The top image is from the Historic England archive and dates from around 1951. It is a low level aerial photograph of Leckhampton Hill Fort, above the town of Cheltenham. What is interesting, is the image shows a rare glimpse of a RDF or HF/DF station. © Historic England Archive, Harold Wingham Collection (reference HAW/9390/08), permission number 7614.
The start of the twentieth century was a post-industrial technological arms race, and the traces left behind are like a timeline of pioneering developments in an attempt to outsmart our adversaries. Two of those boundary breaking technologies were wireless telegraphy, and aviation. The story of Radio Direction Finding, or RDF, joins these two in pioneering a technology that would help to win the Battle of Britain.
There is a bit of confusion online surrounding DF during the second world war, and I will try to logically clarify my understanding of the systems without going into too much technical detail. Some of the terms include Direction Finding (DF), Radio Direction Finding (RDF), Cathode Ray Direction Finding (CRDF), twin path DF, Watson-Watt, Adcock / Watson-Watt DF and finally High-Frequency Direction-Finding (HFDF) also known as huff-duff. It is the latter term in relation to locating friendly fighter aircraft that I will focus on in this article, and in this context the terms RDF and HFDF are interchangeable.
Aircraft location finding started with the pre-war system of direction finding, pioneered at Orford Ness. Two high powered transmitters would transmit on the same frequency (288.5 kHz), from different but fixed and known locations, at different time intervals, from a highly directional and rotating antenna (6 rpm). The transmitter would broadcast a continuous signal, and a morse letter V (dot dot dot dash) each time the antenna rotated past north. The design of the antenna also meant that the signal was strongest at the poles (0 or 360 and 180 degrees), and weakest (or null) at 90 and 270 degrees. The navigators on board the aircraft would listen for the (…-) and time how long it would take for the signal to fade to null (due east or west). This time was multiplied by six to give the angle of the aircraft (or receiver) from the transmitter. It was a simple system when there were few aircraft in the sky, and accuracy of up to 1 degree was possible, but in unfavourable conditions this could drop to 20 degrees accuracy.
The next form of direction finding that took place at the same time, was that to identify the bearing of unknown radio transmissions. A number of organisations undertook this form of direction finding; the General Post Office (GPO) was tasked with locating illegal transmitters, this led to the formation of the Radio Security Service (RSS) who sought illegal transmissions from spies operating in England and across Europe. And just as DF technology developed on land, the Royal Air Force (RAF) developed what is often credited (incorrectly) as the forerunner of RADAR to detect enemy aircraft, the Royal Navy (RN) also operated direction finding equipment for the purposes of detecting enemy submarines and surface vessels. All of these methods relied on tuning in to a transmission, establishing a bearing to it, not only from one station, but from multiple stations so the source could be triangulated. On a slight tangent, it led to the myth across German forces that if they kept their transmissions to sub-30 seconds the enemy (British forces) would not be able to obtain a bearing. This was untrue, and wireless DF operators could often pinpoint a transmission within seconds of it coming on the air.
It was as early as the 1930’s that the Royal Air Force (RAF) Air Defence Great Britain (ADGB) Command established the requirement for an early warning system to protect the country from air attack. This initial system consisted of a number of acoustic ‘sound’ mirrors constructed around the south east coast, along with observers from the Royal Observer Corps (ROC). London was the perceived main target, so most protective measures were designed to protect the Capital. Aircraft of the era were relatively slow moving, and noisy, so this method could provide a sufficient early warning to stand-up the anti-aircraft crews who would have formed a number of defensive anti-aircraft gun lines inland. While planes could be detected and tracked as they approached the coast, there was a massive capability gap inland – how do we track our own aircraft sent to intercept the attackers? Radio sets provided ground-to-air and air-to-ground communications, and it was decided to exploit this existing technology to provide friendly force tracking.
Fighters of the early 1940’s were fitted with a battery powered 2-channel high-frequency (HF) radio, the TR9. With a range of around 35 miles, and 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 the purposes of direction finding the friendly aircraft once airborne. This would be done by a network of directional receiver stations dotted around the country, all linked to the Fighter Command network.
The Transmitter-Receiver TR9 consisted of a two-valve radiotelephony transmitter and a six-valve receiver contained in one case. It was designed primarily for use in single-seater fighter aircraft and was intended to provide two-way communication to a range of 35 miles air/ground and 5 miles air-to-air. Frequency coverage was 4300-6000 kc/s (author comment: 4.3-6 MHz) and the entire power supply was derived from an HT dry battery (author comment: HT stands for High Tension, also known as a B battery and was used to provide a high voltage output for vacuum tube radio equipment) and a secondary cell. In single-seat fighters the set was installed behind the pilot’s cockpit The pilot was provided with headphones, microphone and a remote control unit which operated the send-receive switch, receiver tuning and volume control.Imperial War Museum collection, exhibit COM 496.
To enable this automatic transmission, akin to a beacon, a number of modifications were required. A model-D TR9 radio (TR9D) was introduced into service to facilitate the simultaneous transmission of voice and DF signal. In addition was a mechanical timer and remote contactor. Of these two devices, the timer would facilitate the allocation of a transmission time slot of 14 seconds out of every minute, and the contactor would activate the radio for the broadcast of a 1 kHz signal for ground-to-air direction finding. The facility to accommodate four transmission time slots out of every minute would enable up to four fighter sections within a squadron to be tracked, each one allocated a different time slot. Very similar in principle to modern time division digital signals – just much slower.
Shortly after forming up during a scramble, the squadron leaders would be asked to ready their pip-squeak clocks. In the original system this required them to turn the “wind” knob that moved the single second-hand counter-clockwise around the face of the clock. There were up to four sections of aircraft in each squadron, although most squadrons had two or three sections at any given time. Each section had its own position for the hand; red section had the 12 o’clock location, yellow was at 9 o’clock, blue at 6 o’clock and green at 3 o’clock. Once the clocks were properly positioned, the sector controller would initiate a countdown, Synchronize time, 5, 4, 3, 2, 1, mark. At mark, the pilots would turn on the clock, which would start the second hand moving clockwise. When the hand reached the 12 o’clock position the oscillator was automatically turned on, and it turned off again just before the 3 o’clock position, broadcasting for 14 seconds per minute.Westley, Max (October 2010). “Pip–Squeak – The Missing Link”. Duxford Radio Society Journal.
HFDF was redundant in 1942 with the increase in overland radar coverage from the Chain Home stations, and with the widespread use of IFF (Interrogate Friend or Foe). This system broadcast a return identifying the aircraft as friendly when interrogate by the radar, semi-automating the process. It also explains how some sites appear not to have been finished or made operational. The HFDF site at Garway Hill was constructed in 1942 but never became operational, evidence of the demise of the system.
HFDF (Pip-Squeak) Network
The 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 HFDF and Chain Home radar to provide the full picture.
The Chain Home radar system, 1939 – 1940.
Working initially independently, each HFDF station would tune into the transmissions from friendly squadrons. Once a bearing was established, this would have been passed to the immediate sector control room where the bearings from at least two HFDF stations would have been plotted one the sector plotting table. This would allow a grid square to be identified, that could then be passed to Fighter Control.
HFDF Plotting Table example
In Fighter Control sector headquarters, an air picture was gathered. All of this information was passed via telephone from visual observers, CH radar operators and RDF (or HFDF) locations. As information was gathered, tactical pieces were placed on a paper map of the sector and each operator responsible for a particular aircraft type and sector would move their groups of aircraft around the board. A reproduction of this is on display at the RAF Duxford museum.
The British had developed an air defence network that gave them a critical advantage during the Battle of Britain. The Dowding System – named for Fighter Command’s Commander-in-Chief Air Chief Marshal Sir Hugh Dowding – brought together technology, ground defences and fighter aircraft into a unified system of defence. It not only controlled the fighter force, but other elements of the defence network as well, including anti-aircraft guns, searchlights and barrage balloons.The system had a clearly defined chain of command, enabling control of both the flow of intelligence on incoming raids and the communication of orders.Imperial War Museum article on What was the Dowding System?
The capability of the TR9 radio sets limited their air-to-ground range to 35 miles. This had a direct correlation to the design the HFDF network, as no two stations could be more than 35 miles apart without risking gaps in coverage. In actual fact, 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.
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 south west 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.
Mapping the Sites
Using a combination of sources I have been able to identify 26 extant or sites with elements still standing and 10 further demolished sites. Of the 26 standing, 3 appear to have their associated support building still present. The octagonal blast walls exist in many of these, but in some only the concrete base remains. I am sure there are more sites that I have not been able to identify and the list may well grow in the coming months and years.
The main problem with identification has been inconsistent classification of the sites, and the use of a variety of terminology. Perhaps some sites had double roles, especially given that the pip-squeak huff-duff technique appeared to cease in 1942, some may have been adjusted for other RDF work or been incorporated into the Y-station network of wireless intercept stations. Perhaps all HFDF stations fulfilled multiple roles during their operational life.
Below is a table of known HFDF locations where there is still visible evidence of the direction finding tower. While none of the sites have the wooden tower, most have the octagonal blast walls (I call this the the snail enclosure) still standing. It is assumed that all sites would have had the ancillary building nearby, but there are only three known sites where this is still extant; Llannerch-y-medd on Anglesey, Dunfermline in Scotland and Wittersham in Kent. Clicking on the place name should link you to the Google Maps location.
|Fea Hill||Orkney Islands||-2.632982594||59.24540784|
|Halnaker Hill||West Sussex||-0.692528703||50.87947859|
|Heanish||Argyll and Bute||-6.813929281||56.49146935|
|Hill of Swinister||Shetland||-1.18233406||60.44045794|
|Ventnor||Isle of Wight||-1.227357244||50.59476007|
The following list is of sites that are believed to have once sited a HFDF station. This is based from reported evidence, either photographs, aerial images, or local heritage records. Clicking on the place name should link you to the Google Maps location.
|Ronaldsway||Isle of Man||-4.595443903||54.10281492|
There are a couple of locations that I have come across reference to a possible HFDF site, or the reference has not been clear.
Interactive Map of Sites
HFDF Station Design
These sites were simple in construction and layout, with a common design being used across the country. There was the main HFDF 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 a number of reasons for this:
- To support the structure in high winds. Many were located on the coast or on high points in remote areas.
- The tower may be hollow and lack internal support above the level of the walls, therefore external support must be given.
- 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.
There appears to be a variant on the wooden tower structure, and this is a light weight version which appears online when searching for HFDF sites. Most likely of 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 HFDF 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 HFDF 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.
Leckhampton Hill HFDF Site
Garway Hill HFDF Site
Multiple Tower Sites
There are a number of sites identified that have more than one HFDF 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 HFDF 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 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.
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 HFDF 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 Grounp 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.