Site Records

Site Name: RAF Holmpton R7 Mk III ROTOR Radar Bunker

off South Leys Road

OS Grid Ref: TA35482424

Sub Brit site visit: 24th July 2004

[Source:RAF Holmpton Archive & Nick Catford]

Originally all the radars at RAF Holmpton were to be clustered round the R3 control centre apart from the Type 7 which was remotely sited at South Leys, 2km north west of the technical site.

A Type 7 aerial array over an R7 Mk III bunker

The transmitter, receiver and motor for turning the aerial array were located underground in a bunker designated as an R7 and known as a 'well'. During the ROTOR period two different types of R7 bunker were utilised. Where the Type 7 radar was located close to the R3 operations block it was housed in an R7 Mk II bunker which consisted of a single room. If the R7 was at a dispersed location a larger R7 Mk III was built which consisted of three rooms. Because of the distance from the main site, an R7 Mk III required its own IFF and an Mk 10 IFF was mounted on a Type 14 plinth, turntable and cabin this combination was known as a Type 79.. This was located a short distance to the north of the R7 bunker with a small brick built electricity sub station alongside.

Plan of an R7 Mk III bunker

The Type 7 was developed in the early years of WW2 as parallel development of the Chain Home Low (CHL) radar by the addition of a height-finding capability and a Plan Position Indicator (PPI) display.

One of the two T3705 transmitters in an R7 Mk II bunker; with fan assembly to the right. The metal frame to protect the feed to the antenna pedestal is in the foreground.

Early stations were designated 'transportable' with a fixed version of the final Type 7 equipment being developed in 1942. The final stations comprised an Operations Building (Happidrome), a radar well and an aerial mounted over the well; the well housed the transmitter and the receiver.

Continuous tracking of targets was essential to interception procedure and the aerial system had to provide gap-free cover. To do this an array consisting of 32 centre-fed full wave dipoles was used, mounted in four stacks, each with eight dipoles. For transmission, the top four dipoles and the bottom four dipoles in each stack could be combined either in phase or in antiphase under the operator's control. This achieved overlapping beam positions and provided adequate gap filling.

For reception the dipoles in each stack could be combined in four different ways providing beams at difference angles of elevation for height finding. Switching between beams was done automatically or on a pulse-to-pulse basis using a capacity switch in the feeders.

Photo: The extant Type 79 plinth and power house
Photo by Nick Catford

The antenna beam width was 15 degrees although a narrower beam width and greater range performance was provided for the ROTOR improvement plan by the addition of more stacks of dipoles on each end of the aerial.

WW2 R7's were known at Mk. I, post war this equipment was upgraded and a small R7 'well' designed for use on the radar site was designated Mk. II and a much larger R7 Mk. III was for remote sites up to 2 km from the main site containing IFF facilities. Both Mks. II and III were later re-engineered to Mks. IV and V standard respectively.

For further information and pictures of the RAF Holmpton R7 bunker click here

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Last updated 19th January 2006

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