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Introduction
Following on from two recent activities centred round the two World Wars there has been renewed interest in the topic of wireless direction finding so this new section has been created to record information and references.
It is currently broadly divided into land- and sea- and air-borne use which will be refined as the content develops. As the most recent activity circa 2020 has been the WW2 development of HFDF which includes a brief general introduction this will be the first entry.
High Frequency Direction Finding
Input by Malcolm Mack
Introduction
High Frequency Direction Finding, usually known by its abbreviation as HF/DF or colloquially Huff-Duff, is a type of Radio Direction Finder (RDF) introduced for World War II. High Frequency (HF), nominally 3MHz to 30MHz, refers to a radio band that can effectively communicate over long distances, via refraction (bending) of the electromagnetic waves by the ionosphere. There are also VHF and UHF radio bands.
HF/DF was one of five main technologies, the others being Radar, Sonar (ASDIC), Signals Intelligence (SIGINT) and Nuclear, that proved critical in defeating the Axis Powers.
Early History and WWI
RDF used a radio receiver and antenna system for determining the direction of the source of a radio signal. A direction finder (DF) can be used by an aircraft or ship as a navigational aid. This technique was widely used even before WW1. This was accomplished by measuring the direction (bearing) of at least two transmitters whose locations are already known. When the measured directions from each transmitter are plotted on a map, the intersection of the two plotted lines gives the location of the aircraft or ship carrying the DF. This technique, using the directions to two or more transmitters of known location is called triangulation. Because of the its early use in navigation, the DF was sometimes called a radio compass. Its use as a navigational aid has been almost entirely replaced by more modern navigational systems e.g. GPS.
Early DF systems used a loop antenna that could be mechanically rotated. The operator would tune in a known radio station and then rotate the antenna until the signal disappeared. This meant the antenna was now at right angles to the broadcasting station, although it could be on the either side of the antenna. By taking several such measurements, or using other forms of navigational information to eliminate one of the ambiguous directions, the bearing to the broadcaster could be determined.
In 1907 an improvement utilising induction coils was introduced by Ettore Bellini and Alessandro Tosi that greatly simplified the DF system in some setups. The Bellini-Tosi direction finder (B-T) was widely used on ships, although rotating loops remained in use on aircraft as they were normally physically smaller.
During WW1, Henry Joseph Round who was a Marconi employee before being commissioned, set up a chain of direction-finding stations along the Western Front. Following their success another set of stations were set up in the UK, and these detected the move of the German High Seas fleet from Wilhelmshaven before Jutland in May 1916. A very comprehensive description of a typical Naval Station is here.
Inter-war Years
After the war in 1921 H.J. Round became the Chief Engineer for the Marconi Wireless Telegraph Company, where he was heavily involved with the first wireless broadcasts. He eventually left to form his own consultancy, but was called upon to be a member of the Sonar (ASDIC) steering committee in WWII.
Following WWI, and the work that he had carried out as a commissioned officer during this period, Frank Adcock raised his patent in 1919 on the design of the Adcock array; based on the use of 4 monopoles or dipoles arranged in an orthogonal pattern. This was a method of amplitude comparison to determine the direction of arrival of radio signals.
Further progress on HF transmission was achieved in 1923 and 1924 when Guglielmo Marconi and Charles Samuel Franklin carried out experiments from Poldhu Wireless Station in Cornwall and Marconi’s yacht Elettra off the Cape Verde Islands and Beirut at 3MHz and 12.4 MHz respectively.
Other experimentation in Direction Finding was being carried out by the Admiralty HM School of Signals at Portsmouth, during WWI and through the 1920s and 1930s, with changes in antenna shapes and positioning on various types of warships, submarines and land-based systems. The Marconi Wireless Telegraph Company was fundamental in providing Transmitters and Receivers for these systems. Further research work was being undertaken into ionospheric propagation at the Marconi Research Laboratories by personnel such as T.L. Eckersley. Plus improvements in the HF/DF System.
In 1925 Robert Watson-Watt, whilst working in the Atmospherics Department of the National Physics Laboratory Radio Section at Ditton Park near Slough, made a breakthrough in the use of the Adcock antenna and an oscilloscope. This work was in the study of lightning strikes, but led to progress in the design of HF/DF. Watt and Herd wrote an extensive paper on the system in 1926, but as the publicity showed it was used for lightning strikes, there was a lack of interest, which allowed the concept to be developed in secret in the late 1930s.
Some work on automating the B-T system was carried out prior to the opening of WWII, especially by the French engineers Maurice Deloraine and Henri Busignies working in the French division of the ITT Corporation. The system motorized the indication of the direction. The work was destroyed before the German invasion of France, and the development continued in the USA.
WWII
Waclaw Struszynski was the head of the Direction-Finding Division of the Polish state Telecommunication Establishment in 1939 when the country was invaded. He was evacuated from Warsaw and reached England in 1940. His work for HM School of Signals (later called the Admiralty Signals Establishment) proposed a solution to the problem of signal reflection from the ship’s superstructure, but also introduced ‘sense’ into the antenna. He also led a team that developed a practical antenna, which enabled effective HF/DF systems to be installed on Royal Navy convoy escort ships. Struszynski subsequently worked at the Marconi Research Laboratories at Great Baddow, where he was a consultant in communications research. HF/DF was a valuable part of the Allies’ armoury in detecting German U-Boats and commerce raiders during the Battle of the Atlantic.
During the rush to install the Chain Home radar systems prior to the Battle of Britain, CH stations were located as far forward as possible, along the shoreline, in order to provide maximum warning time. This meant that the inland areas of the British Isles did not have radar coverage, relying on the Royal Observer Corps (ROC) for visual tracking in this area. As the entire Dowding system of air control relied on ground direction, some solution to locating their own fighters was needed. The ROC being unable to definitively provide this information. The expedient solution was to provide HF/DF Stations to tune into the fighter’s radio through triangulation. To aid this a system known as Pip-Squeak was installed in at least two fighters per section to send out a trackable tone.
During the 1930s and 1940s there was an expansion into the VHF and UHF bands for both communications, television and HF/DF techniques.
Specially constructed Y-stations across Britain and a number of countries overseas undertook direction finding on wireless transmissions during both WWI and WWII. Thousands of wireless operators especially during WWII, many of them civilians, but also Wrens, WAAF personnel and members of the ATS, tracked the enemy radio nets up and down the dial, carefully logging every letter or figure. The messages were then sent back to the Admiralty Room 40 in WWI, and Bletchley Park (Station X) in WWII. These intercepts were deciphered, translated and fitted together like a gigantic jigsaw puzzle to produce as complete a picture as possible of what the enemy was doing. There was a station at Marconi Research Laboratories at Great Baddow, the organisation being taken over by the RAF and RN for the duration of WWII. Work continued in this period to improve the HF/DF systems and address other perceived threats.
Input by Roy Simons
Most of the later wartime DF work was done at Baddow in Section E and Section K in which T.L.Eckersley was the prime mover - he invented the spaced frame system that did not need to be installed on a conducting earth or mat. His staff included G.Millington, K.Tremellen and S.B.Smith. G.Isted ran the laboratory work of Section E and experimental work in a large hut in the field with WREN assistance. R.J.Kemp ran Section C with R.F.O’Neill, R.G.Waring, D.J.Fewings and me. Others involved at some time included F.P.Best, J.H.Moon, J.Vickers, S.A.W.Jolliffe, J.S.Forbes. We had a detachment of RAF wireless operator mechanics attached to us to build some equipment etc. We had a number of flight trials over the site which required a lot of night time working.
Earlier DF equipment was designed in New Street over many years and an Adcock system with a Type DFG12 receiver, the latest pre-war version, in which J.F.Hatch was involved, was that used during the war. A later version was the Type DFG24 using a CRT. I developed a VHF automatic version for later detection of V2 missiles in case they had radio guidance but they did not. I do not know if any of the Eckersley spaced frame HF equipments were used in the UK - I know several were used in the desert and in the Far East.
There were many more other developments in progress in these early days at Baddow following on from the research carried out at Broomfield, Hall Street and New Street.
Just to illustrate how important DF continued to be is shown by this extract from Practical Wireless June 1951:
Jet Aircraft Radio at the Festival
A replica of the cockpit of the Comet, the world's first jet airliner, showing its complete Marconi wireless communication and navigational aid equipment will be exhibited in the Transport Pavilion, at the South Bank Exhibition of the Festival of Britain. This Pavilion houses not only exhibits of great historical value, showing the development of transport and communications, but also some of the most up-to-date equipment. . The Comet equipment consists of two high-power high-frequency transmitters, two high- and medium-frequency high-discrimination receivers and two automatic direction finders. Voltage regulators for this installation are also of Marconi design and manufacture.
The Comet is also fitted with Marconi suppressed aerials which do not protrude above the skin of the aircraft but which are built into the structure as an integral part of aerodynamic design. One of the greatest aeronautical advances of today is embodied in this new technique which also embraces the loop aerials associated with the automatic direction finder. All that is necessary to navigate with the automatic direction finder is for one member of a crew to tune the receiver to any transmitting station. Bearings of that station can then be read off, continuously and instantaneously, by members of the crew. Repeater bearing indicators and receiver control boxes can be installed and repeated at any crew position.
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