A collection of walks, discoveries, insights and pictures of exploring Dartmoor National Park
September 29, 2024
High Frequency Direction Finding (HFDF) Station at Merripit Hill – part of the WWII ‘Pip Squeek’ Network
Near to the summit and on the south-eastern flanks of Merripit Hill, just to the east of Postbridge, are the ruins of a World War II installation, which some observers have recorded as the remains of a searchlight installation. The ruins appear too complex to be such an installation and coupled with the fact there is no mention in an April 1944 list that gives the location of the twenty searchlight units on Dartmoor, this casts further doubt on this theory.
The principal feature found on Merripit Hill is a raised octagonal plinth of shuttered concrete, which is key to establishing the possible use of the site. Additionally to this, there are the remains of at least 2 possibly 3 more buildings, which add to the mystery and interpretation for the site . After some research, the conclusion is that the installation was part of a chain of High Frequency Direction Finding (HFDF) ‘fixer’ stations across the UK from the early 1940s, which were used to provide aircraft detection and direction of ‘friendly aircraft’. The system was known as the ‘Pip-Squeek’ network and integrated with Fighter (Bomber) Command. By plotting some known ‘fixer’ sites onto the map, a pattern associated with the placement of these sites can be established. It seems very likely that the site at Merripit Hill may have worked in conjunction with three known sites in Devon and Cornwall, namely at Baxworthy, Clyst Honiton and Looe, which were all within the St. Eval fighter command sector.
Radio Direction Finding (RDF), was developed at Orford Ness and was a pioneering technology in the 1930s that would eventually help to win the Battle of Britain Aircraft. In July 1934, at the Royal Air Force Annual Air exercise it was concluded that the existing system of aircraft detection and fighter direction was woefully inadequate. Following trials in January 1935, it was demonstrated that the direction of transmission from the standard fights Radio Telegraphy set (the TR9), could be determined with surprising accuracy, provided that the direction finding (DF) set was properly sited and well away from sources of metallic interference. It was from these beginnings the direction finding system was further developed. By the early 1940s aircraft were equipped with an upgraded battery-powered 2-channel high-frequency (HF) radio, the TR9D, with a range of around 35 miles. Using the capabilities of this radio, through the broadcast a 1 kHz tone for direction finding the friendly aircraft once airborne. This form of direction finding was redundant in 1942 due to the increase in overland radar coverage from the Chain Home stations and with the widespread use of IFF (Interrogate Friend or Foe). It is known that some HFDF site across the UK never became operational – Merripit Hill may be one of those.
This post would not have been possible without information gleaned from the website frontlineulster.co.uk and through correspondence with that website author, who is confident the Merripit Hill site was a HFDF ‘fixer’ station. Some of the descriptions in this post have been para-phrased from this source
Royal Commission for the Historical Monuments of England, 1987-1993, Duchy Farms Project Survey Visit, S. Probert (Report – Survey). SDV350839
Dobinson, C. S., 2000, Searchlight sites in WWII, 36-8
This annotated map from circa 1948, shows an interpretation of the Merripit Hill HFDF site. Each feature annotated is recorded in this post. The TR9D radio sets in aircraft had a range of around 35 miles. It was a High Frequency (two channel) radio operating between 4.3 MHz and 6 MHz, one channel was utilised for voice communication, and the second as a backup. It was decided to use this secondary channel to broadcast a 1 kHz tone for direction finding the friendly aircraft once airborne. The map would indicate that the three original HFDF sites may well have been adequate for coverage, however, with an additional site at Merripit Hill near the epicentre of these, a more comprehensive coverage could be achieved. The TR9D radio was introduced into service to facilitate the simultaneous transmission of voice and DF signal. The direction finding (DF) stations consisted essentially of a rotating aerial formed by two aluminium plates mounted on a vertical spindle, and connected to a modified receiver. The evolution to full integration into the UK’s air defence network wasn’t immediate with the initial use of the system being used for homing aircraft at night or in poor weather.All the known HFDF sites across the UK seem to have the same octagonal base, some of which had additional blast protection. To reduce interference and optimise signal performance the sites were chosen to be “in open flat country, free from obstructions such as buildings or clumps of trees and 200 yards from main roads and traffic”. Merripit Hill meets all these requirements.The Fixer Station was simple in construction and layout, with a common design being used across the country. The direction-finding tower comprised a 30-foot, octagonal wood-clad structure. Some towers were set in a brick blast wall for the protection of the radio operator who would have worked from the base. The photograph is a superimposed picture of the tower at Southwold which was a Naval Communications Station atop the Merripit Hill ruined base.The photograph is a superimposed picture of the tower Garway Hill (taken in 1942) atop the Merripit Hill base. Garway Hill was never operationalThe Merripit Hill base measures 4 metres across with 0.25 metre highwalls. There is a central channel and two side chambers. Of note are a series of threaded iron rods embedded in the walls upon which formerly secured was the upper (wooden) structure. The centre of the structure is at SX65821 80293The direction finding of friendly aircraft was never intended to work in isolation of other technologies. As with the sound mirrors of the 1930s, radar coverage of the Chain Home network at the time was mainly out to sea covering the approaches to the United Kingdom. This was adequate to provide an early warning, but once aircraft were over land, tracking became difficult. In order to facilitate timely and accurate fighter intercepts of approaching aircraft, it was key to direct airborne fighters to their target. It would take a combination of DF and Chain Home radar to provide the full picture.Close to the tower foundations, a number of concrete fragments can be foundConcrete fragment near to the tower foundationsTo the south west of the tower at SX65828 80273 is some disturbed ground which looks like a possible trench or a gun emplacementContextual view of the possible trench / gun emplacement in relation to the tower ruinsThis Merripit Hill installation is approached from the B3212 road by a straight track, which is still depicted on the OS maps. Some foundations remain, the surface having been removed by the Dartmoor National Park Authority in about 1975.Downhill from the Tower ruins (and next to the track), there are some concrete fragments and much ground disturbance, perhaps indication the site of a former building. If this was indeed the location of a building, due to the lack of building stone, one would conclude it was a wooden structureThe disturbed ground (at SX65885 80226) is annotated as Building 1 on the map in this postConcrete fragment and Dartmoor Pony near the possible building foundations. Of interest, the map shows a ‘circle’ at this location interrupting the track between B3212 and the tower ruins. Brick near the track showing perhaps some evidence of a building (or foundations) About 20 metres to the east of the possible building by the track is a turf covered mound which is about 10 metres in diameter and 1.2 metres high. There are traces of brick walling and a reinforced concrete roof. This almost certainly would have been a (bomb) shelterClose up of the (bomb) shelter. The shelter is located at SX65909 80243The entrance into the (bomb) shelter can be discerned in the turf as shown on the annotated picture.The entrance and internal walls of the (bomb) shelter above the turf. It is annotated at Building 2 on the mapDownhill from the (bomb) shelter next to the track are the foundations of another building (annotated as Building 3 on the map)The foundations of this structure the author has interpreted as an accommodation block as its furthest away from the tower and closest to the road. At RAF Sharpitor, on Peek Hill the technical site is on top of the hill and the domestic site down the hill next to the road, therefore one assumes albeit on a smaller scale the Merripit site follows the same layout. As with other buildings, this one would have no doubt been wooden. These foundations are located at SX65949 80160The track between the B3212 and the tower measures between 1.2m and 3.5m wide and consists of angular limestone stones and boulders. The track where it meets the B3212Next to the track (in the direction of Postbridge) is a large quarry, which is now used as a car park. This quarry is shown on late 19th century mapsOf interest is that next to the 19th century quarry, an extension to this appears to have been dug around 1940s (it appears on 1948 map). The author interprets this as being where stone was excavated for the track foundations to the Merripit Hill siteThe 19th century quarry and its 20th century extension have softened into the 21st century landscape
