Eleventh draft - Dunwich Museum

RADAR AT DUNWICH
(large photographs are available in the photo gallery)
(if you do not know what a MHz is, try the appendix first)
(Another article " People in Radar at Dunwich " deals with individuals' experiences)
Introduction
Timeline
Minsmere Cliffs:
Chain Home Low
Cliff House, Dunwich: Accommodation
Greyfriars, Dunwich : Ground Controlled Intercept : AMES Type 16
Greyfriars, Dunwich : Air Control Radar : MEW AN/CPS-1, Type 24, Type 26, Type 28
Minsmere Cliffs:
Radio Counter Measures : Ground Grocer
Radars near Dunwich : Chain Home, Mobile Radar Unit, Ground Cigar, CHEL, CDR
Summary of Radar at Dunwich
Appendix - Newcomers guide to frequencies and their common uses today
Introduction
This article is a compilation of information provided by visitors to the Dunwich Museum. In
particular the late Dr Ernest Putley who was eminent in the radar field from early days , who
died in 2010 and without whose help little headway could have been made. His original
contributions and photos are at the Dunwich Museum, the RAF Air Defence Museum and as
downloadable files on this site. Thanks are also due to Maurice Clarkson from New Zealand who
provided details of his father's wartime work on Dunwich radar, provided much detail from his
own research, clarified with photographs some points and whose contributions are available as
downloads. Thanks are due to David Sullivan for locating RAF sites in and around Dunwich.
Timeline
1904 - First radar device demonstrated in Holland for detecting approaching ships in fog
1935 - Dunwich mooted as a possible Chain Home radar site
1940 - Chain Home Lowcover : two separate aerials on 25ft timber gantries, Minsmere Cliffs
1942 - CHL single aerial atop 185ft timber tower with second on ground gantry, Minsmere Cliffs
1942 - Ames Type 16 (Ground Controlled Intercept) and Type 24 Height-finder, Greyfriars
1943 - "Ground Grocer", Radio Counter Measures jammer, Minsmere Cliffs
1943 - MEW AN/CPS-1 GCI, and Type 24 Height-finder, Greyfriars, Dunwich until June 1944
1944 - AMES Type 26 (MEW) GCI, and Type 24 and 28 Height-finders at Greyfriars, Dunwich
1945 - At war's end, Dunwich's radars were taken to Heathrow Airport and became the first
civilian air traffic control radars there
1958 - or later, the 185ft wooden CHL tower was removed from Minsmere Cliffs.
2011 - Two original wartime billets (Tower Cottages) and Cliff House are all that remain.
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Minsmere Cliffs: Chain Home Low
The first mention of Dunwich as a radar station was in July 1935. Preliminary radar experiments
at Orfordness had been so encouraging that plans were formed for a coastal chain of radar
stations. Wimperis, Air Ministry Director of Scientific Research, in a minute dated 13/8/35 gives
a list of 20 possible sites, based on map surveys, and Dunwich was one of them.
The first such radars, were called Chain Home. In the end, after a personal
inspection by Watson Watt, this CH station was built at Darsham High
Street, TM409720, about five miles inland from Dunwich. CH stations
worked but were not good at detecting low-flying aircraft.
In June 1939, the Army adapted its coast defence radars for the RAF.
These were designed to detect submarines but were also efficient at
detecting low-flying mine-laying aircraft and these became the basis for
the Chain Home Lowcover (CHL) which were often paired up with
existing CH sites. Between September 1939 and July 1940 a total of fiftyseven CH and CHL stations were completed, as shown on the right one at
Minsmere Cliffs, Dunwich.
All aircraft and shipping movements detected by the Dunwich CHL were plotted, and these plots
were sent by special telephone lines to the nearby Darsham High Street CH Radar Station. The
CH station added information detected by their equipment, and telephoned all plots to the Area
Operations Rooms at Bentley Priory in Stanmore, under the command of Lord Dowding. At
Stanmore, information of enemy movements from all sources was filtered to remove duplication
before forwarding on to the Fighter Group 11 Operations Centre at RAF Uxbridge, under
command of Air Vice Marshall Keith Park. The Operations Centre assessed what aircraft were
needed and available before contacting the appropriate Sector Aerodrome(s) to arrange for the
enemy fighters/ bombers to be intercepted. This sophisticated integrated structure was
instrumental in maintaining air superiority despite the relatively primitive radar systems.
Dunwich CHL chain became operational on the 1st January
1940. Two aerials were built about 600 yards north of the
Minsmere Coastguard cottages (Cassini coordinates
wM931861). The Dunwich pair of broadside array aerials were
each mounted on a 25ft timber gantry 35ft from the cliff edge.
The transmitter hut was 60ft above sea level and the receiver
50ft, so that with the heights of the gantries the aerials were 85
and 75 ft respectively above sea level. At right is a similar
aerial (at Prestatyn CHL)
It should be remembered that there was originally no infrastructure on the Minsmere Cliffs. The
four terraced Victorian coastguard cottages, the only building, had no electricity, water or
sewage. A CHL radar station requires personnel to operate, maintain, guard and supply it
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together with the associated control rooms, transmitter huts, accommodation, guardhouses,
generators, food, fuel, AA artillery, ammunition, electricity, water and sewage systems that
accompany them.
Note also that at this time both CH and CHL stations used separate transmitting and receiving
aerials. Switching alternatively between transmit and receive, so that only one aerial is needed,
was still being developed. Additionally two displays were needed for range and bearing. Height
was not then measured. The modern rotating aerials and map-like display (called a Plan Position
Indicator) which provide range and bearing together were still in the pipeline. All these
refinements were not ready until 1941 at the earliest, and then it took a year or two before
everyone got them.
The CHL system operated at 200Mhz and was thus less susceptible to rain effect than the CH's
20-50MHz. The CHL pulse repetition frequency was variable, of the order of 400 pulses/sec and
peak pulse power was 150kw. The original aerials were 5 bay, 4 stack arrays of end-fed
horizontal half-wave aerials placed at 1/8λ in front of a wire screen of netting, the whole unit
being rotatable at speeds between 1 - 2.3rpm. Both aerials could be 'inched', stopped or reversed.
In May 1940,the Dunwich CHL performance was discussed by the RAF and Telecommunication
Research Establishment with the RAF claiming ranges/aircraft height : 18miles/500ft,
27miles/1000ft, 35miles/2000ft and 50miles/4000ft. Surface craft
(including E boats) were detected at 18miles. The TRE maintained that
Dunwich performance was better : 30miles/500ft, 50miles/2000ft,
60miles/4000ft.
The performance of CHLs placed on relatively low seashore sites like
Dunwich fell well below what was possible when a more elevated site such
as the cliff tops of Beachy Head was available. The answer to this was to
put the aerials on a tower. Experiments putting a CHL on the 200ft gantries
of a transmitter tower at the Douglas Wood CH station, Scotland proved
this to be feasible, so a programme was started to mount on towers those
CHLs on low sites to improve detection of low-flying aircraft (100ft or
less).
An Air Ministry note dated 7/1/1942 states that Dunwich was one of several CHLs to be
provided with a 185ft timber tower. This was in place by June 1942. By this time, techniques for
switching between transmit and receive had been developed, so that only one aerial was required
and fully rotatable powered turning gear had been developed so that the aerial could be
continuously rotated. The receiver would now have been fitted with the familiar PPI display
giving a map-like presentation showing range and bearing at the same time. A second ground
aerial gave height information.The photo above right shows the type of installation.
An official TRE photographer Murray Hardy accidentally included the wooden Dunwich CHL
tower in a photograph in 1943 (see Green Grocer below). The 185ft wooden tower was built
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250 yards inland from the cliff and 600 yards north of the
Coastguard cottages; therefore roughly behind the old pair.
The new high timber tower remained until at least the OS 1958
map but does not appear on 1982 maps. The photo right shows
1950s campers on Dunwich Heath with the 185ft CHL mast
behind.
The Dunwich CHL remained in service throughout the war,
though development on the 200MHz system stopped. New types of radar came to Dunwich.
Cliff House, Dunwich, Accomodation
Accommodation for the WAAFs was at
Cliff House, as was the canteen, and the
recreation room.
Other accommodation was in huts under
the trees north of Dunwich Heath. Google
Earth's zoomable 1945 picture at right
shows Nissen huts at Greyfriars, the
coastguard cottages and on Dunwich
Heath.
The 1945 RAF aerial photo at right (RAF
106G/UK/929 4166 NMR) has been
annotated to show the two CHL
compounds and various ground features.
The two square structures are either
Ground Grocer structures or the remains
of two control rooms which once had a
concrete roof. The parallelogram is a
chain-link fence with concrete posts
around the later CHL compound. The
white edged long lines are anti-glider
ditches (which are visible in the photo
above and even in 2007 photos). At the
centre with a long shadow is the 185ft
wooden CHL tower. Near the cliff edge is
another shorter tower shadow which may
be one of the original CHL gantries. The Nissen huts are clearly visible as is the generator hut
(two Listers) in the first CHL compound near the cliffs.
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At right are some of the concrete huts for
accomodation in the forest just North of the Cliff
House complex, between Dunwich Heath and the
village. These and Cliff House are the only
structures which still remain from this period. The
cottages are now holiday lets (Tower Cottages)
Greyfriars, Dunwich : Ground Controlled
Intercept Type 16
The CHL was admirable for defence but it was too
slow for offensive operation.The answer was to
modify the GCI (Ground Controlled Interception)
system developed for dealing with the night bombers
in the Blitz. This worked very well — so well in fact
that enemy jamming disabled it. When this problem
arose, two answers were suggested. The first, to
operate in the same waveband (50cm, 600MHz) as
the enemy jammers were using, so that they could
not interfere with it without hurting themselves. The
second was to use the new centimetric waveband
(3GHz) made possible by the invention of the Cavity
Magnetron.
The first solution produced the AMES (Air Ministry
Experimental Station) Type 16 radar which was
installed in a field on the cliffs at Greyfriars,
Dunwich, about 750 yards south of the pub and a mile
north of the CHL mast. The Control station was in a
nearby Nissen hut. The Greyfriars equipment was
used mainly for initial trials to complete the design of
the Type 16. The photo above was taken at Greyfriars
by Murray Hardy in 1943.
The AMES Type 16 operated at 600MHz, 50cm with
a pulse repetition frequency of 500 pulses/sec and a
peak power of 100kW. It was often used with a Type
24 height-finder. The single aerial is a photogenic 30
foot diameter parabolic dish that rotates at 33 rpm.
Range was between 120-200 miles.
The TRE map right was made in 1943 and shows the
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Greyfriars Type 16 range, over a southeasterly arc of
110deg, at 100 miles just covering all the North Sea
to the Belgium and Dutch coast and on 200 miles,
reaching beyond Amiens and Brussels.
Enlarged photographs of the inside of the Greyfriar's
control room (right) show plots centred on the
Scheldt and Antwerp. The transmitter control units
are at the back. The plan of the operation was
plotted on one wall chart whilst the actual radar plot
was plotted on the other, so the controllers could see
if the plan was being followed.
Greyfriars, Dunwich : Air Control Radar : MEW AN/CPS-1, Type 24, Type 26, Type 28
In the meantime 10cm equipment (3GHz) was developed both by us and the Americans. One of
the best of these was the American MEW (Microwave Early Warning) AN/CPS-1, an airtransportable early warning radar designed for long range and high angle coverage in areas of
high traffic density. The first MEW went to Devon but the US Eighth Airforce, operating
largely over East Anglia, requested one to help control flights over the North Sea. This second
MEW was deployed at Greyfriars next to the Type 16 site.
Since it was a fully mobile installation it merely required a field
for its vast convoy of trucks. A complete MEW system
weighed about 60 tonnes, required eight trucks for equipment
transport alone, and drew 23 kilowatts of DC power from a
portable generator. It took 100 troops two to three days to pick
up and move a MEW.
The range was 200 miles. The MEW used two antennas joined backto-back, with one antenna covering low altitudes and the other covering
high. Each of the two antenna consisted of a linear array with 106
dipoles in front of a solid reflector, in the form of a section of cylinder
with parabolic curvature, laid horizontally. Each reflector was 25 feet
wide. The low-coverage reflector was 8 feet tall, the high-coverage
was 5 feet tall. They could form a beam only 0.8 degrees wide that
could provide extremely precise location of intruders, at least in the
horizontal plane. Each aerial had its own transmitter, receiver and PPI.
The indicators were ten 12inch diameter PPIs and a skiatron, a PPI
projection plotting table.
One thing which the MEW could not do was find the height of its targets. This problem was
solved by using the latest British 10cm Height-finder, AMES Type 24, photo above right.
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According to the 60 Group records, an experimental lightweight 10cm Height-finder, an AMES
Type 28, was also installed at Greyfriars, operating at 3GHz.
The Dunwich MEW was operational in Britain by the summer of 1943 and later the two Mews
were very useful in helping to deal with the V-1 Blitz, since MEW's longer range gave greater
advance warning of flying bombs, allowing more effective fighter
interceptions. Since the V-1s flew at preset low altitudes, MEW's
inability to compute heights was not a problem.
After June 1944, the US 8th Airforce MEW and Type 24 Heightfinder were moved to France and the British MEW equivalent, the
AMES Type 26, was moved from Fairlight to Greyfriars. An
AMES Type 28 experimental height-finder was set up at Dunwich
to work with this AMES (MEW) Type 26. The plot at right is a
Greyfriars radar PPI plot of the airborne force, both aircraft and
gliders, crossing the North Sea towards the Dutch coast for the
unsuccessful Operation Market Garden (aka "A Bridge too Far")
on 17th September 1944.
Minsmere Cliffs: Radio Counter Measures
On the 16th April 1943, a system called Ground Grocer was set up on the Minsmere Cliffs
designed to jam the airborne radar called Lichtenstein, carried by enemy nightfighters attacking
our bombers. One answer to this threat was to set up at Dunwich a
powerful transmitter on 500MHz to interfere with the German airborne
radar. The installation included a special direction finding receiver to
aim our transmitter in the most effective direction. The photo at right by
Murray Hardy shows the transmitter aerial and over to the left, part of
the Dunwich CHL wooden tower and ground aerial at bottom.
Operating on a frequency between 486 and 501 megacycles per second,
a paraboloid aerial was used to pick up the Lichtenstein pulses, which
were panoramically displayed to the operator. The latter, by means of
remote control, tuned the jammer transmitter to the same frequency. The
transmitter fed into a similar aerial, orientated in the same direction, thus
modulating a beam 16° wide. It was estimated that Grocer would reduce the range of
Lichtenstein B.C. to 500 yards if the aircraft were as far distant as 140 miles, at a height of
12,000 feet and within the beam of the paraboloid and flying towards the station. With the
aircraft flying away from the station, however, the effectiveness of Grocer was greatly reduced,
the power required to jam as effectively as head-on, for a given range, being some 250 times as
great as for the head-on case. Thus Ground Grocer was mainly a cover for Allied bombers on
their return journey. In order not to give the enemy early warning of the approach of Allied
bombers, Ground Grocer was not switched on until the leading aircraft were within 30 miles of
the enemy coast. The beam was swung to cover both the outward and return journeys.
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Shortly after Ground Grocer was built, a much more
powerful transmitter valve -- the Resnatron — was
designed in America and used in another jamming
installation called "Tuba". This, a mobile equipment
employing a fleet of ten trucks, was first set up
alongside the Ground Grocer. It was capable of
delivering up to 60kW power CW at about 500MHz.
Unfortunately, changes in the operational conditions
in the air war reduced the need for this and Ground
Grocer, so that full use of them was not employed.
The photo right shows the Ground Grocer receiving aerial.
Radars near Dunwich : Chain Home, Mobile Radar Unit, Ground Cigar, CHEL, CDR
Darsham High Street : Chain Home
The first coastal radar stations were called Chain Home. One such was built in 1939 at
Darsham High St, about five miles from Dunwich. The wooden receiving towers had to be
separated by hundreds of yards from the steel transmitting towers. The transmitters were north
of the A144 to Brampton, the receivers south and
behind Willow Marsh crossing cottage; both being
west of the A12.
A CH station consisted of four 360 foot steel towers
with wire aerials depended between them in a
continuous curtain array of stacks of horizontal halfwave aerials with reflectors. A switchable lower
curtain array dealt with a gap in the lobe coverage .
A pulsed signal - at a pulse repetition frequency of
12½ - 25 pulses per sec and frequencies of 2050Mhz - was used to floodlight the wide area ahead
(whereas later radars use rotating beams), transmitted at originally a peak power of 200-800
kWatts. The range was about 100-200 miles and halved in rain. Photo shows transmitter towers.
Any radio energy reflected from a target was
received by a separate aerial system consisting of
four wooden 240ft towers in a rhomboid shape,
fitted with six vertically stacked crossed half-wave
arrays which permitted determination of range and
bearing, and to some extent altitude using
goniometers. Photo right and below show the
wooden receiving aerials.
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They were built in three stages. First, after the site had been surveyed, a temporary "advanced"
equipment similar to the MRU (see below) was set up. This enabled the station to start operating
whilst more permanent equipment was installed.
The next step was to instal two 240ft wooden towers similar to
the receiver towers used in the "final" design, one for the
transmitter and the other the receiver. With this "intermediate"
station in operation, work could now proceed with the erection
of the permanent buildings and the final transmitter and
receiver towers . Photo at right shows the Darsham receiving
towers behind the Willow Farm crossing. No other radar
equipments were placed here, but in 1940, when the threat of
invasion was great High Street was one of several CH stations asked to watch inland as well as
out to sea to detect landings of parachutists. This caused difficulties in carrying out its principal
task, requiring the use of a second receiver
Hinton : Mobile Radar Units
The CH stations were built in three stages. After the
initial chain had been complete, various types of
emergency standby stations were provided to guard
against damage or breakdown of the main station.
For High Street, an MRU (Mobile Radio Unit) was
provided at Hinton (TM437718), about half-way
between High Street and Dunwich. The photo shows
105ft wooden MRU aerial tower being raised,
A Mobile Radio Unit as provided as a standby for
High Street would be mounted on vehicles and have
two temporary towers. The wooden towers were up
to 105ft high, one being used for the transmitter and
the second for the receiver aerials.
This equipment worked in a similar way to the
original CH but was originally an all round transmission, not
directional. The MRU (AMES Type 9) worked at 40-50MHz at
300kW peak power . When mounted on a flat site at sea level it had a
range of 50 miles for an aircraft at 5,000 ft or 100 miles for one at
20,000 ft. The photo at right shows the crossed dipoles at the top of
the receiving tower, The crossed receiver dipoles enabled it to
determine bearing whilst it had another set of receiver dipoles lower
down the mast for finding height. If on a permanent site it might have
had huts, but the masts might not have been erected until needed.
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a
Sizewell Radio Counter Measures : Ground Cigar
At Sizewell a few miles away, a powerful transmitter was set up to jam the German nightfighters' communications radio on about 40MHz. Care had to be taken in using this (called
Ground Cigar) to avoid it jamming our own communications, but it was also found to interfere
with the CHL at Dunwich on 200MHz through Ground Cigar's fifth harmonic (5x40=200 ). They
had to cure this by adjusting the CHL's frequency. Later, an airborne version of Cigar was used;
reducing the importance of the ground installation.
Covehithe : Chain Home Extra Low, MRU, Coastal Defence Radar
Covehithe is listed as having, well behind the beach and possibly called Beach Farm or Benacre,
a Chain Home Extra Low Radar and a Coastal Defence Radar which might have been temporary
installations.
Summary of Radar at Dunwich
The war was known as the Wizard War because of the continuous electronic battle for
ascendancy. This story has wandered somewhat from the small coastal village that suddenly
saw a huge influx of personnel from all round the world; and from the quiet cliffs at Dunwich
which were the testing ground for some of the most primitive to some of the most advanced
radars of that early period. When the war was over, a further use was found for the latest radars
at Dunwich. They became the first Air Traffic Control Radars at Heathrow airport.
Appendix - Newcomers guide to frequencies and their common uses today
For those unfamiliar with normal radio frequencies, here is an introduction. Frequencies are
measured in cycles of the radio wave per second, with the unit being Hertz (Hz). A million is
called a Megahertz (MHz) and a thousand million cycles per second is a Gigahertz (GHz) and so
on. The important point is that the higher the frequency the smaller the aerial need be. And
smaller aerials are good. So, development is always towards higher frequencies. For example
the original CH radars had 300ft long vertical aerials at 20MHz; not very portable or compact.
In two years they developed to 3GHz with short aerials that would fit in aircraft and truck-borne
ground aerials only 25ft long. The modern GSM3 mobile phone has two aerials within the
plastic case and operates at about 900MHz.
Long wave BBC Radio 4
Medium wave BBC Radio 5
Short wave BBC World Service
VHF BBC Radio 4 FM
TV analog frequencies
TV digital frequencies
Mobile phone GSM3
Modern air traffic control radars
Modern radar devices
Light
0.2MHz,
0.7MHz,
3-30MHz (also known as HF bands ie high frequency)
94 MHz
50-450 MHz
450-750 MHz
880 MHz - 1.9 GHz
1 - 4GHz
3MHz - 110GHz includes ground radar, radar speed-guns,
through-the-wall radar devices, car control radar
450-750 THz
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