High-frequency direction finding, usually known by its abbreviation
HF/DF or nickname huff-duff, is a type of radio direction finder (RDF)
introduced in World War II.
High frequency (HF) refers to a radio band
that can effectively communicate over long distances; for example,
between U-boats and their land-based headquarters. HF/DF was primarily
used to catch enemy radios while they transmitted, although it was
also used to locate friendly aircraft as a navigation aid. The basic
technique remains in use to this day as one of the fundamental
disciplines of signals intelligence, although typically incorporated
into a larger suite of radio systems and radars instead of being a
HF/DF used a set of antennas to receive the same signal in slightly
different locations or angles, and then used those slight differences
in the signal to display the bearing to the transmitter on an
oscilloscope display. Earlier systems used a mechanically rotated
antenna (or solenoid) and an operator listening for peaks or nulls in
the signal, which took considerable time to determine, often on the
order of a minute or more. HF/DF's display made the same measurement
essentially instantaneously, which allowed it to catch fleeting
signals, such as those from the
The system was initially developed by
Robert Watson-Watt starting in
1926, as a system for locating lightning. Its role in intelligence was
not developed until the late 1930s. In the early war period, HF/DF
units were in very high demand, and there was considerable
inter-service rivalry involved in their distribution. An early use was
RAF Fighter Command
RAF Fighter Command as part of the
Dowding system of
interception control, while ground-based units were also widely used
to collect information for the Admiralty to locate U-boats. Between
1942 and 1944, smaller units became widely available and were common
Royal Navy ships. It is estimated HF/DF contributed to 24%
of all U-boats sunk during the war.
The basic concept is also known by several alternate names, including
Cathode-Ray Direction Finding (CRDF), Twin Path DF, and for its
inventor, Watson-Watt DF or Adcock/Watson-Watt when the antenna is
1.1 Before HF/DF
1.3 Battle of Britain
1.4 Battle of the Atlantic
3 See also
5 External links
Radio direction finding
Radio direction finding was a widely used technique even before World
War I, used for both naval and aerial navigation. The basic concept
used a loop antenna, in its most basic form simply a circular loop of
wire with a circumference decided by the frequency range of the
signals to be detected. When the loop is aligned at right angles to
the signal, the signal in the two halves of the loop cancels out,
producing a sudden drop in output known as a "null".
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 that the
antenna was now at right angles to the broadcaster, although it could
be on either side of the antenna. By taking several such measurements,
or using some other form of navigational information to eliminate one
of the ambiguous directions, the bearing to the broadcaster could be
In 1907 an improvement was introduced by Ettore Bellini and Alessandro
Tosi that greatly simplified the DF system in some setups. The single
loop antenna was replaced by two antennas, arranged at right angles.
The output of each was sent to its own looped wire, or as they are
referred to in this system, a "field coil". Two such coils, one for
each antenna, are arranged close together at right angles. The signals
from the two antennas generated a magnetic field in the space between
the coils, which was picked up by a rotating solenoid, the "search
coil". The maximum signal was generated when the search coil was
aligned with the magnetic field from the field coils, which was at the
angle of the signal in relation to the antennas. This eliminated any
need for the antennas to move. The Bellini–Tosi direction finder
(B-T) was widely used on ships, although rotating loops remained in
use on aircraft as they were normally smaller.
All of these devices took time to operate. Normally the radio operator
would first use conventional radio tuners to find the signal in
question, either using the DF antenna(s) or on a separate
non-directional antenna. Once tuned, the operator rotated the antennas
or goniometer looking for peaks or nulls in the signal. Although the
rough location could be found by spinning the control rapidly, for
more accurate measurements the operator had to "hunt" with
increasingly small movements. With periodic signals like Morse code,
or signals on the fringe of reception, this was a difficult process.
Fix times on the order of one minute were commonly quoted.
Some work on automating the B-T system was carried out just prior to
the opening of World War II, especially by French engineers Maurice
Deloraine and Henri Busignies, working in the French division of the
US's ITT Corporation. Their system motorized the search coil as well
as a circular display card, which rotated in sync. A lamp on the
display card was tied to the output of the goniometer, and flashed
whether it was in the right direction. When spinning quickly, about
120 RPM, the flashes merged into a single (wandering) dot that
indicated the direction. The team destroyed all of their work in the
French office and left France in 1940, just before Germany invaded,
and continued the development in the US.
It had long been known that lightning gives off radio signals. The
signal is spread across many frequencies, but is particularly strong
in the longwave spectrum, which was one of the primarily radio
frequencies for long-range naval communications. Robert Watt (the
"Watson" was not added until 1942) had demonstrated that measurements
of these radio signals could be used to track thunderstorms and
provide useful long-range warning for pilots and ships. In some
experiments he was able to detect thunderstorms over Africa, 2,500
kilometres (1,600 mi) away.
However, the lightning strikes lasted such a short time that
traditional RDF systems using loop antennas could not determine the
bearing before they vanished. All that could be determined was an
average location that produced the best signal over a long period,
incorporating the signal of many strikes. In 1916 Watt proposed
that a cathode ray tube (CRT) could be used as an indicating element
instead of mechanical systems, but did not have the ability to test
Watt worked at the RAF's
Met Office in Aldershot, but in 1924 they
decided to return the location to use for the RAF. In July 1924 Watt
moved to a new location at
Ditton Park near Slough. This site already
hosted the National Physical Laboratory (NPL) Radio Section research
site. Watt was involved in the Atmospherics branch, making basic
studies in the propagation of radio signals through the atmosphere,
while the NPL were involved in field strength measurements in the
field and direction finding investigations. NPL had two devices used
in these studies that would prove critical to the development of
Adcock antenna and a modern oscilloscope.
Adcock antenna is an arrangement of four monopole masts that act
as two virtual loop antennas arranged at right angles. By comparing
the signals received on the two virtual loops, the direction to the
signal can be determined using existing RDF techniques. Researchers
had set up the antenna in 1919 but had been neglecting it in favour of
smaller designs. These were found to have very poor performance due to
the electrical characteristics of the
Slough area, which made it
difficult to determine if a signal was being received on a straight
line or down from the sky. Smith-Rose and Barfield turned their
attention back to the Adcock antenna, which had no horizontal
component and thus filtered out the "skywaves". In a series of
follow-up experiments they were able to accurately determine the
location of transmitters around the country.
It was Watt's continuing desire to capture the location of individual
lightning strikes that led to the final major developments in the
basic huff-duff system. The lab had recently taken delivery of a
WE-224 oscilloscope from Bell Labs, which provided easy hook-up and
had a long-lasting phosphor. Working with Jock Herd, in 1926 Watt
added an amplifier to each to the two arms of the antenna, and sent
those signals into the X and Y channels of the oscilloscope. As hoped,
the radio signal produced a pattern on the screen that indicated the
location of the strike, and the long-lasting phosphor gave the
operator ample time to measure it before the display faded.
Watt and Herd wrote an extensive paper on the system in 1926,
referring to it as "An instantaneous direct-reading radiogoniometer"
and stating that it could be used to determine the direction of
signals lasting as little as 0.001 seconds. The paper describes
the device in depth, and goes on to explain how it could be used to
improve radio direction finding and navigation. In spite of this
public demonstration, and films showing it being used to locate
lightning, the concept apparently remained unknown outside the UK.
This allowed it to be developed into practical form in secret.
Battle of Britain
Battle of Britain 
Main article: Pip-squeak
During the rush to install the
Chain Home (CH) 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 over the
British Isles did not
have radar coverage, relying instead on the newly formed Royal
Observer Corps (ROC) for visual tracking in this area. While the ROC
were able to provide information on large raids, fighters were too
small and too high to be positively identified. As the entire Dowding
system of air control relied on ground direction, some solution to
locating their own fighters was needed.
The expedient solution to this was the use of huff-duff stations to
tune in on the fighter's radios. Every Sector Control, in charge of a
selection of fighter squadrons, was equipped with a huff-duff
receiver, along with two other sub-stations located at distant points,
about 30 miles (48 km) away. These stations would listen for
broadcasts from the fighters, compare the angles to triangulate their
location, and then relay that information to the control rooms.
Comparing the positions of the enemy reported by the ROC and the
fighters from the huff-duff systems, the Sector Commanders could
easily direct the fighters to intercept the enemy.
To aid in this process, a system known as "pip-squeak" was installed
on some of the fighters, at least two per section (with up to four
sections per squadron).
Pip-squeak automatically sent out a steady
tone for 14 seconds every minute, offering ample time for the
huff-duff operators to track the signal. It had the drawback of
tying up the aircraft's radio while broadcasting its DF signal.
The need for DF sets was so acute that the
Air Ministry initially was
unable to supply the numbers requested by Hugh Dowding, commander of
RAF Fighter Command. In simulated battles during 1938 the system was
demonstrated to be so useful that the Ministry responded by providing
Bellini-Tosi systems with the promise that CRT versions would replace
them as soon as possible. This could be accomplished in the field,
simply by connecting the existing antennas to a new receiver set. By
1940 these were in place at all 29 Fighter Command "sectors", and were
a major part of the system that won the battle.
Battle of the Atlantic
"Super Duff" equipment on the museum ship HMS Belfast. The
circular indicator provides a direct reading of the relative bearing
from-which signals are received - red numerals for to port of the
ship, green for to starboard
Along with sonar ("ASDIC"), intelligence from breaking German codes,
and radar, "Huff-Duff" was a valuable part of the Allies' armoury in
detecting German U-boats and commerce raiders during the Battle of the
Kriegsmarine knew that radio direction finders could be used to
locate its ships at sea when those ships transmitted messages.
Consequently, they developed a system that turned routine messages
into short-length messages. The resulting "kurzsignale" was then
encoded with the
Enigma machine (for security) and transmitted
quickly. An experienced radio operator might take about 20 seconds to
transmit a typical message.
At first, the UK's detection system consisted of a number of shore
stations in the
British Isles and North Atlantic, which would
coordinate their interceptions to determine locations. The distances
involved in locating U-boats in the Atlantic from shore-based DF
stations were so great, and DF accuracy was relatively inefficient, so
the fixes were not particularly accurate. In 1944 a new strategy was
developed by Naval Intelligence where localized groups of five
shore-based DF stations were built so the bearings from each of the
five 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.
It was intended that other groups would be set up in Iceland, Nova
Scotia and Jamaica. Simple averaging was found to be ineffective,
and statistical methods were later used. Operators were also asked to
grade the reliability of their readings so that poor and variable ones
were given less weight than those that appeared stable and
well-defined. Several of these DF groups continued into the 1970s as
part of the Composite Signals Organisation.
Land-based systems were used because there were severe technical
problems operating 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. As ships were equipped, a complex measurement
series was carried out to determine these effects, and cards were
supplied to the operators to show the required corrections at various
frequencies. By 1942, the availability of cathode ray tubes improved
and was no longer a limit on the number of huff-duff sets that could
be produced. At the same time, improved sets were introduced that
included continuously motor-driven tuning, to scan the likely
frequencies and sound an automatic alarm when any transmissions were
detected. Operators could then rapidly fine-tune the signal before it
disappeared. These sets were installed on convoy escorts, enabling
them to get 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 located by radar if still on the surface or
ASDIC if submerged.
From August 1944, Germany was working on the Kurier system, which
would transmit an entire kurzsignale in a burst not longer than
454 milliseconds, too short to be located, or intercepted for
decryption, but the system had not become operational by the end of
Huff-duff aerial (enlarged) on a Pakistani frigate. Note the
arrangement of the four vertical antennas, which form two loops.
The basic concept of the huff-duff system is to send the signal from
two aerials into the X and Y channels of an oscilloscope. Normally the
Y channel would represent north/south for ground stations, or in the
case of the ship, be aligned with the ship's heading fore/aft. The X
channel thereby represents either east-west, or port/starboard.
The deflection of the spot on the oscilloscope display is a direct
indication of the instantaneous phase and strength of the radio
signal. Since radio signals consist of waves, the signal varies in
phase at a very rapid rate. If one considers the signal received on
one channel, say Y, the dot will move up and down, so rapidly that it
would appear to be a straight vertical line, extending equal distances
from the center of the display. When the second channel is added,
tuned to the same signal, the dot will move in both the X and Y
directions at the same time, causing the line to become diagonal.
However, the radio signal has a finite wavelength, so as it travels
through the antenna loops, the relative phase that meets each part of
the antenna changes. This causes the line to be deflected into an
ellipse or Lissajous curve, depending on the relative phases. The
curve is rotated so that its major axis lies along the bearing of the
signal. In the case of a signal to the north-east, the result would be
an ellipse lying along the 45/225-degree line on the display.
Since the phase is changing while the display is drawing, the
resulting displayed shape includes "blurring" that needed to be
This leaves the problem of determining whether the signal is
north-east or south-west, as the ellipse is equally long on both sides
of the display centre-point. To solve this problem a separate aerial,
the "sense aerial", was added to this mix. This was an omnidirectional
aerial located a fixed distance from the loops about 1/2 of a
wavelength away. When this signal was mixed in, the opposite-phase
signal from this aerial would strongly suppress the signal when the
phase is in the direction of the sense aerial. This signal was sent
into the brightness channel, or Z-axis, of the oscilloscope, causing
the display to disappear when the signals were out of phase. By
connecting the sense aerial to one of the loops, say the north-south
channel, the display would be strongly suppressed when it was on the
lower half of the display, indicating that the signal is somewhere to
the north. At this point the only possible bearing is the north-east
The signals received by the antennas is very small and at high
frequency, so they are first individually amplified in two identical
radio receivers. This requires the two receivers to be extremely well
balanced so that one does not amplify more than the other and thereby
change the output signal. For instance, if the amplifier on the
north/south antenna has slightly more gain, the dot will not move
along the 45 degree line, but perhaps the 30 degree line. To balance
the two amplifiers, most set-ups included a "test loop" which
generated a known directional test signal.
For shipboard systems, the ship's superstructure presented a serious
cause of interference, especially in phase, as the signals moved
around the various metal obstructions. To address this, the ship was
anchored while a second ship broadcast a test signal from about one
mile away, and the resulting signals were recorded on a calibration
sheet. The broadcast ship would then move to another location and the
calibration would be repeated. The calibration was different for
different wavelengths as well as directions; building a complete set
of sheets for each ship required significant work.
Naval units, notably the common HF4 set, included a rotating plastic
plate with a line, the "cursor", used to help measure the angle. This
could be difficult if the tips of the ellipse did not reach the edge
of the display, or went off it. By aligning the cursor with the peaks
at either end, this became simple. Hash marks on either side of the
cursor allowed measurement of the width of the display, and use that
to determine the amount of blurring.
Operation RAFTER – remotely confirming that a superhet radio
receiver is listening to a certain frequency
^ a b Bauer 2004, p. 1.
^ "The development of a high-frequency cathode-ray direction-finder
for naval use"
^ "Adcock/Watson-Watt Radio Direction Finding"
^ a b Bauer 2004, p. 2.
^ Pexee le Vrai (16 October 2006). "Le HF/DF (ou Huff-Duff) : Une
Invention Française" [HF/DF (or Huff-Duff): A French Invention] (in
French). Retrieved 18 July 2014. [permanent dead link]
^ a b c d Bauer 2004, p. 4.
^ a b "The Battle of the Atlantic", near the end and at start of next
^ "Robert Watson-Watt", Biographical Dictionary of the History of
Technology, p. 1280.
^ Gardiner 1962.
^ Watson Watt, R. A.; Herd, J. F. (February 1926). "An instantaneous
direct-reading radiogoniometer". Journal of the Institution of
Electrical Engineers. 64 (353): 611–622.
^ Zimmerman, David (2010). Britain's Shield:
Radar and the Defeat of
the Luftwaffe. Amberley Publishing. p. Chapter 10.
^ "High-frequency direction finding"
^ Judkins, Phil (January 2012). "Making Vision into Power".
International Journal of Engineering and Technology. 82 (1). ,
^ Judkins 2012, p. 107 caption
^ Dirk Rijmenants, "Kurzsignalen on German U-boats", Cipher Machines
^ "Naval Radio Operations During World War II".
^ "The Evesdroppers" (PDF). Time Out: 8–9. 21 May 1976.
^ Bauer 2004, p. 7.
^ Bauer 2004, p. 6.
^ Bauer 2004, pp. 6-7.
^ Bauer 2004, pp. 14-15.
^ Bauer 2004, p. 16.
^ Bauer 2004, pp. 17-19.
Bauer, Arthur O. (27 December 2004). "HF/DF An Allied Weapon against
German U-Boats 1939–1945" (PDF). Retrieved 2008-01-26. : A
paper on the technology and practice of the HF/DF systems used by the
Royal Navy against U-Boats in World War II
Gardiner, G. (15 February 1962). "Radio Research at
Ditton Park - II:
1922–1927". Radio Research Organization Newsletter (10).
Beesly, Patrick (1978). Very
Special Intelligence: The story of the
Admiralty's Operational Intelligence Center in World War II. Spere.
deRosa, L. A. "Direction Finding". In Blyd, J. A.; Harris, D. B.;
King, D. D.; et al. Electronic Countermeasures. Los Altos, CA:
Peninsula Publishing. ISBN 0-932146-00-7.
Williams, Kathleen Broome (1996-10-01). Secret Weapon: U.S.
High-Frequency Direction Finding in the Battle of the Atlantic. Naval
Institute Press. ISBN 1-55750-935-2.
Royal Navy High Frequency Radio Direction Finding, WW2