1936 attention was focused on a production
version, and early 1937 saw the addition of height finding.
The first five stations, covering the approaches to London, were
installed by 1937 and began full-time operation in 1938. Operational
tests using early units in 1937 demonstrated the difficulties in
relaying useful information to the pilots in fighter aircraft. This
led to the formation of the first integrated ground-controlled
interception network, the Dowding system, which collected and filtered
this information into a single view of the airspace.[a] Dozens of CH
stations covering the majority of the eastern and southern coasts of
the UK, along with a complete ground network with thousands of miles
of private telephone lines, were ready by the time the war began in
Chain Home proved decisive during the
Battle of Britain
Battle of Britain in 1940;
CH systems could detect enemy aircraft while they were still forming
up over France, giving RAF commanders ample time to marshal their
entire force directly in the path of the raid. This had the effect of
multiplying the effectiveness of the RAF to the point that it was as
if they had three times as many fighters, allowing them to defeat the
larger German force. With such high efficiency, it was no longer the
case that "the bomber will always get through".
Chain Home network was continually expanded, with over forty
stations operational by the war's end. CH was not able to detect
aircraft at low altitude, and from 1939 was normally partnered with
Chain Home Low
Chain Home Low system, or AMES Type 2, which could detect aircraft
flying at any altitude over 500 ft (150 m). Ports were
Chain Home Extra Low, AMES Type 14, which gave cover down
to 50 ft (15 m) but at shorter ranges of approximately 30
miles (50 km). In 1942 the AMES Type 7 radar began to take over
the job of tracking of targets once detected, and CH was used entirely
in the early warning role. Late in the war, when the threat of
Luftwaffe bombing had ended, the CH systems were used to detect V2
missile launches. After the war, they were reactivated as part of the
ROTOR system to watch for Soviet bombers, before being replaced by
newer systems in the 1950s. Today only a few of the original sites
remain intact in any fashion.
1.1 Prior experiments
Radio research in the UK
1.3 Detection of aircraft
1.4 "The bomber will always get through"
1.5 Tales of a "death ray"
1.6 Tizard commission
1.7 "Less unpromising"
1.9 Experimental system
1.10 Planning the chain
1.11 Into production
1.13 Battle of Britain
1.15 Big Ben
1.17 CH today
2.1 Mechanical layout
2.2 Transmitter details
2.3 Receiver details
2.4 Distance and bearing measurement
2.5 Altitude measurement
2.6 Raid assessment
2.7 Fruit machine
3 Detection, jamming and counter-jamming
3.1 Early detection
3.2 Anti-jamming technologies
3.3 First attempts, halting followup
3.4 Spoofing jammers, jitter
3.5 Klein Heidelberg
4 Comparison to other systems
Chain Home sites
6 See also
9 Further reading
10 External links
From the earliest days of radio technology, signals had been used for
navigation using the radio direction finding (RDF) technique. RDF can
determine the bearing to a radio transmitter, and several such
measurements can be combined to produce a radio fix, allowing the
receiver's position to be calculated. Given some basic changes to
the broadcast signal, it was possible for the receiver to determine
its location using a single station. The UK pioneered one such service
in the form of the
Through the early period of radio development it was also widely known
that certain materials, especially metal, reflected radio signals.
This led to the possibility of determining the location of objects by
broadcasting a signal and then measuring the bearing of any
reflections using existing RDF equipment. Such a system saw patents
issued to Germany's
Christian Hülsmeyer in 1904, and widespread
experimentation with the basic concept was carried out from then on.
These systems revealed only the bearing to the target, not the range,
and due to the low power of radio equipment of that era, were useful
only for short-range warning in fog or bad weather.
The use of radio detection specifically against aircraft was first
considered in the early 1930s. Teams in the UK, US, Japan,
Germany and others had all considered this concept and put at
least some small amount of effort into developing it. Lacking ranging
information, such systems remained of limited use in practical
Radio research in the UK
Watt's position with the National Physical Laboratory placed him at
the center of a network of researchers whose knowledge of radio
physics was instrumental to the rapid development of radar.
In the UK, Robert Watt[b] had developed the most advanced form of
basic RDF in 1922. Since 1915, Watt had been working for the Met
Office in a lab that was colocated at the National Physical
Radio Research Section (RRS) at
Ditton Park in
Slough. He became interested in using the fleeting radio signals given
off by lightning as a way to track thunderstorms. However, existing
RDF techniques were too slow to allow the direction to be determined
before the signal disappeared. He solved this by connecting a cathode
ray tube (CRT) to a directional
Adcock antenna array, originally built
by the RRS but now unused. This system, later known as huff-duff,
allowed the almost instantaneous determination of the bearing of a
Met Office began using it to produce storm warnings for
During this same period, Edward Appleton of King's College, Cambridge
was carrying out experiments that would lead to him winning the Nobel
Prize in Physics. Using a
BBC transmitter set up in 1923 in
Bournemouth and listening for its signal with a receiver at Oxford
University, he was able to use changes in wavelength to measure the
distance to a reflective layer in the atmosphere then known as the
Heaviside layer. After the initial experiments at Oxford, an NPL
Teddington was used as a source, received by Appleton
in an out-station of King's College in the East End of London. Watt
learned of these experiments and began conducting the same
measurements using his team's receivers in Slough. From then on, the
two teams interacted regularly and it was Watt that coined the term
ionosphere to describe the multiple atmospheric layers they
In 1927 the two radio labs, at the
Met Office and NPL, were combined
to form the
Radio Research Station (also RRS), run by the NPL with
Watt as the Superintendent. This provided Watt with direct contact
to the research community, as well as the chief signals officers of
the British Army,
Royal Navy and Royal Air Force. Watt became a
well-known expert in the field of radio technology. It began a
long period where Watt agitated for the NPL to take a more active role
in technology development, as opposed to its pure research role. Watt
was particularly interested in the use of radio for long-range
aircraft navigation, but the NPL management at
Teddington was not very
receptive and these proposals went nowhere.
Detection of aircraft
Arnold Frederic Wilkins
Arnold Frederic Wilkins joined Watt's staff in Slough. As the
"new boy", he was given a variety of menial tasks to complete. One of
these was to select a new shortwave receiver for ionospheric studies,
a task he undertook with total seriousness. After reading everything
available on several units, he selected a model from the General Post
Office (GPO) that worked at what was at that time very high
frequencies. As part of their tests of this system, in June 1932 the
GPO published a report, No. 232 Interference by Aeroplanes. The report
recounted the GPO testing team's observation that aircraft flying near
the receiver caused the signal to change its intensity, an annoying
effect known as fading.
The stage was now set for the development of radar in the UK. Using
Wilkins' knowledge that shortwave signals bounced off aircraft, a BBC
transmitter to light up the sky as in Appleton's experiment, and
Watt's RDF technique to measure angles, a complete radar could be
built. However, while such a system could determine the angle to a
target, it could not determine its range and thereby produce a single
location in space. To locate the target, two such measurements would
need to be made and the location calculated using triangulation, a
time-consuming process that would be subject to any inaccuracies in
either measurement or differences in calibration between the two
The missing technique that made radar practical was the use of pulses
to measure range by measuring the time between the transmission of the
signal and its reception. This would allow a single station to measure
angle and range at the same time. In 1924, two researchers at the
Naval Research Laboratory
Naval Research Laboratory in the United States,
Merle Tuve and Gregory
Briet, decided to recreate Appleton's now-famous experiment using
timed pulsed signals instead of the changing wavelengths. The
application to a detection system was not lost on those working in the
field, and such a system had been prototyped by
W. A. S. Butement
W. A. S. Butement and
P. E. Pollard of the British Signals Experimental Establishment (SEE)
in 1931. The
War Office proved uninterested in the
concept and the development remained little known outside SEE.
"The bomber will always get through"
Stanley Baldwin's 1932 comments on future aerial warfare led to a
"feeling of defencelessness and dismay". it was the UK's concern about
this issue that led to so much support being given to radar
development while other countries had a much more lackadaisical
approach until the war started.
At the same time, the need for such a system was becoming increasingly
pressing. In 1932,
Winston Churchill and his friend, confidant and
scientific advisor Frederick Lindemann travelled by car in Europe,
where they saw the rapidly rebuilding of the German aircraft
industry. It was in November of that year that Stanley Baldwin
gave his famous speech, stating that "The bomber will always get
In the early summer of 1934, the RAF carried out large-scale exercises
with up to 350 aircraft. The forces were split, with bombers
attempting to attack London and fighters, guided by the Observer
Corps, attempting to stop them. The results were dismal. In most cases
the vast majority of the bombers reached their target without ever
seeing a fighter. To address this, the RAF gave increasingly accurate
information to the defenders, eventually telling the observers where
and when the attacks would be taking place. Even then, 70% of the
bombers reached their targets unhindered.
The numbers that reached London suggested any targets in the city
would be completely destroyed. Squadron Leader P. R. Burchall summed
up the results by noting that "a feeling of defencelessness and
dismay, or at all events of uneasiness, has seized the public." In
November of that year, Churchill gave a speech on "The threat of Nazi
Germany" in which he stated the
Royal Navy could not protect Britain
from an enemy who attacked by air.
Through the early 1930s, a debate raged within British military and
political circles about strategic air power. Baldwin's famous speech
led many to believe the only way to prevent bombing of British cities
was to make a strategic bomber force so large it could, as Baldwin put
it, "kill more women and children more quickly than the enemy."
Even the highest levels of the RAF came to agree with this policy,
publicly stating that their tests suggested that "'The best form of
defence is attack' may be all-too-familiar platitudes, but they
illustrate the only sound method of defending this country from air
invasion. It is attack that counts." As it became clear the
Germans were rapidly rearming the Luftwaffe, the fear grew RAF could
not meet the objective of winning such a tit-for-tat exchange and many
suggested they invest in a massive bomber building exercise.
Others felt advances in fighters meant the bomber was increasingly
vulnerable and suggested at least exploring a defensive approach.
Among the latter group was Lindemann, test pilot and scientist, who
The Times in August 1934 that "To adopt a defeatist attitude
in the face of such a threat is inexcusable until it has definitely
been shown that all the resources of science and invention have been
Tales of a "death ray"
What may be a movie recreation of the Grindell-Matthews death ray
The idea of a death ray had been around for a while, mainly as a plot
device inB-movies. In 1923–24 an inventor named Harry Grindell
Matthews repeatedly claimed to have built a device that projected
destructive power over long ranges and attempt to sell it to the
British War Office, but it was deemed to be fraudulent. The RAF
offered a prize to anyone who could demonstrate a working model that
could kill a sheep at 100 yards; it went unclaimed.
Around the same time, a series of stories suggested another radio
weapon was being developed in Germany. The stories varied, with one
common thread being a death ray, and another that used the signals to
interfere with an engine's ignition system to cause the engine to
stall. One commonly repeated story involved an English couple who were
driving in the
Black Forest on holiday and had their car fail in the
countryside. They claimed they were approached by soldiers who told
them to wait while they conducted a test, and were then able to start
their engine without trouble when the test was complete. This was
followed shortly thereafter by a story in a German newspaper with an
image of a large radio antenna that had been installed on Feldberg in
the same area.
Although highly skeptical of these stories, the Air Ministry could not
ignore them as they were theoretically possible. If such a system
could be built, it might render bombers useless. If this were to
happen, the night bomber deterrent might evaporate overnight, leaving
the UK open to attack by Germany's ever-growing air fleet. Conversely,
if the UK had such a device, the population could be protected.
This prompted Harry Wimperis[c] to press for the formation of a study
group to consider the concept of a death ray, and more broadly, the
entire issue of air defence. Lord Londonderry, then Secretary of State
for Air, approved the formation of the Committee for the Scientific
Survey of Air Defence in November 1934, asking
Henry Tizard to chair
the group, which thus became better known to history as the Tizard
When Wimperis sought an expert in radio to help judge the death-ray
concept, he was naturally directed to Watt. He wrote to Watt "on the
practicability of proposals of the type colloquially called 'death
ray'". The two met on 18 January 1935, and Watt promised to
look into the matter. Watt turned to Wilkins for help but wanted to
keep the underlying question a secret. He asked Wilkins to calculate
what sort of radio energy would be needed to raise the temperature of
8 imperial pints (4.5 l) of water at a distance of 5 kilometres
(3.1 mi) from 98 to 105 °F (37 to 41 °C). Wilkins
immediately surmised this was a question about a death ray, to Watt's
bemusement. Wilkins made a number of back-of-the-envelope
calculations demonstrating the amount of energy needed would be
impossible given the state of the art in electronics.
According to R. V. Jones, when Wilkins reported the negative results,
Watt asked, "Well then, if the death ray is not possible, how can we
help them?" Wilkins recalled the earlier report from the GPO, and
noted that the wingspan of a contemporary's bomber aircraft, about
25 m (82 ft), would make them just right to form a
half-wavelength dipole antenna for signals in the range of 50 m
(160 ft) wavelength, or about 6 MHz. In theory, this would
efficiently reflect the signal and could be picked up by a receiver to
give an early indication of approaching aircraft.
Arnold Wilkins carried out most of the theoretical and practical work
that proved radar could work.
Watt wrote back to the committee saying the death ray was extremely
unlikely, but adding:
Attention is being turned to the still difficult, but less
unpromising, problem of radio detection and numerical considerations
on the method of detection by reflected radio waves will be submitted
The letter was discussed at the first official meeting of the Tizard
Committee on 28 January 1935. The utility of the concept was evident
to all attending, but the question remained whether it was actually
possible. Albert Rowe and Wimperis both checked the maths and it
appeared to be correct. They immediately wrote back asking for a more
detailed consideration. Watt and Wilkins followed up with a 14
February secret memo entitled Detection and Location of
Radio Means. In the new memo, Watson-Watt and Wilkins first
considered various natural emanations from the aircraft - light, heat
and radio waves from the engine ignition system - and demonstrated
that these were too easy for the enemy to mask to a level that would
be undetectable at reasonable ranges. They concluded that radio waves
from their own transmitter would be needed.
Wilkins gave specific calculations for the expected reflectivity of an
aircraft; the received signal would be only 10−19 times as strong as
the transmitted one, but such sensitivity was considered to be within
the state of the art. To reach this goal, a further improvement in
receiver sensitivity of two times was assumed. Their ionospheric
systems broadcast only about 1 kW, but commercial shortwave
systems were available with 15 amp transmitters (about 10 kW)
that they calculated would produce a signal detectable at about 10
miles (16 km). They went on to suggest that the output power
could be increased as much as ten times if the system operated in
pulses instead of continuously, and that such a system would have the
advantage of allowing range to the targets to be determined by
measuring the time delay between transmission and reception on an
oscilloscope. The rest of the required performance would be made
up by increasing the gain of the antennas by making them very tall,
focusing the signal vertically. The memo concluded with an outline
for a complete station using these techniques. The design was almost
identical to the CH stations that went into service.
This Morris Commercial T-type van, originally used as a portable radio
reception testbed, was later refitted for the
Daventry Experiment. It
is shown in 1933, being operated by "Jock" Herd.
The letter was seized on by the Committee, who immediately released
£4,000 to begin development.[d] They petitioned Hugh Dowding, the Air
Member for Supply and Research, to ask the Treasury for another
£10,000. Dowding was extremely impressed with the concept, but
demanded a practical demonstration before further funding was
Wilkins suggested using the new 10 kW, 49.8 m
Hill shortwave station in
Daventry as a suitable ad hoc transmitter.
The receiver and an oscilloscope were placed in a delivery van the RRS
had previously used for measuring radio reception around the
countryside. On 26 February 1935,[e] they parked the van in a field
Upper Stowe and connected it to wire antennas stretched across
the field on top of wooden poles. A
Handley Page Heyford
Handley Page Heyford made four
passes over the area, producing clearly notable effects on the CRT
display on three of the passes.
Observing the test were Watt, Wilkins, and several other members of
the RRS team, along with Rowe representing the Tizard Committee. Watt
was so impressed he later claimed to have exclaimed:
Britain has become an island again!
Rowe and Dowding were equally impressed. It was at this point Watt's
previous agitation over development became important; NPL management
remained uninterested in practical development of the concept, and was
happy to allow the Air Ministry to take over the team. Days later,
the Treasury released £12,300 for further development, and a
small team of the RRS researchers were sworn to secrecy and began
developing the concept. A system was to be built at the RRS
station, and then moved to
Orfordness for over-water testing. Wilikins
would develop the receiver based on the GPO units, along with suitable
antenna systems. But this left the problem of developing a suitable
pulsed transmitter. An engineer familiar with these concepts was
Edward George Bowen joined the team after responding to a newspaper
advertisement looking for a radio expert. Bowen had previously worked
on ionosphere studies under Appleton, and was thus well acquainted
with the basic concepts. He had also used the RRS' RDF systems at
Appleton's request and was known to the RRS staff. After a breezy
interview, Watson-Watt and Jock Herd stated the job was his if he
could sing the Welsh national anthem. He agreed, but only if they
would sing the Scottish one in return. They declined, and gave him the
Starting with the
BBC transmitter electronics, but using a new
transmitter tube from the Navy, Bowen produced a system that
transmitted a 25 kW signal at 6
MHz (50 metre
wavelength), sending out 25 μs long pulses 25 times a
second. Meanwhile, Wilikins and L.H. Bainbridge-Bell built a
receiver based on electronics from
Ferranti and one of the RRS CRTs.
They decided not to assemble the system at the RRS for secrecy
reasons. The team, now consisting of three scientific officers and six
assistants, began moving the equipment to
Orfordness on 13 May 1935.
The receiver and transmitter were set up in old huts left over from
World War I
World War I artillery experiments, the transmitter antenna was a
single dipole strung horizontally between two 75 foot (23 m)
poles, and the receiver a similar arrangement of two crossed
The system showed little success against aircraft, although echoes
from the ionosphere as far as 1,000 miles away were noted. The
group released several reports on these effects as a cover story,
claiming that their ionospheric studies had been interfering with the
other experiments at the RRS at Slough, and expressing their gratitude
that the Air Ministry had granted them access to unused land at
Orfordness to continue their efforts. Bowen continued increasing
the voltage in the transmitter, starting with the 5000 Volt
maximum suggested by the Navy, but increasing in steps over several
months to 12,000 V, which produced pulses of 200 kW.
Arcing between the tubes required the transmitter to be rebuilt with
more room between them, while arcing on the antenna was solved by
hanging copper balls from the dipole.
By June the system was working well, although Bainbridge-Bell proved
to be so skeptical of success that Watt eventually returned him to the
RRS and replaced him with Nick Carter. The Tizard Committee
visited the site on 15 June to examine they team's progress. Watt
secretly arranged for a
Vickers Valentia to fly nearby, and years
later insisted that he saw the echoes on the display, but no one else
recalls seeing these.
Watt decided not to return to the RRS with the rest of the Tizard
group and stayed with the team for another day. With no changes
made to the equipment, on 17 June the system was turned on and
immediately provided returns from an object at 17 mi
(27 km). After tracking it for some time, they watched it fly off
to the south and disappear. Watt phoned the nearby Seaplane
Experimental Station at
Felixstowe and the superintendent stated that
Supermarine Scapa flying boat had just landed. Watt requested the
aircraft return to make more passes. This event is considered the
official birth date of radar in the UK.
RAF Martlesham Heath
RAF Martlesham Heath took over the job of providing
targets for the system, and the range was continually pushed out.
During a 24 July test, the receiver detected a target at 40 mi
(64 km) and the signal was strong enough that they could
determine the target was actually three aircraft in close formation.
By September the range was consistently 40 miles, increasing to
80 miles (130 km) by the end of the year, and with the power
improvements Bowen worked into the transmitter, was over 100 mi
(160 km) by early 1936.
Planning the chain
Watson-Watt suggested using
Bawdsey Manor as a development site after
noticing it on a Sunday drive while working at Orfordness.
In August 1935, Albert Percival Rowe, secretary of the Tizard
Committee, coined the term "
Radio Direction and Finding" (RDF),
deliberately choosing a name that could be confused with "Radio
Direction Finding", a term already in widespread use.
In a 9 September 1935 memo, Watson-Watt outlined the progress to date.
At that time the range was about 40 mi (64 km), so
Watson-Watt suggested building a complete network of stations
20 mi (32 km) apart along the entire east coast. Since the
transmitters and receivers were separate, to save development costs he
suggested placing a transmitter at every other station. The
transmitter signal could be used by a receiver at that site as well as
the ones on each side of it. This was quickly rendered moot by the
rapid increases in range. When the Committee next visited the site in
October, the range was up to 80 miles (130 km), and Wilkins was
working on a method for height finding using multiple antennas.
In spite of its ad hoc nature and short development time of less than
six months, the
Orfordness system had already become a useful and
practical system. In comparison, the acoustic mirror systems that had
been in development for a decade was still limited to only 5 mi
(8.0 km) range under most conditions, and were very difficult to
use in practice. Work on mirror systems ended, and on 19 December
1935, a £60,000 contract[f] for five[g] stations along the south-east
coast was sent out, to be operational by August 1936.
The only person not convinced of the utility of RDF was Lindemann. He
had been placed on the Committee by the personal insistence of his
long-time friend, Churchill, and proved completely unimpressed with
the team's work. When he visited the site, he was upset by the crude
conditions, and apparently, by the box lunch he had to eat.
Lindemann strongly advocated the use of infrared systems for detection
and tracking and numerous observers have noted Lindemann's continual
interference with radar. As Bowen put it:
Within a few months of his joining the Committee, what had previously
been an innovative and forward-looking group became riven with strife.
It was strictly Lindemann versus the rest, with his hostility to radar
and his insistence on totally impractical ideas about intercepting
enemy aircraft by means of wires dangled from balloons, or by
infrared, which at that time simply did not have the sensitivity to
detect aircraft at long range.
Churchill's backing meant the other members' complaints about his
behaviour were ignored. The matter was eventually referred back to
Lord Swinton, the new Secretary of State for Air. Swinton solved the
problem by dissolving the original Committee and reforming it with
Appleton in Lindemann's place.
As the development effort grew, Watt requested a central research
station be set up, "of large size and with ground space for a
considerable number of mast and aerial systems". Several members
of the team went on scouting trips with Watt to the north of
Orfordness but found nothing suitable. Then Wilkins recalled having
come across an interesting site about 10 mi (16 km) south of
Orfordness sometime earlier while on a Sunday drive; he recalled it
because it was some 70–80 feet (21–24 m) above sea level,
which was very odd in that area. What was really useful was the large
manor house on the property, which would have ample room for
experimental labs and offices. In February and March 1936, the team
Bawdsey Manor and established the Air Ministry Experimental
Station (AMES). When the scientific team left in 1939, the site became
the operational CH site RAF Bawdsey.
While the "ness team" began moving to Bawdsey, the
remained in use. This proved useful during one demonstration when the
new system recently completed at
Bawdsey failed. The next day, Robert
Hanbury-Brown and the newly arrived Gerald Touch started up the
Orfordness system and were able to run the demonstrations from there.
Orfordness site was not completely shut down until 1937.
The first workable radar unit constructed by
Robert Watson-Watt and
his team. The four widely separated NT46 tubes can be seen. Production
units were largely identical.
The system was deliberately developed using existing commercially
available technology to speed introduction. The development team
could not afford the time to develop and debug new technology. Watt, a
pragmatic engineer, believed "third-best" would do if "second-best"
would not be available in time and "best" never available at all.
This led to the use of the 50 m wavelength, which Wilkins
suggested would resonate in a bomber's wings and improve the signal.
However, this also meant that the system was increasingly blanketed by
noise as new commercial broadcasts began taking up this formerly
high-frequency spectrum. The team responded by reducing their own
wavelength to 26 m to get clear spectrum. To everyone's delight,
contrary to Wilkins' predictions, the shorter wavelength produced no
loss of performance. This led to a further reduction to 13 m,
and finally the ability to tune between 10 and 13 m, in order to
provide some frequency agility to help avoid jamming.
Wilkins' method of height-finding was added in 1937. He had originally
developed this system as a way to measure the vertical angle of
transatlantic broadcasts while working at the RRS. The system
consisted of several parallel dipoles separated vertically on the
receiver masts. Normally the RDF goniometer was connected to two
crossed dipoles at the same height and used to determine the bearing
to a target return. For height finding, the operator instead connected
two antennas at different heights and carried out the same basic
operation to determine the vertical angle. Because the transmitter
antenna was deliberately focused vertically to improve gain, a single
pair of such antennas would only cover a thin vertical angle. A series
of such antennas was used, each pair with a different center angle,
providing continual coverage from about 2.5 degrees over the horizon
to as much as 40 degrees above it. With this addition, the final
remaining piece of Watt's original memo was accomplished and the
system was ready to go into production.
Industry partners were canvassed in early 1937, and a production
network was organized covering many companies. Metropolitan-Vickers
took over design and production of the transmitters,
AC Cossor did the
same for the receivers, the
Radio Transmission Equipment Company
worked on the goniometers, and the antennas were designed by a joint
AMES-GPO group. The Treasury gave approval for full-scale deployment
in August, and the first production contracts were sent out for 20
sets in November, at a total cost of £380,000. Installation of 15
of these sets were carried out in 1937 and 1938. In June 1938 a London
headquarters was set up to organize the rapidly growing force. This
became the Directorate of Communications Development (DCD), with Watt
named as the director. Wilkins followed him to the DCD, and A. P. Rowe
took over AMES at Bawdsey. In August 1938, the first five stations
were declared operational and entered service during the Munich
crisis, starting full-time operation in September.
During the summer of 1936, experiments were carried out at RAF Biggin
Hill to examine what effect the presence of radar would have on an air
battle. Assuming RDF would provide them 15 minutes warning, they
developed interception techniques putting fighters in front of the
bombers with increasing efficiency. They found the main problems were
finding their own aircraft's location, and ensuring the fighters were
at the right altitude.
In a similar test against the operational unit at
Bawdsey in 1937, the
results were comical. As Dowding watched the ground controllers
scramble to direct their fighters, he could hear the bombers passing
overhead. He identified the problem not as a technological one, but in
the reporting; the pilots were being sent too many reports, often
contradictory. This realization led to the development of the Dowding
system, a vast network of telephone lines reporting to a central
filter room in London where the reports from the radar stations were
collected and collated, and fed back to the pilots in a clear format.
The system as a whole was enormously manpower intensive.
By the outbreak of war in September 1939, there were 21 operational
Chain Home stations. After the
Battle of France
Battle of France in 1940 the network
was expanded to cover the west coast and Northern Ireland. The Chain
continued to be expanded throughout the war, and by 1940 it stretched
Orkney in the north to Weymouth in the south. This provided radar
coverage for the entire Europe-facing side of the British Isles, able
to detect high-flying targets well over France. Calibration of the
system was carried out initially using a flight of mostly
civilian-flown, impressed Avro Rota autogyros flying over a known
landmark, the radar then being calibrated so that the position of a
target relative to the ground could be read off the CRT. The Rota was
used because of its ability to maintain a relatively stationary
position over the ground, the pilots learning to fly in small circles
while remaining at a constant ground position, despite a headwind.
The rapid expansion of the CH network necessitated more technical and
operational personnel than the UK could provide, and in 1940, a formal
request was made by the
British High Commission, Ottawa
British High Commission, Ottawa of the
Canadian Government, appealing for men skilled in radio technology for
the service of the defence of Great Britain. By the end of 1941, 1,292
trained personnel had enlisted and most were rushed to England to
serve as radar mechanics.
Battle of Britain
During the battle,
Chain Home stations — most notably the one at
Isle of Wight
Isle of Wight — were attacked several times between 12 and
18 August 1940. On one occasion a section of the radar chain in Kent,
including the Dover CH, was put out of action by a lucky hit on the
power grid. However, though the wooden huts housing the radar
equipment were damaged, the towers survived owing to their open steel
girder construction. Because the towers survived intact and the
signals were soon restored, the
Luftwaffe concluded the stations were
too difficult to damage by bombing and left them alone for the
remainder of the war. Had the
Luftwaffe realised just how essential
the radar stations were to British air defences, it is likely that
they would have expended great effort to destroy them.
Chain Home was the primary radar system for the UK for only a short
time. By 1942, many of its duties had been taken over by the far more
advanced AMES Type 7 GCI radar systems. Whereas CH scanned an area
perhaps 100 degrees wide and required considerable effort to take
measurements, the Type 7 scanned the entire 360 degree area around the
station, and presented it on a plan position indicator, essentially a
real-time two-dimensional map of the airspace around the station. Both
fighters and bombers appeared on the display, and could be
Identification friend or foe
Identification friend or foe (IFF) signals. The
data from this display could be read directly to the intercepting
pilots, without the need for additional operators or control centres.
With the deployment of GCI, CH became the early warning portion of the
radar network. To further simplify operations and reduce manpower
requirements, the job of plotting the targets became semi-automated.
An analogue computer of some complexity, known simply as "The Fruit
Machine", was fed the bearing and range directly from the operator
console, reading the goniometer setting directly, and the range from
the setting of a dial that moved a mechanical pointer along the screen
until it lay over a selected target. When a button was pushed, the
Fruit Machine read the inputs and calculated the X and Y location of
the target, which a single operator could then plot on a map, or relay
directly over the telephone.
The original transmitters were constantly upgraded, first from
100 kW of the
Orfordness system to 350 kW for the deployed
system, and then again to 750 kW to offer greatly increased
range. To aid in detection at long range, a slower 12.5 pulse per
second rate was added. The four-tower transmitter was later reduced to
Heavily camouflaged and highly mobile, attempts to attack the V-2 were
unsuccessful. CH did help provide some early warning, the best
solution to be had.
The arrival of the
V-2 rocket in September 1944 was initially met with
no potential response. The missiles flew too high and too fast to be
detected during their approach, leaving no time even for an air raid
warning to be sounded. Their supersonic speed meant that the
explosions occurred without warning before the sound of their approach
reached the target. The government initially tried to pass them off as
explosions in the underground gas mains. However, it was clear this
was not the case, and eventually, examples of the V-2 falling in its
final plunge were captured on film.
In response, several CH stations were re-organized into the "Big Ben"
system to report the V-2s during launch. No attempt was made to try to
find the location of the launch; the radio-goniometer was simply too
slow to use. Instead, each of the stations in the network, Bawdsey,
Gt. Bromley, High St, Dunkirk and Swingate (Dover) were left set to
their maximum range settings and in the altitude measuring mode. In
this mode, the radar had several stacked lobes where they were
sensitive to signals. As the missile ascended it would pass through
these lobes in turn, causing a series of blips to fade in and out over
time. The stations attempted to rapidly measure these ranges and
forward them by telephone to a central plotting station.
At the station, these range measurements were plotted as arcs on a
chart, known as range cuts. The intersections of the arcs defined the
approximate area of the launcher. Since the missile approached the
target as it climbed, each of these intersections would be closer to
the target. Taking several of these, in turn, the trajectory of the
missile could be determined to some degree of accuracy, and air raid
warnings sent to likely areas.
Success in this task was aided by the missile fuselage profile, which
acted as an excellent quarter-wave reflector for 12 M band HF
RAF Fighter Command
RAF Fighter Command was also informed of the launch in an
effort to attack the sites. However, the German launch convoys were
motorized, well camouflaged and highly mobile, making them extremely
difficult to find and attack. The only known claim was made by
Supermarine Spitfire pilots of
No. 602 Squadron RAF
No. 602 Squadron RAF squadron came
across a V2 rising from a wooded area, allowing a quick shot of
The British radar defences were rapidly run down during the last years
of the war, with many sites closed and others placed on "care and
maintenance". However, immediate postwar tensions with the Soviet
Union resulted in recommissioning of some wartime radars as a stopgap
measure. Specific radars were remanufactured to peacetime standards of
quality and reliability, which gave significant increases in range and
accuracy. These rebuilt systems were the first phase of Chain Home's
replacement system, ROTOR, which progressed through three phases from
1949 to 1958. The very last
Chain Home Type 1 systems were retired
in 1955 along with the wholesale demolition of most of the steel and
Some of the steel transmitter towers still remain, although the wooden
receiver towers have all been demolished. The remaining towers have
various new uses and in some cases are now protected as a Listed
building by order of English Heritage. One such 360-foot-high
(110 m) transmitter tower can now be found at the BAE Systems
Great Baddow in Essex. It originally stood at RAF Canewdon
in Essex. This is the only surviving
Chain Home tower still in its
original, unmodified form with cantilever platforms at 50 ft,
200 ft & 360 ft.
Swingate transmitting station
Swingate transmitting station in Kent
(originally AMES 04 Dover) has 2 original towers (3 up until 2010)
which are used for microwave relay although the towers lost their
platforms in the 1970s.
RAF Stenigot (picture below) in Lincolnshire
has another, almost complete tower, less its top platforms and used
for training aerial erectors.
The only original
Chain Home site which is still used as a military
radar station is
RAF Staxton Wold
RAF Staxton Wold in Yorkshire although there are no
remains on site of the original 1937 equipment as it was completely
cleared and remodelled for the Rotor replacement: Linesman/Mediator
system in 1964.
The 240-foot timber receiver towers were some of the tallest wooden
structures ever built in Britain. Two of these wooden towers were
still standing in 1955, at Hayscastle Cross. Unlike the
transmitter tower pictured here, those at Hayscastle Cross were guyed.
The wooden reception towers at
Stoke Holy Cross
Stoke Holy Cross were demolished in
Wilkins would later repeat the
Daventry Experiment for the 1977 BBC
Television series The Secret War episode; "To See For a Hundred
Three of the four transmitter towers of the
Bawdsey CH station as seen
in 1945. The antennas proper are just visible at the extreme right.
These towers, as all of Chain Home, were built by J. L. Eve
Chain Home radar installations were normally composed of two sites.
One compound contained the transmitter towers with associated
structures, and a second compound, normally within a few hundred
metres distance, contained the receiver masts and receiver equipment
block where the operators (principally WAAF, Women's Auxiliary Air
Force) worked. The CH system was, by modern terminology, a
"bistatic radar", although modern examples normally have their
transmitters and receivers far more widely separated.
The transmitter antenna consisted of four steel towers 360 feet
(110 m) tall, set out in a line about 180 feet (55 m) apart.
Three large platforms were stationed on the tower, at 50, 200 and 350
feet off the ground. A 600 ohm transmission cable was suspended from
the top platform to the ground on either side of the platform (only on
the inside of the end towers). Between these vertical feed cables were
the antennas proper, eight half-wave dipoles strung between the
vertical cables and spaced ½ of a wavelength apart. They were fed
from alternating sides so the entire array of cables was in-phase,
given their ½ wavelength spacing. Located behind each dipole was a
passive reflector wire, spaced 0.18 wavelength back.
The resulting curtain array antenna produced a horizontally polarised
signal that was directed strongly forward along the perpendicular to
the line of the towers. This direction was known as the line of shoot,
and was generally aimed out over the water. The broadcast pattern
covered an area of about 100 degrees in a roughly fan-shaped area,
with a smaller side lobe to the rear, courtesy of the reflectors, and
much smaller ones to the sides. When the signal reflected off the
ground it underwent a ½ wavelength phase-change, which caused it to
interfere with the direct signal. The result was a series of
vertically-stacked lobes about 5 degrees wide from 1 degree off the
ground to the vertical. The system was later expanded by adding
another set of four additional antennas closer to the ground, wired in
a similar fashion.
The receiver consisted of an
Adcock array consisting of four 240 foot
(73 m) tall wooden towers arranged at the corners of a square.
Each tower had three sets (originally two) of receiver antennas, one
at 45, 95 and 215 feet off the ground. The mean height of the
transmitter stack was 215 feet, which is why the topmost antenna
was positioned at the same altitude in order to produce a reception
pattern that was identical to the transmission. A set of motor-driven
mechanical switches allowed the operator to select which antenna was
active. The output of the selected antenna on all four towers was sent
to a single radiogoniometer system (not Watt's own huff-duff
solution). By connecting the antennas together in X-Y pairs the
horizontal bearing could be measured, while connecting together the
upper and lower antennas allowed the same goniometer to be used to
measure the vertical angle.
Two physical layout plans were used, either 'East Coast' or 'West
Coast'. West Coast sites replaced the steel lattice towers with
simpler guy-stayed masts, although they retained the same wooden
towers for reception. East Coast sites had transmitter and receiver
blocks protected with earth mounds and blast walls, along with
separate reserve transmitter and receivers in small bunkers with
attached 120 ft aerial masts. These reserves were in close
proximity to the respective transmitter/receiver sites, often in a
neighbouring field. West Coast sites relied on site dispersal for
protection, duplicating the entire transmitter and receiver buildings.
Chain Home transmitter, RAF Air Defence
Radar Museum (2007)
Chain Home transmitting valve, London Science Museum. The valve was
capable of being dismantled and consequently had to be continuously
vacuum pumped while operating. This was done via the piping to the
Operation began with the Type T.3026 transmitter sending a pulse of
radio energy into the transmission antennas from a hut beside the
towers. Each station had two T.3026's, one active and one standby. The
signal filled space in front of the antenna, floodlighting the entire
area. Due to the transmission effects of the multiple stacked
antennas, the signal was most strong directly along the line of shoot,
and dwindled on either side. An area about 50 degrees to either
side of the line was filled with enough energy to make detection
The Type T.3026 transmitter was provided by Metropolitan-Vickers,
based on a design used for a
BBC transmitter at Rugby. A unique
feature of the design was the "demountable" tubes, which could be
opened for service, and had to be connected to a oil diffusion vacuum
pump for continual evacuation while in use. The tubes were able to
operate at one of four selected frequencies between 20 and
55 MHz, and switched from one to another in 15 seconds. To
produce the short pulses of signal, the transmitter consisted of
Hartley oscillators feeding a pair of tetrode amplifier tubes. The
tetrodes were switched on and off by a pair of mercury vapour
thyratrons connected to a timing circuit, the output of which biased
the control and screen grids of the tetrode positively while a bias
signal kept it normally turned off.
Stations were arranged so their fan-shaped broadcast patterns slightly
overlapped to cover gaps between the stations. To ensure that the
stations did not broadcast at the same time, power from the National
Grid was used to provide a convenient phase-locked 50 Hz signal
that was available across the entire nation. Each station was equipped
with a phase-shifting transformer that allowed it to trigger at a
specific point on the Grid waveform, selecting a different point for
each station to avoid overlap. The output of the transformer was fed
to a Dippy oscillator that produced sharp pulses at 25 Hz,
phase-locked to the output from the transformer. The locking was
"soft", so short-term variations in the phase or frequency of the grid
were filtered out. The system of spacing the transmissions out in
time was known as "running rabbits".
During times of strong ionospheric reflection, especially at night, it
was possible that the receiver would see reflections from the ground
after one reflection. To address this problem, the system was later
provided with a second pulse repetition frequency at 12.5 pps,
which meant that a reflection would have to be from greater than 6,000
miles (9,700 km) before it would be seen during the next
In addition to triggering the broadcast signal, the output of the
transmitter trigger signal was also sent to the receiver hut. Here it
fed the input to a time base generator that drove the X-axis
deflection plates of the CRT display. This caused the electron beam in
the tube to start moving left-to-right at the instant that the
transmission was completed. Due to the slow decay of the pulse, some
of the transmitted signal was received on the display. This signal was
so powerful it overwhelmed any reflected signal from targets, which
meant that objects closer than about 5 miles (8.0 km) could not
be seen on the display. To reduce this period even to this point
required the receiver to be hand-tuned, selecting the decoupling
capacitors and impedance of the power supplies.
The receiver system, built by
A.C. Cossor to a TRE design, was a
multiple-stage superheterodyne. The signal from the selected antennas
on the receiver towers were fed through the radiogoniometer and then
into a three-stage amplifier, with each stage housed in a metal screen
box to avoid interference between the stages. Each stage used a Class
B amplifier arrangement of EF8s, special low noise, "aligned-grid"
pentodes.[h] The output of the initial amplifier was then sent to the
intermediate frequency mixer, which extracted a user-selectable amount
of the signal, 500, 200 or 50 kHz as selected by a switch on the
console. The first setting allowed most of the signal through, and was
used under most circumstances. The other settings were available to
block out interference, but did so by also blocking some of the signal
which reduced the overall sensitivity of the system.
The output of the mixer was sent to the Y-axis deflection plates in a
specially designed high-quality CRT. For reasons not well
explained in the literature, this was arranged to deflect the beam
downward with increasing signal.[i] When combined with the X-axis
signal from the time base generator, echoes received from distant
objects caused the display to produce blips along the display. By
measuring the centre point of the blip against a mechanical scale
along the top of the display, the range to the target could be
determined. This measurement was later aided by the addition of the
calibrator unit or strobe, which caused additional sharp blips to be
drawn every 10 miles (16 km) along the display. The markers
were fed from the same electronic signals as the time base, so it was
always properly calibrated.
Distance and bearing measurement
Chain Home display showing several target blips between 15 and 30
miles distant from the station. The marker at the top of the screen
was used to send the range to the fruit machine.
The operator display of the CH system was a complex affair. The large
knob on the left is the goniometer control with the sense button that
make the antenna more directional.
Determining the location in space of a given blip was a complex
multi-step process. First the operator would select a set of receiver
antennas using the motorized switch, feeding signals to the receiver
system. The antennas were connected together in pairs, forming two
directional antennas, sensitive primarily along the X or Y axis, Y
being the line of shoot. The operator would then "swing the gonio", or
"hunt", back and forth until the selected blip reached its minimum
deflection on this display (or maximum, at 90 degrees off). The
operator would measure the distance against the scale, and then tell
the plotter the range and bearing of the selected target. The operator
would then select a different blip on the display and repeat the
process. For targets at different altitudes, the operator might have
to try different antennas to maximize the signal.
On the receipt of a set of polar coordinates from the radar operator,
the plotter's task was to convert these to X and Y locations on a map.
They were provided with large maps of their operational area printed
on light paper so they could be stored for future reference. A
rotating straightedge with the centrepoint at the radar's location on
the map was fixed on top, so when the operator called an angle the
plotter would rotate the straightedge to that angle, look along it to
pick off the range, and plot a point. The range called from the
operator is the line-of-sight range, or slant range, not the
over-ground distance from the station. To calculate the actual
location over the ground, the altitude also had to be measured (see
below) and then calculated using simple trigonometry. A variety of
calculators and aids were used to help in this calculation step.
As the plotter worked, the targets would be updated over time, causing
a series of marks, or plots, to appear that indicated the targets'
direction of motion, or track. Track-tellers standing around the map
would then relay this information via telephone to the filter room at
RAF Bentley Priory, where a dedicated telephone operator relayed that
information to plotters on a much larger map. In this way the reports
from multiple stations were re-created into a single overall view.
Due to differences in reception patterns between stations, as well as
differences in received signals from different directions even at a
single station, the reported locations varied from the target's real
location by a varying amount. The same target as reported from two
different stations could appear in very different locations on the
filter room's plot. It was the job of the filter room to recognize
these were actually the same plot, and re-combine them into a single
track. From then on the tracks were identified by a number, which
would be used for all future communications. When first reported the
tracks were given an "X" prefix, and then "H" for Hostile or "F" for
friendly once identified.[j] This data was then sent down the
telephone network to the Group and Section headquarters where the
plots were again re-created for local control over the fighters.
The data also went sideways to other defence units such as Royal Navy,
Army anti-aircraft gun sites, and RAF barrage balloon operations.
There was also comprehensive liaison with the civil authorities,
principally Air Raid Precautions.
Plotting and reporting tracks was a manpower intensive operation. This
image shows the receiver station at RAF Bawdsey, the home of CH
development. It is commanded by Flight Officer Wright, on the phone.
The radar operator is just visible in the background, just right of
centre. She communicated with the plotter, in the foreground wearing
headphones, via intercom so the readings could be made out even under
Due to the arrangement of the receiver antennas, the sensitive area
had a number of side lobes that allowed reception at multiple vertical
angles. Typically the operator would use the upper set of antennas at
215 feet (66 m), which had the clearest view of the horizon. Due
to the half-wave interference from the ground, the main lobe from this
antenna was directed at about 2.5 degrees above the horizontal, with
its sensitive region extending from about 1 to 3 degrees. At the
ground the gain was zero, which allowed aircraft to escape detection
by flying at low altitudes. The second lobe extended from about 6 to
12 degrees, and so on. This left a distinct gap in the reception
pattern centred at about 5.2 degrees.
This reception pattern provided CH with a relatively accurate way to
estimate the altitude of the target. To do this, the motorized switch
in the receiver hut was used to disconnect the four receiver masts and
instead select the two vertically displaced antennas on one mast. When
connected to the radiogoniometer, the output on the display was now
effected by the relative signal strength of the two lobes, rather than
the relative strengths in X and Y in the horizontal plane. The
operator swung the radiogoniometer looking for the peak or minimum
reception, as before, and noted the angle.
The number reported by the operator was the line-of-sight range to the
target, or slant range, which included components of both the
horizontal distance and altitude. To convert this to the real range on
the ground, the plotter used basic trigonometry on a right angle
triangle; the slant range was the hypotenuse and the open angle was
the measurement from the radiogoniometer. The base and opposite sides
could then be calculated, revealing the distance and altitude. An
important correction was the curvature of the Earth, which became
significant at the ranges CH worked at. Once calculated, this allowed
the range to be properly plotted, revealing the grid square for the
target, which was then reported up the chain.
When the target was first detected at long range, the signal typically
did not have enough of a return in the second lobe to perform height
finding. This only became possible as the aircraft approached the
station. Eventually this problem would recur as the target centred
itself in the second lobe, and so forth. Additionally, it was not
possible to determine the difference between a signal being compared
between the first and second or second and third lobe, which caused
some ambiguity at short ranges. However, as the altitude was likely
determined long before this, this tended not to be a problem in
Unfortunately this pattern left a set of distinct angles where
reception in both lobes was very low. To address this, a second set of
receiver antennas were installed at 45 feet (14 m). When the two
lower sets of antennas were used, the pattern was shifted upward,
providing strong reception in the "gaps", at the cost of diminished
long-range reception due to the higher angles.
Another critical function of the CH operators was to estimate the
number and type of aircraft in a raid. A gross level of the overall
size could be determined by the strength of the return. But a much
more accurate determination could be made by observing the "beat" rate
of the composite echoes, the way they grew and diminished over time as
they entered into different sections of the antenna reception pattern.
To aid this, the operator could reduce the pulse length to 6
microseconds (from 20) with a push-button. This improved the range
resolution, spreading the blip out on the display at the cost of lower
Raid assessment was largely an acquired skill and continued to improve
with operator experience. In measured tests, experimenters found that
acquired skill was so great that experienced operators could often
pick out targets with returns less than the current signal-to-noise
ratio. How this was accomplished was a great mystery at the time–the
operators were spotting blips in static that were larger than the
signal. It is currently believed this is a form of stochastic
The fruit machine greatly simplified measurement and calculation,
driving the plotter directly.
Operating a CH station was a manpower-intensive situation, with an
operator in the transmitter hut, an operator and assistant in the
receiver hut, and as many as six assistants in the receiver hut
operating the plotters, calculators and telephone systems. In order to
provide 24-hour service, multiple crews were needed, along with a
number of service and support personnel. This was then multiplied by
the reporting hierarchy, which required similar numbers of WAAFs at
each level of the
Dowding system hierarchy.
Plotting the angle of the target was a simple process of taking the
gonio reading and setting a rotating straightedge to that value. The
problem was determining where along that straightedge the target lay;
the radar measured the slant range straight-line distance to the
target, not the distance over the ground. That distance was affected
by the target's altitude, which had to be determined by taking the
somewhat time-consuming altitude measurements. Additionally, that
altitude was affected by the range, due to the curvature of the Earth,
as well as any imperfections in the local environment, which caused
the lobes to have different measurements depending on the target
As no small part of the manpower required was dedicated to calculation
and plotting, a great reduction could be made by using as much
automation as possible. This started with the use of various
mechanical aids; these were eventually replaced by the fruit machine,
an electromechanical analogue computer of some complexity. It
replicated all of these devices and tables in electrical form. An
electrical repeater, or synchro, was added to the gonio dial. To
measure the range, a new dial was added that moved a mechanical marker
to a selected blip on the display. When a particular target was
properly selected, the operator pushed a button to activate the fruit
machine, which then read these inputs. In addition to the inputs, the
fruit machine also had a series of local corrections for both angle
and altitude, as measured by calibration flights and stored in the
machine in telephone uniselectors. These corrections were
automatically added to the calculation, eliminating the time-consuming
lookup of these numbers from tables. The output was the altitude,
which then allowed the plotters to determine the proper over-ground
distance to the target.
Later versions of the fruit machine were upgraded to directly output
the position of the aircraft with no manual operation. Using the same
buttons to send settings to the machine, the operator simply triggered
the system and the outputs were used to drive a T-square-like
indicator on the chart, allowing the operator to read the calculated
location directly. This reduced the number of people needed at the
station and allowed the station to be reorganized into a much more
compact form. No longer did the operator call readings out to the
plotters; now they sat directly beside the plotting table so they
could see if the results looked right, while the tellers could see the
plot and call it into the area plotting room. A further upgrade
allowed the data to be sent to the local plotting room automatically
over the phone lines, further reducing the required manpower.
Detection, jamming and counter-jamming
From May to August 1939 the LZ130 Graf Zeppelin II made flights along
Britain's North Sea coast to investigate the 100-metre-high radio
towers that were being erected from
Portsmouth to Scapa Flow. LZ130
performed a series of radiometric tests and took photographs. German
sources report the 12 m
Chain Home signals were detected and
suspected to be radar; however, the chief investigator was not able to
prove his suspicions. Other sources are said to report different
During the Battle of France, the Germans observed 12 m pulse
signals on the western front without being able to recognize their
origin and purpose. In mid-June 1940, the Deutsche Versuchsanstalt
für Luftfahrt (DVL, German Aeronautic Research Institute) set up a
special group under the direction of Professor von Handel and found
out that the signals originated from the installations on the coast of
the English Channel.
Their suspicions were finally proven in the aftermath of the Battle of
Dunkirk, when the British were forced to abandon a mobile gun-laying
radar (GL Mk. I) station in Normandy. Wolfgang Martini's team of
specialists was able to determine the operation of the system. GL was
a rather crude system of limited effectiveness, and this led the
Germans to have a dim view of British radar systems. However, an
effective system requires more than just the radar; plotting and
reporting are equally important, and this part of the system was fully
developed in Chain Home. The German's failure to realize the value of
the system as a whole has been pointed to as one of their great
failings during the war.
The British had been aware that the Germans would determine the
purpose of the system and attempt to interfere with it, and had
designed in a variety of features and methods in order to address some
of these issues even as the first stations were being built. The most
obvious of these was CH's ability to operate on different frequencies,
which was added to allow the stations to avoid any sort of
continuous-broadcast interference on their operating frequency.
Additionally, the Interference Rejection Unit, or IFRU, allowed the
output of the intermediate stages of the amplifiers to be clipped in
an attempt to finely tune the receiver to the station's own signals
and help reject broadband signals.
More complex was a system built into the CH displays, implemented in
order to remove spurious signals from unsynchronized jamming pulses.
It consisted of two layers of phosphor in the CRT screen, a
quick-reacting layer of zinc sulphide below, and a slower "afterglow"
layer of zinc cadmium sulphide on top. During normal operation the
bright blue signal from the zinc sulphide was visible, and its signal
would activate the yellow zinc cadmium sulphide layer, causing an
"averaged" signal to be displayed in yellow. To filter out jamming
pulses, a yellow plastic sheet was placed in front of the display,
rendering the blue display invisible and revealing the dimmer yellow
averaged signal. This is the reason many radars from the War through
to the 1960s have yellow displays.
Another method was to use range-only measurements from multiple CH
stations to produce fixes on individual targets, the "Chapman method".
To aid this task, a second display would be installed that would be
fed the Y-axis signal from a distant CH station over telephone lines.
This system was never required.
First attempts, halting followup
When jamming was first attempted by the Germans it was handled in a
much more clever fashion than had been anticipated. The observation
that the transmissions of the individual stations were spread out in
time, in order to avoid mutual interference, was exploited. A
system was designed to send back spurious broadband pulses on a chosen
CH station's time slot. The CH operator could avoid this signal simply
by changing their time slot slightly, so the jamming was not received.
However, this caused the station's signals to start overlapping
another's time slot, so that station would attempt the same cure,
affecting another station in the network, and so forth.
A series of such jammers were set up in France starting in July 1940,
and soon concentrated into a single station in Calais that affected CH
for some time. However, the timing of these attempts was extremely
ill-considered. The British quickly developed operational methods to
counteract this jamming, and these had effectively eliminated the
effect of the jamming by the opening of the
Battle of Britain
Battle of Britain on 10
July. The Germans were well on their way to develop more sophisticated
jamming systems, but these were not ready for operation until
September. This meant that the CH system was able to operate
unmolested throughout the Battle, and led to its well-publicized
By the opening of the Battle in July the German
units were well aware of CH, and had been informed by the DVL that
they could not expect to remain undetected, even in clouds. However,
Luftwaffe did little to address this and treated the entire topic
with some level of disdain. Their own radars were superior to CH in
many ways, yet in actions they had proven to be only marginally
useful. During the Air Battle of the Heligoland Bight in 1939, a
Freya radar detected the raid while it was still an hour away
from its target, yet had no way to report this to any of the fighter
units that could intercept it. Getting the information from the radar
to the pilots in a useful form appeared to be a difficult problem, and
the Germans believed the British would have the same problems and thus
radar would have little real effect.
Some desultory effort was put into attacking the CH stations,
especially during the opening stages of the Battle. However, British
engineers were able to quickly return these units to service, or in
some cases simply pretend to do so in order to fool the Germans into
thinking the attacks failed. As the pattern of these attacks became
clear, the RAF began to counter them with increasing effectiveness.
Junkers Ju 87
Junkers Ju 87 dive bombers were subjected to catastrophic losses
and had to be withdrawn from battle. The Germans gave up trying to
attack CH directly on any reasonable scale.
Thus, CH was allowed to operate throughout the Battle largely
unhindered. Although communications were indeed a serious problem, it
was precisely this problem that the
Dowding system had been set up to
address, at great expense. The result was that every British fighter
was roughly twice as effective, or more, than its German counterpart.
Some raids were met with 100% of the fighters dispatched successfully
engaging their targets, while German aircraft returned home over half
the time having never seen the enemy. It is for this reason that
Chain Home with winning the Battle.
Spoofing jammers, jitter
This second jamming system was eventually activated at
Cap Gris Nez
Cap Gris Nez in
September, using a system that triggered its signal in response to the
reception of a pulse from CH. This meant that the system responded to
the CH station even if it moved its time slot. These systems, known as
Garmisch-Partenkirchen were used during
Operation Donnerkeil in 1941.
Further improvements to the basic concept allowed multiple returns to
be generated, appearing like multiple aircraft on the CH display.
Although relatively sophisticated, CH operators quickly adapted to
these new jammers by periodically changing the pulse repetition
frequency (PRF) of their station's transmitter. This caused the
synchronized jamming signals to briefly go out of synch with the
station, and the blips from the jammers would "jitter" on the screen,
allowing them to be visually distinguished. The "Intentional Jitter
Anti-Jamming Unit", IJAJ, performed this automatically and randomly,
making it impossible for the German jammers to match the changes.
Another upgrade helped reject unsynchronized pulses, supplanting the
two-layer display. This device, the "Anti-Jamming Black-Out" unit,
AJBO, fed the Y-axis signal into a delay and then into the brightness
control of the CRT. Short pulses that appeared and disappeared were
muted, disappearing from the display. Similar techniques using
acoustic delay lines, both for jamming reduction and filtering out
noise, became common on many radar units during the war.
Main article: Klein Heidelberg
The Germans also made use of CH for their own passive radar system,
known as Klein Heidelberg. This used CH's transmissions as their
source, and a series of receivers along the Channel coast. By
comparing the time of arrival of the signals from a selected aircraft,
its range and direction could be determined with some accuracy. Since
the system sent out no signals of its own, the allies were not aware
of it until they overran the stations in 1944. Most of the stations
had only just been built when they were overrun.
Comparison to other systems
Modern texts are often dismissive of Chain Home, viewing it as "dead
end technology with serious shortcomings".
In many respects, CH was a crude system, both in theory and in
comparison to other systems of the era. This is especially true when
CH is compared to its German counterpart, the Freya. Freya operated on
shorter wavelengths, in the 2.5 to 2.3 m (120 to 130 MHz)
band, allowing it to be broadcast from a much smaller antenna. This
meant that Freya did not have to use the two-part structure of CH with
a floodlight transmission, and could instead send its signal in a more
tightly focused beam like a searchlight. This greatly reduced the
amount of energy needed to be broadcast, as a much smaller volume was
being filled with the transmission. Direction finding was accomplished
simply by turning the antenna, which was small enough to make this
relatively easy to arrange. Additionally, the higher frequency of the
signal allowed higher resolution, which aided operational
effectiveness. However, Freya had a shorter maximum range of
100 mi (160 km), and could not accurately determine
It should be remembered that CH was deliberately designed specifically
to use off-the-shelf components wherever possible. Only the receiver
was truly new, the transmitter was adapted from commercial systems and
this is the primary reason the system used such a long wavelength. CH
stations were designed to operate at 20–50 MHz, the "boundary
area" between high frequency and
VHF bands at 30 MHz, although
typical operations were at 20–30
MHz (the upper end of the HF
band), or about a 12 m wavelength. The detection range was
typically 120 mi (190 km; 100 nmi), but could be
The main limitation in use was that
Chain Home was a fixed system,
non-rotational, which meant it could not see beyond its sixty-degree
transmission arc or behind it once the targets had flown overhead, and
so raid plotting over land was down to ground observers, principally
the Observer Corps (from April 1941 known as the Royal Observer
Corps). Ground-based observation was acceptable during the day but
useless at night and in conditions of reduced visibility. This problem
was reduced on introduction of more advanced surveillance radars with
360-degree tracking and height-finding capability and, more important,
aircraft fitted with Airborne Intercept radar (AI), which had been
developed in parallel with
Chain Home from 1936 onwards. This new
equipment began to appear in late 1940 fitted to Bristol Blenheim,
Bristol Beaufighter and
Boulton Paul Defiant
Boulton Paul Defiant aircraft.
Even as the CH system was being deployed, a wide variety of
experiments with newer designs was being carried out. By 1941 the Type
7 Ground Control Intercept
Radar (GCI) on a wavelength of
1.5 m was entering production, and reached widespread service in
Chain Home sites
Chain Home Map shows modern aerial photographs of the
locations of AMES Type 1 Chain Home.
Chain Home Low
Chain Home Low Map shows modern aerial photographs of the
locations of AMES Type 2
Chain Home Low.
Chain Home Extra Low Map shows modern aerial photographs of
the locations of
Chain Home Extra Low.
Radar site locations in this period are complicated due to the rapid
growth in technology 1936–45 and the changing operational
requirements. By 1945 there were 100+ radar sites in the UK. One of
the primary objectives of post war
ROTOR was to streamline and manage
an unwieldy network that grew rapidly 'as required' in the war years.
Individual sites are listed below:
Suffolk (grid reference TM336380) 
Western Isles (NA9910024250) 
Isle of Man
Isle of Man (NX4604) 
Western Isles (NB5314034470)
Castell Mawr: Near Llanrhystud, Ceredigion, AMES No. 67 (SN5369)
Isle of Man
Isle of Man (SC2178) 
Danby, North Yorkshire
Danby, North Yorkshire (NZ732097)
Suffolk (grid reference TM408718) 
Douglas Wood: Monikie, Angus (NO4862041515)
Drone Hill: Near Coldingham, Borders (NT8447066535)
Drytree: Goonhilly Downs,
Kent (TR076595) 
Greystone: County Down, Northern Ireland, AMES No. 61
Hawks Tor: Plymouth, Devon
High Street, Darsham
Kilkeel: County Down,
Northern Ireland AMES No. 78
Argyll and Bute
Argyll and Bute (NL9408045570) 
Sutherland (NC9590009600) 
Orkney (HY4621104396) 
Nefyn: Gwynedd, AMES No. 66 (SH2704037575) 
North Cairn: Near Stranraer, Dumfries, AMES No. 60 (NW97107074)
Shetland Islands (HU3613015575)
Otterburn, Northumberland (NY944896)
East Sussex (TQ644073)
West Sussex (TQ043052)
Port Mor: Tiree,
Argyll and Bute
Argyll and Bute (NL9442) 
Rhuddlan: Denbighshire, AMES No. 65 (SJ012764)
Ringstead: Ringstead Bay,
East Sussex (TQ968232)
St. Lawrence, Isle of Wight:
Isle of Wight
Isle of Wight (SZ530760) 
Saligo Bay: Islay,
Argyll and Bute
Argyll and Bute (NR2116066740)
Sutherland (NC4170067500) 
Saxmundham, Suffolk, IP17 3QD (TM411720)
Isle of Man
Isle of Man (SC2566) 
Shetland Islands (HP6634016805)
North Yorkshire (TA023778)
Louth, Lincolnshire (TF256827)
Stoke Holy Cross:
Tower: Blackpool, Lancashire, AMES No. 64 (SD306357)
Trelanvean: Goonhilly Downs,
Isle of Wight
Isle of Wight (SZ568785):
West Beckham, Norfolk
Whale Head: Sanday,
Orkney Islands (HY7590546125)
Worth Matravers: Swanage,
Wylfa: Isle of Anglesey, AMES No. 76 (SH3522093385)
History of radar
Battle of the beams
British military history of World War II
Chain Home Low
Civilian Technical Corps
RAF Air Defence
^ Older works generally refer to the entire network as
Chain Home as
well, RAF wartime materials clearly separate the radar network from
the reporting chain.
^ He added "Watson" to his name when he was knighted in 1942, and thus
was known only as Watt during the period covered in this article.
^ Bowen suggests Tizard was the original impetus for the formation of
the Committee and had approached Wimperis to back him up.
^ Some sources say £2,000.
^ This, coincidentally, was the same day Hitler officially created the
^ Bowen puts the sum at £1,000,000.
^ Gough says seven
^ Introduced in 1938, the EF8 was not technically a pentode as it had
4 grids making it a hexode. However, the purpose of the fourth grid
and the alignment of the remaining grids was to reduce the partition
noise from which pentodes generally suffer. Since the device exhibited
pentode characteristics, all literature generally describes it as a
'pentode. It is not clear whether the device was specifically
developed for the chain home system.
^ The image of the operator console on this page appears to offer the
solution; the line is not being drawn across the top of the display,
but the middle, where it is the widest and thus provides the greatest
resolution. The tube is then placed in a box with the upper section
covered, so the line on the middle of the CRT appears at the top of
the resulting opening. Of course this could also be operated upward.
^ Other codes may have been used as well, this is not intended to be
an exhaustive list.
^ Claims have been made that the LZ130 missions (1) failed to detect
any radio emissions of interest at all; (2) failed to identify the
true purpose of the new British stations, concluding the towers were
for long-range naval radio communication, not radio location; and (3)
failed to identify the origin of the signals as the towers that had
aroused the interest in the first place. It is agreed that German
scientists were not certain of British radar defences, and these
claims may reflect the debate among those scientists.
^ "The prototype CH system – 1939… Chain, Home… Operational".
Bournemouth University. 1995–2009. Retrieved 23 August 2009.
^ Neale 1985, p. 73.
^ Connor, Roger (5 June 2014). "D-Day and the Wizard War". National
Air and Space Museum.
^ Jones 1978.
^ "Federal Standard 1037C, Glossary Of Telecommunication Terms".
General Services Administration. 1996.
^ Sitterly, B.; Davidson, D. (1948). The LORAN System (PDF). McGraw
Hill. p. 4.
^ a b Bauer, Arthur (15 January 2005).
Christian Hülsmeyer and about
the early days of radar inventions (PDF). Foundation Centre for German
Communications and Related Technologies.
^ Bowen 1998, p. 6.
^ Nakajima, Shigeru (1988). "The history of Japanese radar development
to 1945". In Burns, Russell.
Radar Development to 1945. Peter
Peregrinus. pp. 245–258. ISBN 978-0863411397.
^ a b Hollmann. "
Radar Development in Germany". Radarworld.org.
Retrieved 10 February 2013.
^ a b c Bowen 1998, p. 7.
^ Watson 2009, p. 39.
^ Clark, Robert (2013). Sir Edward Appleton G.B.E., K.C.B., F.R.S.
Elsevier. pp. 39–45, 51, 53.
^ a b c d Bowen 1998, p. 9.
^ a b c Watson 2009, p. 44.
^ Clark 1997, p. 30.
^ Seitz & Einspruch 1998, p. 91.
^ Home, R.W. (2007). "Butement, William Alan (1904–1990)".
Australian Dictionary of Biography.
^ Mukerjee, Madhusree (29 September 2011). "Lord Cherwell: Churchill's
Confidence Man". Historynet.
^ Middlemas, Keith; Barnes, John (1969). Baldwin: A Biography.
Weidenfeld and Nicolson. p. 722.
^ a b c "The Air Attacks on London". The Spectator. 2 August 1934.
Winston Churchill (16 November 1934). The Threat of Nazi Germany
(Audio recording). Retrieved 19 May 2017.
^ "Mr Baldwin on Aerial Warfare – A Fear for the Future". The Times.
London, ENG, UK: 7 column B. 11 November 1932. .
^ a b Clark 1997, p. 28.
^ Clark 1997, pp. 28-29.
^ Philip Kaplan, The Few: Preparation for the Battle of Britain: Rare
Photographs from Wartime Archives, Pen and Sword - 2014, page 73
^ David Clarke, Britain's X-traordinary Files, Bloomsbury Publishing -
2014, pages 48-51
^ Heazell, Paddy (2011). Most Secret: The Hidden History of Orford
Ness. The History Press. ISBN 9780752474243. Retrieved 8 March
^ Jones 1978, p. 50.
^ Bowen 1998, p. 4.
^ Zimmerman, David (1996). Top Secret Exchange: The Tizard Mission and
the Scientific War. McGill-Queen's Press. p. 23.
^ Jones 1978, p. 19.
^ Watson 2009, pp. 44-45.
^ Austin, B.A. (1999). "Precursors To
Radar — The Watson-Watt
Memorandum And The
Daventry Experiment" (PDF). International Journal
of Electrical Engineering Education. 36: 365–372. Archived from the
original (PDF) on 25 May 2015.
^ a b c d e f Watson 2009, p. 45.
^ Jones, Reginald Victor (2009). Most Secret War. Penguin.
^ Allison, David (29 September 1981). New Eye for the Navy: The Origin
Radar at the
Naval Research Laboratory
Naval Research Laboratory (PDF) (Technical report).
Naval Research Laboratory. p. 143.
^ Bowen 1998, p. 10.
^ a b c d Watson 2009, p. 46.
^ Gough 1993, p. 2.
^ "Hitler organizes Luftwaffe". History Channel.
^ a b Gough 1993, p. 3.
^ a b c Bowen 1998, p. 8.
^ a b Watson 2009, p. 47.
^ Bowen 1998, pp. 11–13.
^ a b Watson 2009, p. 48.
^ a b Bowen 1998, p. 14.
^ Bowen 1998, p. 13.
^ Bowen 1998, p. 15.
^ a b c Bowen 1998, p. 16.
^ a b Watson 2009, p. 50.
^ a b c d e Watson 2009, p. 51.
^ a b c Bowen 1998, p. 21.
^ a b c d Bowen 1998, p. 20.
^ Watson 2009, p. 52.
^ Heazell, Paddy (2011). Most Secret: The Hidden History of Orford
Ness. The History Press. p. 280.
^ Waligorski, Martin (10 April 2010). "From Peace to War – Royal Air
Force Rearmament Programme, 1934–1940". Spitfiresite.com. Retrieved
10 February 2013.
^ a b "Longwave
Radar At War / Early American
Vectorsite.net. Retrieved 10 February 2013.
^ a b Gough 1993, p. 5.
^ Gough 1993, p. 6.
^ "Sir Henry and the 'Biggin Hill Experiment'".
Histru.bournemouth.ac.uk. Retrieved 10 February 2013.
^ Grande, George Kinnear (2000). Canadians on Radar: Royal Canadian
Air Force, 1940-45. The Canadian
Radar History Project.
^ a b Neale 1985, p. 83.
^ Dick Barrett (19 March 2002). "Chain Home". The
Retrieved 10 February 2013.
^ "The Spitfire and the hunt for the V2". The Scotsman. 14 November
ROTOR Project". TheTimeChamber. 24 January 2013. Retrieved 10
^ An aerial photograph (accessed 2009-06) shows these towers.
^  poringlandarchive.co.uk
^ a b c d e f g Neale 1985, p. 74.
^ Neale 1985, pp. 74-75.
^ Neale 1985, p. 78.
^ Neale 1985, pp. 78-79.
^ Neale 1985, p. 80.
^ a b Neale 1985, p. 79.
^ Article describing the EF8
^ Neale 1985, pp. 79-80.
^ a b c d e Neale 1985, p. 81.
^ Neale 1985, p. 75.
^ "The RAF Fighter Control System". RAF. 6 December 2012. Retrieved 10
^ a b c Neale 1985, p. 76.
^ Pritchard, p.55. Many of the German experts believed radar at
12 m wavelengths was not likely, being well behind the current
state of the art in Germany.
^ Gerhard Hepcke, "The
^ a b c "The
Radar War by Gerhard Hepcke Translated into English by
Hannah Liebmann page 8-9" (PDF). Retrieved 10 February 2013.
^ Willis, Nicholas; Griffiths, Hugh.
Klein Heidelberg – a WW2
bistatic radar system that was decades ahead of its time (Technical
^ Clark, Gregory C. (12 April 2010). "Deflating British
Radar Myths of
World War II". Spitfiresite.com. Retrieved 10 February 2013.
^ Pritchard, p.49
^ "The First Airborne Radar". R-type.org. Retrieved 10 February
^ "Starlight, Southern
Radar and RAF Sopley". Winkton.net. Retrieved
10 February 2013.
^ Dick Barrett (22 September 2003). "Type 7 air defence search radar".
Radarpages.co.uk. Retrieved 10 February 2013.
^ These show the locations of all 'Mainland' UK
Chain Home Type 1 /
Type 2 sites.
Northern Ireland had comprehensive Type 1 / Type 2 cover
but these stations are not shown on the maps.
^ "RAF Bawdsey' ('PKD') R3 GCI
Radar Station". Subterranea
Britannica. 27 April 2004. Retrieved 10 February 2013.
^ Pictures of Brenish Archived 2 November 2005 at the Wayback Machine.
^ a b c "
Isle of Man
Isle of Man
Radar Stations". Subterranea Britannica. 4
January 2011. Retrieved 10 February 2013.
^ Dunwich Museum –
Radar at Dunwich[dead link]
^ "Dunkirk". Subterranea Britannica. Retrieved 10 February 2013.
^ Pictures of Kilkenneth Archived 16 May 2006 at the Wayback Machine.
^ Pictures of Loth Archived 23 November 2005 at the Wayback Machine.
Helmsdale site Archived 20 June 2006 at the Wayback Machine.
^ "Netherbutton". Sub Brit. Retrieved 10 February 2013.
^ "Raf Netherbutton,
Radar Station" Archived 19 July 2011
at the Wayback Machine. scotlandsplaces.gov.uk. Retrieved 29 November
^ Pictures of
Nefyn Archived 6 August 2009 at the Wayback Machine.
^ "Nefyn". Homepage.ntlworld.com. Archived from the original on 18
October 2012. Retrieved 10 February 2013.
^ Pictures of Port Mor Archived 4 November 2005 at the Wayback
^ "St Lawrence". Subterranea Britannica. Retrieved 10 February
^ Pictures of Sango Archived 3 August 2009 at the Wayback Machine.
^ "Schoolhill". Subterranea Britannica. 29 June 2004. Retrieved 10
^ "Ventnor". Subterranea Britannica. Retrieved 10 February 2013.
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