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Duxford Radio Society

Imperial War Museum, Duxford, England 

Equipment Summary


Updated 24-09-2009

Radar Anti-Aircraft No. 3 MK. 7 - Blue Cedar (UK)

Radar AA3 MKVII after external restoration

AA3 MK7 mobile radar cabin after external restoration, June 2007

AA3 MKVII with AEC Matador towing tractor

The mobile radar unit pulled by an AEC Militant Medium Recovery Vehicle.  When in service, the radar unit was towed by an AEC Matador Medium Artillery Tractor

Equipment information

The AA3 MK7 was a mobile trailer-mounted, second level, anti-aircraft gun control radar station, the original design of which dates from the mid 1940s.  The equipment design was in use with various UK and Commonwealth Armed Forces until the mid 1970s.

The UK code name for this equipment was Blue Cedar.

The radar was originally used to control anti-aircraft guns Type 5.25 inch and Type 3.7 inch in elevation, height and range via an intermediate mechanical computing device known as an AA Predictor. AA3 MK7 is capable of detecting a Spitfire at 25,000 yards and planes the size of a Beaufighter or larger at 36,000 yards.

The radar station is housed in a steel, air-conditioned,  four-wheel trailer which carries a rotating 5ft parabolic dish antenna on the roof.  The radar trailer weighs approx 5.125 tons and has dimensions of length 15 ft, height 12 ft 9 ins, width 12 ft.

The frequency range of operation is 3.0-3.1G Hz (9.7-10cm) with a transmit output power of 200kW peak, which gives a detection range between  950-36,000 yards.  The antenna rotates at 20RPM and also executes an elevation scan every 4 seconds.

The system  can operate in either search mode or tracking mode.   This means that the station can both acquire and track moving targets in range, elevation and azimuth bearings.

Electrical power is obtained from a separate trailer-mounted three-cylinder Lister/Tilling-Stevens diesel generator, type X2/X8/17472, which can supply 17k VA three phase AC at 240 Volts 50c/s.
Equipment background story

This particular radar unit was built by the British Thomson Houston Company and  has serial number 223 on its identification plate.

After use by the British Army Anti-Aircraft Command it was supplied to the Netherlands Armed Services.

After service in Holland it was subsequently transferred to the Air Defense Regiment of Ireland and was used as a mobile air defence radar.

Following completion of its active service in Ireland it was donated to the Winbolt Collection.  It has spent approx the last 8 years or more stored in the open at Bletchley Park.

It is complete with its trailer mounted diesel generator (dated 1952 with 12 hours operation on the clock), target simulator equipment and some of the original documentation.

 IWM Duxford already has on display two examples of the type of large anti-aircraft gun which was controlled by this radar unit (AA Gun Type 3.7 inch).

One can be seen in Hangar 4 finished in deep bronze green, and a second in the Land Warfare Hall.

An informative public display would be the radar station exhibited alongside the matching 3.7 inch AA gun to show the complete 1940s/1950s mobile Anti-Aircraft station.

Conservation & restoration process

This equipment was kindly donated to DRS by Dr. G. E. Winbolt.

The external conservation of this complete mobile radar station  began with maintenance work by DRS Volunteers on the diesel engine of the  generator trailer and work to repair the roof of the main Radar cabin.  As the equipment has to stand outside for the foreseeable future, the priority was to protect the exterior of the station.

During August 2006 the diesel generator was moved to the Duxford Aviation Society Military Vehicles Section in the Land Warfare Hall at Duxford to be overhauled, stripped, resprayed and serviced by the DAS Volunteers from this Section.  Volunteers from the Radio Section began work on the electrical components.  The generator exterior restoration was finished in December 2006.  Work continues on the electrical, control and metering equipment.

On 18 September 2006 the radar trailer was also moved to the DAS Military Vehicles Section ready to begin its mechanical restoration and overhaul.   The exterior restoration was finished in early February 2007.  See photos below.

After the external mechanical restoration, if a covered area to work can be found, it intended that the power supplies will be gradually applied to the main radar trailer to enable the heaters and air conditioning plant to operate and therefore maintain the cabin interior in a dry state.  Conservation work can then start on the internal equipment modules.

On 24 January 2008 the custom  made canvas cover or 'Tilt' for the diesel generator arrived  for first fitting.

Photographs of Radar AA3 MK7 Blue Cedar taken in the afternoon light, January 2008
 AA3 MK7 Radar january 2008
View of AA3 MK7 from the other side 
Close-up view of the radar cabin 
View of generator with its cover fitted 
Radar AA3 MK7 with matching 17KW diesel generator
View of the radar station from the other side
Close-up view of radar cabin 1year after re-painting
Close-up of generator with custom cover fitted
AA3 MK7 Restoration Project Sequence Photographs
AA3 MK7 radar trailer with dish
AA3 MK7 radar trailer front view

AA3 MK7 interior PPI display & controls
AA3 MK7 Lister Diesel generator trailer

Rear view of radar cabin  while in storage at Bletchley Park
Front view of radar cabin
Part of radar cabin interior
17 kVA diesel generator trailer
AA3MK7 Radar being delivered to Duxford
AA3MK7 Radar cabin with DRS crew
AA3MK7 radar cabin and Generator trailer being delivered to Duxford from Bletchley Park
Radar cabin with the DRS team of "youngsters"
 From left to right: Arthur Llewellyn, Denis Willis, Dennis Harvey
Radar AA3 MKVII before restoration

First starting of the generator

Mobile radar cabin before restoration, September 2006 Working on the diesel generator March 2006
Diesel generator in MT Section 
Generator front view 
Generator rear view 
Electrical control panel partly dismantled 
Diesel generator in the DAS Military Vehicles Section workshop
Diesel generator front view
Rear view of diesel generator
Electrical control panel partly dismantled
AA3 diesel generator paint1
Diesel generator paint2 
Diesel generator paint3  Diesel generator paint4 
Mechanical assembly after top coat of Deep Bronze Green paint
Diesel generator front view
Rear side view of diesel generator
Other side view
Radar trailor with AEC Matador tractor called Milly 
Radar Cabin outside Land Warfare Hall 
Adding some paint details 
Investigating the engine
Radar trailer pulled by the DAS MV Section AEC Militant Medium Recovery Vehicle (called "Milly").  Since this shot was taken, Milly has been finished in a nice deep bronze green paint finish.
Radar trailer parked outside the IWM Duxford Land Warfare Hall in the queue of vehicles waiting for attention
Adding some of the fine paint details to the generator engine paint finish
Now the engine has to be made to start well and run reliably
AA3 Radar trailer in DAS MV Section workshop 
DAS MV Section volunteers at work
View through emergency exit of antenna drive
Rear view of Radar trailer in primer finish 
AA3 MK7 radar trailer in the DAS MV Section workshop ready for dismantling and restoration to start
 DAS MV Section Volunteers working on the radar trailer.  Note the emergency exit door at right.
View through the emergency exit door showing the antenna mounting, the main support shaft, waveguides and cableforms etc Rear view of radar trailer showing progress made on the cleaning, sanding, de-greasing and primer stages
AA3 MK7 right hand side painted
AA3 nearly finished painting now
Onr of the electronics modules inside
Some corrosion present inside
Right hand side view of the new paint finish.  The Military Vehicles Section do a really excellent job. 
 The exterior restoration is nearly finished now.  Just the small details to be highlighted and the wooden decking on the roof to be re-fitted
View inside one of the electronics modules inside the radar cabin
 Close-up view of the same module - some corrosion to be gently dealt with in here
Side view of finished Radar cabin
Side-rear view of Radar cabin
Rear view of finished Radar cabin
Front view of finished Radar cabin
Side view of finished radar cabin after restoration taken on a damp morning in February 2007  Rear-side view.
The wooden decking has now been painted and re-fitted on the roof
Rear view showing the motor  drive assemblies for the dish
 Front view showing the towing and steering gear
Radar unit and diesel generator behind
Side view of radar with dish raised
Three-quarter side view of finished unit
Side view of radar and generator
  Radar cabin with diesel generator  behind  Side view of radar unit with dish mount raised and tilted over into the operating position
Three-quarter side view of finished unit  Radar and generator unit from side
Work is still ongoing on the electrical control gear of the mobile generator trailer.  The diesel engine needs further attention.  A replica of the correct canvas cover for the diesel generator has now been fitted.
Work has now started to conserve the electronics modules inside the radar cabin.
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27-03-2006 V2.00 Updated 24-09-2009