HIGH-FREQUENCY DIRECTION FINDING, usually known by its abbreviation
HF/DF or nickname HUFF-DUFF, is a type of radio direction finder (RDF)
World War II
HF/DF used a set of antennas to receive the same signal in slightly
different locations or angles, and then used the 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 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
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 considered.
* 1 History
* 2 Description * 3 See also * 4 References * 5 External links
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 determined.
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 mechanical systems tended to be used 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
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
Henri Busignies , working in the French division of the
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 this.
Watt worked at the RAF\'s
Met Office in
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
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
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
Main article: Pip-squeak
During the rush to install the
Chain Home (CH) radar systems prior to
Battle of Britain
The expedient solution to this was the use 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
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 Atlantic .
At first, the UK's detection system consisted of a number of shore
stations in the
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 the war.
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
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 one.
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