At the same time as development of Chain Home, Chain Home Low had been developed to fill the gaps left by the CH. CHL stations were able to detect low flying aircraft that could not be detected by Chain Home stations. There were two CHL stations proposed for the Island. To the south Cregneish was built on top of Meayll Hill, construction starting in 1939; this initially reported to Scarlett and subsequently Dalby. A CHL at Maughold was selected in the north to work with Bride. The Chain Home stations reported to the 9 Group filter room at Longley Lane, Preston.
Cregneish came on-line in July 1940, two months ahead of Scarlett but at this time Maughold hadn’t been started and was later abandoned.
Cregneish was developed as a major CHL site, initially equipped with two AMES Type 2 radars. These were later supplemented with an AMES Type 31, Coastal Defence (CD) No.1 MkV manned and used by the Navy to monitor shipping and an AMES Type 52. The station was an important surveillance site monitoring the primary shipping lanes from North America to Liverpool.
As Creigneish & Dalby formed an important component of the British early warning network they both remained operational for a few years after the war.
Today little remains at Meayll Hill although the bases of a number of buildings can be easily identified. There are still substantial remains at Bride although one of the technical blocks is located on a land-fill site.
Chain Home radar stations were static installations which ‘floodlit’ a fixed wide area with radio waves of wavelength around 6 - 15m (20 - 46MHz). It was soon realised that CH was unable to detect low-flying aircraft approaching at an angle of elevation less than two degrees due to the reflection of radio waves from the sea. This problem was overcome by the development of Chain Home Low (CHL) Radar which used a shorter wavelength of 1.5 Metres (200 MHz) and was based on a system originally designed for gun laying army coastal defence sites. CHL was the first radar to use one aerial for both transmission and reception. The official designation of the first CHL stations was Air Ministry Experimental Station Type 2 (AMES Type 2).
Because of the shorter wavelength, the aerial array for CHL was much smaller than for CH and it was designed to produce a very narrow beam. The small aerial array could be steered in a horizontal plane, thereby providing, in effect, an accurate radar ‘searchlight’ with a range of 50 miles. The signals were presented on a PPI display.
CHL however, did not provide any facility for finding height. Chain Home Low represented the first generation of modern radar systems. At first the aerials were steered by hand and did not cover the full 360 degrees. The aerials were, however, soon motorised and continuous rotation and allowed the use of a Plan Position Indicator (PPI) display. In a PPI display the cathode ray tube is scanned radially from the centre of the rim of the tube face. The angle of the scan is synchronised to the rotating aerial and the intensity of the beam is modulated by the signals received from the aerial.
CHEL or Chain Home Extra Low (AMES Type 52) radar was similar to CHL but worked at an even shorter wavelength of 10cm (3,000MHz). The shorter wavelength allowed the use of the rotating reflector dish aerial, which is now a common sight at airports. AMES Type 52 was derived from a Naval Surveillance Radar, AMES Type 31 Coastal Defence No. 1 Mk V.
Three types of radar were installed at Meayll Hill:
- AMES Type 2 - the original type of CHL. Two sets were installed, an early set at the south of the site and a later set at the north of the site. The latter set was designated Channel A and the earlier set was designated Channel B. The sets were operated by RAF personnel and used to monitor aircraft movements in the Irish Sea.
- AMES Type 31 - naval radar. 1 set was installed, mounted on top of the now demolioshed pillbox at the south of the site and operated by the navy. It was used to monitor shipping in the Irish Sea.
- AMES Type 52 - (CHEL) One set was located south of the pillbox at the northern end of the site. It was operated by RAF personnel and used to monitor aircraft movements in the Irish Sea.
- IFF - The term IFF derives from the words ‘Identification Friend or Foe. IFF allowed a radio operator to identify friendly aircraft. Before the outbreak of war a simple IFF system was developed in Britain. Aircraft were fitted with aerials incorporating motor-driven tuners that caused the reflected signal received by ground radar stations to vary in amplitude. Later models employed an electronic unit that detected the presence of friendly radar and then transmitted a coded signal causing the ground radar display to indicate a friendly aircraft on the PPI display.
The IFF system at Meayll Hill was of the later electronic type. It was located at the north side of the site. The characteristic cross base for the aerial tower and associated rectangular base for the equipment cubicle indicate the system was an Interrogator (IFF Mk III).
The site was secured by barbed wire and guarded by the military. The plan shows the location of the surface remains with only bases now remaining. The locations of the technical buildings, IFF, pillboxes, machine-gun posts (mounds of earth probably eroded by motorcycle scrambling) and the sentry box at the entrance to the site are fairly certain. The identification of other remains is tentative.
A radar mechanic from Scarlett CH tells of being invited to visit Cregneish CHL site on 5th May 1942. He caught the bus to Port St. Mary and then walked up to the site. He was surprised to find how small an area it occupied compared with CH and he was impressed by the PPI display that showed the positions of planes, ships, the Mountains of Mourne in Ireland and nearby mountains in England, Scotland and Wales. The radar mechanic at Cregneish told him that CHL was reliable and needed much less maintenance than CH.