Introduction
Land and
shipboard high-frequency radio direction finding (HF D/F or Huff Duff), is now
generally recognised as being with radar and code-breaking, a primary factor in
the allied victory in the Battle of the Atlantic against the German and Italian
submarine onslaught against shipping. Perhaps this is evenly balanced?
However little has been published about the background of the work carried out
after 1918 to provide this capability. This Paper sets out to record some of
the various steps taken at HM Signal School, Portsmouth to make this possible. It
seems essential that some details must be given of the transfer of effort from
the improvement of direction finding on the lower frequency transmissions then
in use, to the provision of the same service on signals over 2 MHz. Theoretical
explanations and detailed technical descriptions have been kept to a minimum as
this type of information is available in the references.
The Inter-War Years
This Paper sets out to record some
of the various steps taken at HM Signal School, Portsmouth to make this
possible, despite the limited resources and lack of appreciation of the value
of this operational capability. It does
however seem essential that some details must be given of the transfer of
effort from the improvement of direction finding using the radio transmissions
on lower frequencies, then in use, to the provision of the same service on
signals over 2 MHz. 1.
Use of radio equipment for
communication was well established before the outbreak of war in 1914. Part of the
development of this capability had shown that it was possible to determine with
some accuracy, the relative bearing of a radio transmitter whose signals on
lower frequencies were being received. However further investigation by HM
Signal school before the end of hostilities in 1918 had not provided any
significant results.2 Despite
the lack priority given to other work,
such as production of suitable electronic valves for use in radio transmitters
on shorter wavelengths, work was continued to further develop equipment for direction
finding at higher frequency transmissions.
It had been established that the ‘Bellini-Tosi’ system was best suited to use on board ships. The most
important element in any Radio Direction Finding equipment is the design of the
aerial system used. Between 1920 and 1930 several different designs were used.
These ranged from use of two frame coils fitted within the bridge structure of
large ships which were later replaced by two large loops. One was of rectangular
shape in the fore and aft plane and the other of triangular shape was fitted athwartships.
Both were sited in positions as high as possible in the foremast structure, rigged
at right angles and bisecting each other. The total area covered by these some
what extempore arrangements was about 2,500 Sq. Ft to suit reception of long
wave signals at any distance. It was recognised that effect of adjacent land
masses on transmitter sites and also that of atmospheric conditions at Sun rise
and sunset had to be considered. 1
The various alternatives using
different type of shaped loops can be identified on photographs of many
contemporary warships including HM Battleship WARSPITE 3. In early
trials, some warships these large aerials were at lower level and fitted between
the funnels or in one funnelled ships between the funnel and bridge structure.
The beam loop was suspended from the triatic stay and
connected to stump masts or booms. This arrangement presented difficulties in
the physical handling of the portable lengths of wire which had to be kept
rigid using bottle screws2. Operational efficiency was affected by the
effects of radiation from the main transmitting aerial and bearing errors were
also caused by proximity to fixed structures as well as the movement of boats
or gun mountings. Different sizes at varying heights in and on both masts may
have been used in some ships for conduct of trials3.
The main advantage of fixed rigid loops
was to avoid the problems of rotating such a large assembly. It also would suit
reception of signals transmitted from positions over a long distance. Although
the aerial circuit could be tuned this would require adjustment for each
received signal and introduce large errors since the exact size of each loop
would not be identical. Use of tuned aerial circuits was not continued.
Signals received by aerials of this
design would be of maximum strength when the loops were pointing directly towards
the position of the transmitter and be at their minimum when pointing 90o away from
its actual direction1.T his had to established by turning the ships
through a full circle which was inevitably a long process and could involve
several time-consuming operations involving significant man-power. It follows
that by use of two loops there are two positions in which maximum and minimum
signals from the bearing of the transmitter site are received. In order to
overcome this ambiguity the two loops are connected to a Sensefinder1.
This is an angle measuring device, known as a ‘goniometer’, fed by a separate
input of the same signal. Accuracy of
the determination of the actual bearing of the received signal depends on there
being no mutual coupling between the loops and that the received aerial current
fed from the loops produces a uniform magnetic field1.
A goniometer comprises two fixed
Field Coils, connected to the two loop aerials and a separate rotating Search
Coil fed from a separate Sense aerial. This Sense signal will not vary and is 90o out of
phase with that from the loops. As result of this, the combined signal sent to
the main receiving circuits is modified by movement of the rotating component
There are two positions when the
received signal is at maximum during one complete rotation. When turned
clockwise the true bearing as indicated on the Bearing Indicator Dial is being
received. However due to the combination of the Sense aerial input with that
from the two loops, the reciprocal bearing is identified when the signal
strength increases (See Figure 21 in Reference 1).
Transmissions on frequencies about
600 KHz were a more complex matter because of the effect due to movement of
layers in the ionosphere. Use of a super-heterodyne receiver was more efficient
at higher frequencies. Shore trials at a special establishment exclusively for
aerial design were carried out first. During trials at sea using one of the
loop aerials to instead of a separate Sense Aerial proved unsatisfactory
compared with results obtained when a completely separate wire aerial was used.1.
Amongst the factors considered during these early trials was the effect of
coincident wireless transmissions on trials ships when direction finding was
being carried out and these were forbidden. It was also found that during
reception of signals on higher frequencies, significant bearing errors were
caused by ship’s rigging, ship structure and the movement of gun mountings or
boats. In addition it was discovered that the physical length of the ship also
had an influence on the accuracy of bearing errors on frequencies whose
wavelength was related to the received signals. It was therefore evident that
improvement had to be made to the aerial design in order to reduce bearing
errors although this could not be eliminated on some frequencies2
After 1930 a single rotating loop
aerial of circular design were installed in some warships3 (HMS
BARHAM)
instead of these large loops. It was then realised that smaller ‘diamond
shaped’ rotating doubled loops would be more successful on higher frequencies
than the some what extempore fittings previously used on large ships. Clearly
they would also be much more suitable for fitting on smaller vessel, especially
destroyers. This proved to be most significant step and the fixed Frame coil
design used had two rotatable smaller loops and became a standard fit in TRIBAL
Class destroyers on build before September 19393. The final design
used one of two different designs of aerial units, identified as Frame Coils
S16 and S17. These were of different sizes, that of S16 being 3ft square and
S17 of 4ft 6” square. The size of aerial used depended on the structure of
structure of particular ships mast. The aerial unit was mounted on a pole mast
fitted at the top of foremast of the ship. Both Frame Coil and Pole-mast could
be struck using a separate beam to lower or raise the complete assembly when
necessary to allow passage under a bridge or for replacement.
Much more attention was paid to
further development of H/F DF after 1931 and trials were carried out in HM
Cruiser CONCORD using a rotating aerial fitted on the roof of the spotting top
sited at the top of the foremast. Results using high frequencies revealed
errors in bearing varied on various frequencies were due to ship’s rigging, and
the length of the ship. In addition, efficient earth connections had to be used
for the aerial cabling. It was essential to site the receiving equipment as
near as possible to the aerial loops and the receiving equipment was fitted on
the mast structure for the installation in HMS CONCORD2.
Trials were carried out on an
aircraft carrier, probably HMS
ARGUS in 1938 for further investigation. As this
vessel had no masts or rigging it was possible to establish the effects of
factors other than re-radiation from these. By this time an improved design of goniometer to suit use on higher frequencies had been developed. Installation
of D/F equipment had been completed on many other warships before the outbreak
of war. HM Battlecruiser
RENOWN had fixed loops; HM Battleship
BARHAM retained fixed loops of the mainmast. as well
as the fixed loops on the mainmast. HM Cruiser
AURORA also had two rigid loops whilst HM Cruiser
MANCHESTER was fitted with a
rotating Frame Coil similar to the TRIBAL Class destroyers. Soon after the
outbreak of war HM Cruisers EURYALUS and
BONAVENTURE.had
rotating frame coil aerials3. In all these ships, existing receiving
units, modified to accept separate inputs from two
loops, were used with Sense Aerial Tuners, Heterodyne Receivers and Amplifiers.
The receiving circuits of the these items did not
incorporate provision for elimination of ambiguous bearing indication when used
for higher frequencies because of the errors caused by structure and other factors.
However all low frequency receiver units used a goniometer.
D/F in Submarines
The trials after 1930 included work
on submarines which particularly relied on reception of long wave
transmissions. Initially, frame aerials sited within the conning tower
structure were used and it was found that these good results on low
frequencies. Subsequent trials were carried out using a two fixed loops
attached to the periscope but this was replaced by fixed loops fitted on the
hull of the boat aft of the conning tower. The main aerial used for the main transmitter
was used for sense finding3.
Developments after 1939
Before the outbreak of war in 1939
it had been decided that D/F aerials used for frequencies above 2MHz must be
sited at the highest point possible to minimise errors due to the surroundings.
A new design of Bellini-Tosi aerial using two rigid
loops was required with the Sense aerial positioned at exact centre of its
framework. A counterpoise ‘earth’ was
incorporated in the design of the aerial unit4. By introducing an
adjustable capacitance it was possible to reduce amount of current induced in
the Sense Aerial by the support mast thus reducing the amount of unnecessary
content provided to the receiver for sense finding.
This was a most vital discovery and
enabled an operationally effective system identified as FH1 to be produced in
HM Signal School.2 During March 1941, the
first RN HF D/F Outfit was installed in HM Destroyer HESPERUS as Outfit FH1
after the completion of trials on the new Frame Coil S25 in cruisers4.
Further trials had to be arranged in convoy escorts with priority for fitting given
to those deployed for ant-submarine defence. However the selection of position
introduced conflict between ship designers and users which was hard to resolve
satisfactorily. In both cases, choice of the site to
be used had to take due account of the arcs of fire of weapons and the
positions of other wireless aerials. Mandatory requirements associated with
topside stability and electronic interferences, in the early stages of
development, made it impossible to fit both
radar and Huff Duff on the same ship2. Most escorts had the unit fitted on the mainmast aft
in order to enable a surface warning radar set to be
fitted on the foremast. In post war years the aerial was generally fitted on a
pole mast forward, above the radar aerial. An improved version of FH1 was
introduced during July 1941, identified as Outfit FH3 and installed in HM
Destroyers GURKHA and LANCE2. These equipments all used the ‘aural-null’
method to establish the true bearing of signals but work was in hand to
introduce a method of visual presentation instead. This would enable less
skilled operators to rapidly establish and interpret the true bearing of
signals received. Twenty five escorts and rescue ships had been fitted with FH3
by January 1942.
Distance from
the transmitter was impossible to determine accurately, but operators soon
learned to distinguish HF ground waves from sky waves. Since ground waves could
only be detected 12 to 14 miles from the transmitter, FH3 operators knew when
an intercepted signal represented a dangerously close U-boat. The FH3
incorporated the B21B receiver which had a frequency
coverage from 1 to 20 Mc. By the end of January
1942 25 escorts and some rescue ships were fitted with
an improved version.
The improved version, FH4, used a cathode ray tube display
and was fitted in HM Cutter CULVER, the ex US Navy Coastguard Cutter USS
MENDOSA during October 1941 Production of the FH4 Outfit was developed and
produced by Plessey Company which included a ‘twin channel’ receiver developed
by the company.
In this improved equipment each of the loops were connected
to separate identical amplifying receiver units which could be balanced for
phase shift and gain in order to provide outputs to the deflecting plates of
the cathode ray tube.. This ensured that the resulting display would show when any
phase differences existed in the signals being received from the two loops. If ‘in
phase’ a single line will be displayed, but if the phase from one loop differs
an elliptical trace will be shown., A Sense aerial is incorporated in the
design of the Frame Aerial, S25B as shown in the diagram4 and
ensures that the bearing display is ‘true’5. The visual display of
signals made it easier to distinguish ground wave signals since these were much
stronger and the length of the displayed single line would be greater and
indicate the range of the position from which the transmission was being made.
This was a vital factor in successful defence against submarine attack as
U-Boat transmission were of very short duration and visual display provided in
FH4 was easier for an experienced operator to assess. Although Sky wave signals
reflected from ionospheric layers were of lower strength and made bearing and
range determination unreliable. Ground wave signals at ranges up to 12 miles made
both factors much easier to determine since the signals were much stronger. A
Test facility is included in FH4 design to balance the operation of the two
receiver systems.
right - HD/DF aerial believed on
destroyer escort USS Francis M Robinson DE220 (US Naval Historical Centre)
The first
of 30 production models was available in March 1942 and fitted on board HM
Destroyer LEAMINGTON (Ex USS TWIGGS.) and later that year on a few other convoy
escorts and rescue ships. This outfit because a standard fit in all RN vessels
requiring a direction finding capability and was in use for the rest of the
21st century.2. During service this equipment also proved invaluable
for other use than convoy defence. It was used for tactical purposes to establish
positions of ships in a Task Group which were required to change position
during a current operation. For example, during the
Home Fleet operation in
December 1943 to intercept the German battle cruiser SCHARNHORST HF D/F was
very effectively used to engage the enemy ship
and also cover the passage of vital supply convoy. It also provided a more
reliable method of establishing ship’s position from Shore Beacons when in poor
weather.
Shore stations
The development of suitable equipment for locating the
position of U-Boats which proved to be essential requirement in meeting the threat to
convoys. The bearings of transmissions by U-Boats could be determined by two or
more shore stations and passed to the convoy escort commander. This information
enabled course of convoys to be changed to avoid the threat and for escorts fitted
with HF D/F to search for transmissions and carry out search and destroy
operations by following the bearings indicated by transmissions. Soon after the
more important work by HM Signal School on ship equipment allowed it was
decided that the most suitable aerial design for shore station equipment would be an
Adcock system. Vertical dipoles were used for detection of signals on both low
and high frequency transmissions Initially wire aerials suspended from high
masts were used for earlier outfits but later arrangements used smaller
rotating aerials sometimes in mobile vans, After introduction of the S25 Frame
Coil for FH4 this was used at shore stations and in mobile units for high
frequency requirements.
HF D/F Research and Development in allied countries
In France, pre-war R&D on Huff Duff was carried out at
the Laboratoire Telephonique
in Paris, a subsidiary of International Telephone and Telegraph (ITT).This work
paralleled that
of the British, with whom the French shared electronic secrets until the
Germans overran France in May 1940. Some of the engineers were able to get to
the USA via Spain early in 1941. They were allowed to work on naval HF D/F
equipment, but were not fully trusted.
The US. Navy, experimenting with its own shipboard Huff Duff models
as well as the British FH3, was slow to embrace the new ideas. Like the Royal
Navy, US warship captains preferred radar to ‘Huff Duff’. However by 1942 after
comparative trials between use of FH3 and an a US
designed outfit, a US design, Outfit DAQ was chosen. Over four thousand outfits were produced
and fitted in all new destroyers and escorts.
Once the development of satisfactory equipment for use at
sea on high frequency transmissions had been completed it was possible to begin
work on development of suitable outfits for use an
Very High Frequencies (VHF). There had been a most significant increase in use
of VHF equipment in aircraft and in escorts for communication during convoy
defence and other operations. In aircraft carriers, immediate knowledge of
positions of ships and aircraft was essential and made possible by the
availability these new facilities. By 1945 direction finding outfits for use on
VHF transmissions was available on some aircraft carriers deployed in the Far
East and proved invaluable for navigation and the safety of aircraft during
Fleet operations as well as in inclement weather. Several Dipole aerials arranged
in a circular formation are used for these D/F outfits.
Post War
Development
In the immediate post war years direction finding on Ultra
High Frequencies (UHF) was successful. This band is now in general use for all inter-ship
communications as well as for all aircraft radio communication requirements. As
a natural progression similar progress has been made on the provision of
equipment suitable for use on radar transmissions and is now widely used for
many defence requirements. Since the much shorter wavelengths are used four
individual ‘Horn Type’ aerial units are used for each different radio frequency band. These are installed in the Fore and Aft and
Athwartships planes.
Conclusions
No better
confirmation of the satisfactory development of HF D/F equipment in Great
Britain before and during WW2 is the German admission after September 1945 that
their own work on this particular research had been
inadequate. It was admitted that during WW2 they had experienced great
difficulty in locating the position of U-Boats. Pre war work in UK before
September 1939 had been severely restricted by the resources provided by the Admiralty
despite the recognition by a few senior officers that any work on this subject
was an obvious essential to the conduct of war at sea..
References:
1. ADMIRALTY HANDBOOK
OF WIRELESS TELEGRAPHY (HMSO 1938)
2. AS WE WERE Unpublished
account of the work of HM Signal School 1896-1946
(Later Admiralty Signal
and Radar Establishment.)
3. Photographs showing Aerial Units
4. Drawing of FH4 Frame Coil (S25).
Other Sources:
HITLER’S U-BOAT
WAR by C Blair, Volume 1, Appendix 8 (1999)
SECRET WEAPON by
KM Adams (1996)
RADAR AT SEA by
D Howse (1993)
Acknowledgement:
The Curator, Radar and Communications Museum, HMS COLLINGWOOD,
Fareham, Hants.
