High-frequency direction finding, usually known by its abbreviation HF/DF (nicknamed huff-duff) is the common name for a type of radio direction finding
Radio direction finder
A radio direction finder is a device for finding the direction to a radio source. Due to low frequency propagation characteristic to travel very long distances and "over the horizon", it makes a particularly good navigation system for ships, small boats, and aircraft that might be some distance...

 employed especially during the two World War
World war
A world war is a war affecting the majority of the world's most powerful and populous nations. World wars span multiple countries on multiple continents, with battles fought in multiple theaters....


The idea of using two or more radio receivers to find the bearings of a radio transmitter and with the use of simple triangulation find the approximate position of the transmitter had been known and used since the invention of wireless communication. The general principle is to rotate a directional aerial and note where the signal is strongest. With simple aerial design the signal will be strongest when pointing directly towards and directly away from the source, so two bearings from different positions are usually taken, and the intersection plotted. More modern aerials employ uni-directional techniques.

HF/DF was used by early aviators to obtain bearings of radio transmitters at airfields by rotatable aerials above the cockpit, and during World War I shore installations of all protagonists endeavoured to obtain information about ship movements in this way. The requirement both to tune a radio and rotate an aerial manually made this a cumbersome and slow business, and one which could be evaded if the radio transmission were short enough. Films depicting World War II spies transmitting covertly will sometimes show detection vans attached to patrols performing this activity.

Finding the location of radio and radar transmitters is one of the fundamental disciplines of Signal Intelligence SIGINT
Signals intelligence is intelligence-gathering by interception of signals, whether between people , whether involving electronic signals not directly used in communication , or combinations of the two...

. In the World War II context, huff-duff applied to direction-finding of radio communications transmitters, typically operating at high frequency
High frequency
High frequency radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters . Frequencies immediately below HF are denoted Medium-frequency , and the next higher frequencies are known as Very high frequency...

 (HF). Modern direction finding of both communications and noncommunications signals covers a much wider range of frequencies.

Within communications intelligence (COMINT), direction finding is part of the armoury of the intelligence analyst. Sister disciplines within COMINT include cryptanalysis
Cryptanalysis is the study of methods for obtaining the meaning of encrypted information, without access to the secret information that is normally required to do so. Typically, this involves knowing how the system works and finding a secret key...

, the analysis of the content of encrypted messages, and traffic analysis
Traffic analysis
Traffic analysis is the process of intercepting and examining messages in order to deduce information from patterns in communication. It can be performed even when the messages are encrypted and cannot be decrypted. In general, the greater the number of messages observed, or even intercepted and...

, the analysis of the patterns of senders and addressees. While it was not a significant World War II tool, there are a variety of Measurement and Signal Intelligence (MASINT) techniques that extract information from unintentional signals from transmitters, such as the oscillator frequency of a superheterodyne radio receiver.

Battle of the Atlantic

Along with ASDIC (sonar)
Sonar is a technique that uses sound propagation to navigate, communicate with or detect other vessels...

, Ultra code breaking
Ultra was the designation adopted by British military intelligence in June 1941 for wartime signals intelligence obtained by "breaking" high-level encrypted enemy radio and teleprinter communications at the Government Code and Cypher School at Bletchley Park. "Ultra" eventually became the standard...

 (COMINT) and radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

, "Huff-Duff" was a valuable part of the Allies' armoury in detecting German U-boat
U-boat is the anglicized version of the German word U-Boot , itself an abbreviation of Unterseeboot , and refers to military submarines operated by Germany, particularly in World War I and World War II...

s and commerce raiders during the Battle of the Atlantic.

The origins of the technique lay in the work of the National Physical Laboratory, Slough, in the late 1920s. A team of scientists had been investigating the Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...

 and part of the work involved tracking thunderstorm
A thunderstorm, also known as an electrical storm, a lightning storm, thundershower or simply a storm is a form of weather characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere known as thunder. The meteorologically assigned cloud type associated with the...

s. Due to the very short duration of the lightning flashes the scientists found the normal loop aerial
Loop antenna
A loop antenna is a radio antenna consisting of a loop of wire, tubing, or other electrical conductor with its ends connected to a balanced transmission line...

 was not capable of being rotated quickly enough to obtain a bearing
Bearing (navigation)
In marine navigation, a bearing is the direction one object is from another object, usually, the direction of an object from one's own vessel. In aircraft navigation, a bearing is the actual compass direction of the forward course of the aircraft...

 pinpointing the electromagnetic pulse of the lightning. The laboratory however contained one of the then-new oscilloscope
An oscilloscope is a type of electronic test instrument that allows observation of constantly varying signal voltages, usually as a two-dimensional graph of one or more electrical potential differences using the vertical or 'Y' axis, plotted as a function of time,...

s, and in conjunction with a new aerial design called the Adcock aerial
Adcock antenna
The Adcock antenna is an antenna array consisting of four equidistant vertical elements which can be used to transmit or receive directional radio waves. It was invented and patented by British engineer Frank Adcock in 1919 as British Patent No. 130,490, and has been used for a variety of...

, consisting of two large loop aerials at right-angles aligned north-south, and east-west, which didn't have to rotate, these two innovations allowed an instantaneous indication of the direction of any signal's source. This equipment formed the basis for "Huff-Duff".

The Royal Navy
Royal Navy
The Royal Navy is the naval warfare service branch of the British Armed Forces. Founded in the 16th century, it is the oldest service branch and is known as the Senior Service...

 designed a particularly sophisticated apparatus that could take bearings on the high frequency radio transmitters employed by the German Kriegsmarine
The Kriegsmarine was the name of the German Navy during the Nazi regime . It superseded the Kaiserliche Marine of World War I and the post-war Reichsmarine. The Kriegsmarine was one of three official branches of the Wehrmacht, the unified armed forces of Nazi Germany.The Kriegsmarine grew rapidly...

 in World War II. There were severe technical problems of engineering effective high frequency direction finding systems for operation on ships, mainly due to the effects of the superstructure on the wavefront of arriving radio signals. However, these problems were overcome under the technical leadership of the Polish engineer Wacław Struszyński, working at the Admiralty Signal Establishment.

Many shore based installations were constructed around the North Atlantic and whenever a U-boat transmitted a message, "Huff-Duff" could get bearings on the approximate position of the boat. Because it worked on the electronic emission and not the content of the message, it did not matter that the content was encrypted using an Enigma machine
Enigma machine
An Enigma machine is any of a family of related electro-mechanical rotor cipher machines used for the encryption and decryption of secret messages. Enigma was invented by German engineer Arthur Scherbius at the end of World War I...

. This information was then transmitted to convoys at sea, and a complex chess game developed as Royal Navy controllers tried to manoeuvre wide convoys past strings of U-Boats set up by the Kriegsmarine controllers.

A key feature of the British "Huff-Duff" was the use of an oscilloscope display and fixed aerial which could instantaneously reveal the direction of the transmission, without the time taken in conventional direction finding to rotate the aerial—U-boat transmissions were deliberately kept short, and it was wrongly assumed by the U-boat captains that this would avoid detection of the sender's direction.

Another feature was the use of continuously motor-driven tuning, to scan the likely frequencies to pick up and sound an automatic alarm when any transmissions were detected.

In 1942 the allies began to install Huff-Duff on convoy escort ships, enabling them to get much more accurate triangulation fixes on U-boats transmitting from over the horizon, beyond the range of radar. This allowed hunter-killer ships and aircraft to be dispatched at high speed in the direction of the U-boat, which could be illuminated by radar if still on the surface and by ASDIC if it had dived. It was the operation of these technologies in combination which eventually turned the tide against the U-boats.
The distances involved in locating U-boats in the Atlantic were so great that d/f accuracy was relatively inefficient and in 1944 a new strategy was developed by Naval Intelligence where localised groups of 5 shore based d/f stations were built so that the bearings from each of the 5 stations could be averaged to gain a more reliable bearing. Four such groups were set up in Britain at Ford End in Essex, Anstruther in Fife, Bower in the Scottish Highlands and Goonhavern in Cornwall. Traces of some of the last two groups can still be seen in Google satellite images. It was intended that other groups would be set up in Iceland, Nova Scotia and Jamaica but confirmation is needed. Simple averaging was found to be ineffective and later statistical methods were used. Operators were also asked to grade the reliability of their readings so that poor and variable ones were given less importance than those that appeared stable and well defined. Several of these df groups continued into the 1970s as part of Composite Signals.

Battle of Britain

During the Battle of Britain the Royal Air Force
Royal Air Force
The Royal Air Force is the aerial warfare service branch of the British Armed Forces. Formed on 1 April 1918, it is the oldest independent air force in the world...

 (RAF) deployed an Identification Friend or Foe
Identification friend or foe
In telecommunications, identification, friend or foe is an identification system designed for command and control. It is a system that enables military and national interrogation systems to identify aircraft, vehicles, or forces as friendly and to determine their bearing and range from the...

 (IFF) system codenamed "pipsqueak", a term also used for a small calibre German shell during World War I. RAF fighters had a clockwork mechanism that regulated the broadcast of a signal over an HF channel for fourteen seconds of every minute. Each Fighter Command sector had Huff-Duff receiving stations that would monitor the "pipsqueak" broadcasts and telephone the bearings back to the sector control rooms where they could be triangulated and the squadron's location plotted.

See also

  • Elephant Cage
  • Operation RAFTER
    Operation RAFTER
    RAFTER was a code name for the MI5 radio receiver detection technique, mostly used against clandestine Soviet agents and monitoring of domestic radio transmissions by foreign embassy personnel from the 1950s on.- Explanation :...

    – remotely confirming that a superhet radio receiver is listening to a certain frequency
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.