Second World War Aircraft Direction Finding Part II – Interpreting the Remains

In the second part of this article, I will pull together information and images from several sources in an attempt to interpret some of the cryptic remains from what I believe to be direction-finding sites across the country.

Update 2024: Prompted to reevaluate my interpretation of these sites by author and historian Ian Brown, I have amended this article to reflect the results of our research. I had originally interpreted the brick and concrete hexagonal towers as being blast walls for HFDF antennas, but based on a mixture of documents in the National Archives, it is more likely that these were used predominantly for VHF direction-finding stations. In Part III of this research, I have plotted all known sites to help confirm or otherwise our hypothesis.

Rediscovering the HFDF Network

Understanding the capability and function of the HFDF system has enabled a greater element of interpretation and discovery of these sites across the country.

The capability of the TR9 radio sets limited their air-to-ground range to 35 miles. This had a direct correlation to the design of the HFDF network, as no two stations could be more than 35 miles apart without risking gaps in coverage. The stations would be within around 30 miles of each other, typically located in a group of three, with two subordinate stations and one control site plotting the sector and reporting to sector control at Fighter Command.

In May 1937 during the development of the HFDF system, the Biggin Hill Sector was provided with two additional stations at Chatham and Wittersham, with the Sector control being located at Biggin Hill. These would be the original HFDF fixer stations and one of the main structures of the Wittersham site remains. Coverage across the remainder of the country would be a greater challenge.

Plotting the known sites onto the map, it’s possible to establish some patterns associated with the placement of these sites. A good example is the southwest of England, where there are three sites in Devon and Cornwall at Baxworthy, Clyst Honington and Looe, all within the St. Eval fighter command sector.

Siting the Stations

Great care was taken in May 1937 when two additional stations were sited in the Biggin Hill Sector. To reduce interference and optimise signal performance the sites had to be “in open flat country, free from obstructions such as buildings or clumps of trees and 200 yards from main roads and traffic.”¹Signals Volume V, Fighter Control and Interception, Chapter 1, The Biggin Hill Experiments, pp15-16. Air Historical Branch, 1952 Before construction began the performance of each site was confirmed against long-range aircraft and then calibrated using signals from closer aircraft. While relatively small patches of land were required for the new HFDF stations, this was at a time when the Royal Air Force was undergoing expansion and the pressure for purchasing new sites was huge. Such importance was placed on the provision of HFDF stations that the Chief of the Air Staff at the time gave personal attention to the issue “by all means possible.”

1937 was a key year for the deployment of HFDF stations, by the end of the year several Fighter Control Sectors in the south of England and around London had three HFDF stations each:

1. Biggin Hill
2. North Weald
3. Hornchurch
4. Northolt
5. Duxford

It was intended that every Fighter Control Sector across the country would have three DF stations, but this was expected to take 2 years to complete.

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Fixer Station Design

These sites were simple in construction and layout, with a common design being used across the country. There was the main direction-finding tower; a 30-foot, octagonal wood-clad structure, set in a brick blast wall for the protection of the radio operator who would have worked from the base. I have noticed two variations in the construction of this tower, one being the wood-clad variant with wooden buttresses, as in the example at Southwold. There may have been several reasons for this:

1. To support the structure in high winds. Many were located on the coast or at high points in remote areas.
2. The tower may be hollow and lack internal support above the level of the walls, therefore external support must be given.
3. I have read that the walls may have been filled with gravel for the protection of the crews. If this was the case, the walls would have been very heavy and would have needed external support. I do not rate this theory, not at least in the upper layers of the tower as gravel would have greatly attenuated (reduced) the incoming signal strength. However, there is a site in Scotland recorded as an experimental VHF station which did have gravel in the walls, but this site had no brick blast wall and the antenna was external to the tower structure.

There appears to be a variant of the wooden tower structure, and this is a lightweight version which appears online when searching for direction-finding sites. Most likely the station at Hoxne, the four-element directional antenna is exposed, and the tower may be clad in tarred felt. The site also lacks the recognised octagonal blast wall of others. Perhaps an early design.

Another site existed at Leckhampton Hill in Gloucestershire. In this post-war (circa 1951) aerial photograph, the tower can be seen without the wooden buttresses, and a variation on the tower design with a more open blast wall than has been seen with the existing snail enclosures, but the tower also does not appear to be wood clad. Of note, while the tower has been removed, the concrete base remains on the hill, as do the concrete sockets for the buttresses, suggesting at one stage they were present.

Two images on Geograph have been uploaded showing the site at Nedge Hill, now demolished. The tower is the recognised wooden-clad design, with wooden buttresses and a brick blast wall. Also in the images is a glimpse of the associated crew quarters and probable generator building.

From the ruins on site, as well as photographs from the two existing sites with their ancillary buildings still standing, I have been able to recreate the design of this building. It was constructed out of brick, with a concrete render and four brick buttresses, suggesting the walls were single-skin. There appear to be two main living rooms, most likely with accommodation for a small crew of RAF personnel to maintain a 24 hour watch. To the rear of the building is a probable generator room to charge batteries or run the receivers in the direction-finding tower. A single stove seems to have been provided, and with what seems to be a coal store on the side of the building, it is likely this was a solid fuel burner.

On most sites, all that remains today is the brick blast wall, or the concrete base of the direction-finding tower. With eight equal sides, and associated buttresses, there was a gap to allow the operator to enter the tower. Given the side of the tower it was likely only one operator would have been required and they would have worked the receiver set from ground level, behind the protection of the blast wall.

Ballyhalbert Fixer Station, County Down

Garway Hill Fixer Station, England

Hillquoy Fixer Station, Orkney

Leckhampton Hill HFDF Site, Gloucestershire

Pocklington Fixer Station, Yorkshire

I was sent a series of images from Geoff Harris who contacted me through the website about a previously unknown station at Pocklington in Yorkshire. They show the RAF personnel of the fixer station standing outside the buildings on the perimeter of the airfield. The first image from around 1943 they are in front of the accommodation building, and in the second image they are in front of the blast walled direction finding enclosure. Geoff also includes a RAF aerial photograph from 1946 in which the station is visible. But what is apparent from these images, is the blast wall enclosure is square, and inside it appear four masts; what I believe is an Adcock direction finding array. A variation from the evidence I have seen at other stations, and something I will investigate further.

Multiple Tower Sites

There are a number of sites identified that have more than one direction-finding tower, some as many as four towers located around the central crew admin building. This may have increased the capacity of the site to track aircraft in the area, and given their location in the south of England this is likely. With each fixer station being able to tune into a single frequency, this would facilitate tracking of a fighter squadron (one time-slot per section within the squadron, up to four). With the air battle raging overhead, it must have been necessary to have the capacity to track more than one squadron at a time, so the construction of multiple receiver towers at single sites.

• Farningham, Kent
• Habchester, Northumberland
• Halnaker Hill, East Sussex
• Minster, Kent
• Wittersham, Kent

Conclusion

These unique sites spread across the country were short lived but their importance can’t be underplayed. They filled a vital gap in capability to not only identify but guide our fighters towards the enemy aircraft. It was a rudimentary system devised when manpower was stretched, and equipment was limited, and while the role of monitoring the direction-finding stations would have been relatively safe and away from enemy bombing, it must have been solitary and tedious at times.

I would like to visit and record as many of these sites as I can in an effort to further document and understand them. Keep your eyes peeled, and I hope you can make use of the list of locations I have already been able to record.

Amendments since publication

• Multiple tower site at Minster, Kent added. Information from the Pillbox Study Group MikeofDorset, visible on 1946 aerial images. All structures now removed.
• Scarborough HFDF image obtained from the Britain from Above archive confirming the presence of a site on the headland. Added to database.
• Further sites identified from the extended Defence of Britain archive, listed as removed but with remains still evident: Duxford and Lyme Regis; no trace of sites at Whaddon, Farningham, Hyde, Southwell, and Weston; with possible sites at Inverness and Portsoy.
• Site at Shropham added, most likely serving Snetterton airfield. All remains removed.
• Photographs of the site at Ballyhalbert added.
• Millfield HFDF site location discovered and submitted by J Towill.
• Bury Down site added thanks to a contribution on Facebook by C Counter.
• St Davids site added, accidentally omitted from first version.
• Coverack site identified by S Davies on Facebook.
• Thank you to Ian Brown to has contributed valuable context and information to the operation of these sites, as well as highlighting a number of new sites to the database.
• C Jackson identified the site at Hoxne as a FV10 equipped VHF direction finder installed at Royal Naval Air Stations.
• Geoff Harris submitted evidence of a now removed site at Pocklington.
• I visited the site at Hillquoy on Orkney. An album has been uploaded to Flickr and linked to this page.
• Continued work with Ian Brown has increased the likelihood these sites are the remains of V.H.F. and not H.F.D.F. sites. Archived documents from the National Archives have been listed and plotted in Part III of this research.

Contribute

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  Signals Volume V, Fighter Control and Interception, Chapter 1, The Biggin Hill Experiments, pp15-16. Air Historical Branch, 1952