The Spitfire Receiver
by Steven B. Johnston, WD8DAS
A Hamfest Tale
In the late 70s I was a teenage ham with a big appetite for cheap radio gear,
including, of course, military surplus. At a northeastern Ohio hamfest one
summer day I came upon something quite "different". It was a medium-sized
black box with a few interesting knobs and cables sitting perfectly in a matching,
felt-lined wooden transit case. The labels called out Receiver Type R.1147A
with the British "A-crown-M" logo. To my young techie eyes, it was beautiful.
My interest was further aroused by a yellowed scrap of paper inside the case which
identified the cable pinouts and referred to the unit as a "Spitfire Receiver".
My blood began to pound in my veins -- a radio from the famed Spitfire fighter plane?
It couldn't be true! I asked the seller for the story of this radio, but he was selling
it for someone else, didn't really know much, etc -- you know the rap. Trying to
conceal my excitement, I asked the price. My heart sank when he said 50 bucks. Not
a lot of money, of course, but much more than I had in my jeans. But I HAD to have
this radio! I opened my wallet, counted the bills again, cursed my earlier, more
mundane purchases, and offered him all that I had: $18.
He hemmed and hawed, but at this point I laid it on the line. I really wanted that
radio, I would give it a good home where it would be appreciated, and I was willing
to give him every last dollar. I'm sure no Electric Radio reader has ever felt this sort
of compulsion. The seller saw my enthusiasm, and with a smile accepted my offer.
He even returned three dollars of my money so I could buy some lunch! This unknown
soul certainly earned my "Hamfest Hero" award.
Getting it Home
When I got it home, the faded note told me what voltages I needed, so I tapped them
out of another receiver (if memory serves it was a Gonset G-66) and fired it up.
A reassuring hiss sang out of the earphones, and as I tuned around with a cliplead
on the antenna connector I heard TV video buzz. This, plus the funny-looking acorn
tubes, told me it was a VHF receiver -- probably tuning somewhere in the hi-VHF TV band.
My first efforts to research this receiver ran into dead ends, but when I wrote to the
Royal Air Force Museum in London I hit paydirt. The museum was kind enough to send
excerpts from the official war histories and photocopies of the declassified (yet still
marked TOP SECRET - cool!) R.1147 manual. The manual, dated March, 1941, had
markings from the Air Ministry in Whitehall and Radio Aids Navigation School on
H.M.S. "Northney". I learned a lot from those few pages.
History of the R.1147
Great Britain's struggles against German U-boats stimulated the development of a
number of radio systems intended as airborne navigation aids. Two types of
direction-finding equipment were considered: HF direction finding (HF-DF, also
known as Huff-Duff) for tracking and locating enemy ships by eavesdropping on
their routine communications, and VHF/UHF receivers homing on friendly beacons
to on land bases or aircraft carriers.
The possibility of leaving HF direction finding to escort vessels and relying on them
to pass on the intelligence to Royal Navy Fleet Air Arm (F.A.A.) aircraft had been
considered but generally rejected in February 1942. It was apparent from the start
that development of suitable equipment would be challenging, but the urgency was
great and any improvement to U-boat suppression by aircraft was considered worthwhile.
Likewise, the difficulties faced by naval aircraft returning from long reconnaissance
missions called for some form of homing system to reduce losses and improve efficiency.
Royal Air Force Photographic Reconnaissance Units (P.R.U.) faced similar challenges,
and sought similar relief. Standard V.H.F. radiotelephone systems were used for a
sort of primitive "talk-in", but this was of limited effectiveness. Installation of
more advanced homing equipment in P.R.U. aircraft was first suggested in August
1940. P.R.U. aircraft had a special need for a homing device in that the heights at
which they operated added to the difficulties of accurate wind velocity forecasting.
The existing Fighter Command V.H.F. TR.1133 was initially found to be unacceptable
to Headquarters Coastal Command (in charge of P.R.U.), due to the size and weight of the
equipment. A system called "Rebecca" (which I need to find out more about) was suggested,
but the Telecommunications Research Establishment (T.R.E.) suggested as an alternative
a simple beacon with a searchlight beam rotating clockwise, to be used with a stopwatch
and simple receiver in the aircraft; Naval aircraft used the system as an aid to returning
to carriers. The Royal Aircraft Establishment (R.A.E.) designed a basic homing receiver,
the R.1110, then the more advanced R.1147, which was about to be produced for the
Admiralty's Fleet Air Arm.
In March 1941 the Admiralty loaned two R.1147 receivers for tests in P.R.U. Spitfires.
On 29 September 1941 installation of an R.1147 in a P.R.U. Spitfire had been completed
by the R.A.E., ground and air tests of the equipment had been carried out, and the range
and characteristics obtained were considered by the R.A.E. to be satisfactory for
operational use, particularly in adverse weather. Installation in other P.R.U. aircraft
was recommended. However, the Officer Commanding No. 1 P.R.U., at whose unit
the trials had taken place, thought the recommendations were premature, as the
installation was being tested.
A report on the results of further tests was made on 11 October. Successive homing
bearings had varied by as much as 10 degrees and required considerable concentration
by the pilot, which would not be possible when he was flying on instruments during
actual operations. Considerable errors resulted from flying on an incorrect bearing for
only a few minutes when letting down at a high ground speed. Results obtained by a
navigator in a Fulmar aircraft were far more accurate, partly due to the slower speed,
but primarily because of the increased concentration possible by the navigator/observer.
HQ Coastal Command considered that the main purpose of the equipment (to be an aid in
adverse weather) had been overlooked in the later report, but in any case the significant
problem remained: in adverse conditions it was impossible, or at least unwise, to
concentrate on anything other than the normal flying instruments. Nevertheless, HQ
Coastal Command recommended that development should continue and that receivers be
installed in all P.R.U. aircraft. On 27 October it was confirmed that installation was
required in all operational P.R.U. Spitfires, as well as on new production line aircraft.
Further trials of the R.1147 revealed additional weaknesses, and a test of the TR.1133
carried out in a P.R.U. Spitfire by unit personnel on 16 December 1941, Headquarters
Coastal Command suspended provisioning of the R.1147 until further TR.1133 trials.
They were carried out at Duxford in the same month, after which the Officer
Commanding No. 1 P.R.U. reported that installation of the larger TR.1133 was
possible if the number of oxygen bottles carried was reduced from six to three.
On 6 January 1942, installation of the R.1147 were canceled. Use of sector V.H.F.
homing stations adjacent to photographic reconnaissance units was arranged with
Headquarters Fighter Command and P.R.U. aircraft began to use the TR.1133 installation
in April 1942, eighteen months after the original requirement had been raised, during
which time they had been operated with no radio installation of any kind.
The Fleet Air Arm continued use of the R.1147 and the more sensitive R.1147a in
Seafires as well as various reconnaissance aircraft.
Description of the R.1147
The R.1147 was intended to supersede the R .1110 previously employed in aircraft
for the reception of S.F. modulated (a type of modulation I am unfamiliar with)
V.H.F. homing signals from rotating beacon transmitters. The frequency coverage is
from 180 to 220 Mc/s, and the signal range up to 100 miles when the aircraft altitude
permits a line-of-sight path. Provision is made for operation of the receiver either
directly, or from one or two remote control positions. The remote control positions are
equipped for power, volume, and tuning by spline shaft.
The receiver is a superhet, with seven tubes, namely a pentode (type VR95) mixer
with a triode (type VR59) local oscillator, two pentode (type VR95) I.F. amplifiers
operating at 25 Mc/s, a double-diode-triode (type VR55) acting as I.F. detector,
heterodyne oscillator and A.F. detector, and two pentodes (type VR53 or 56) acting
as separate S.F. and A.F. amplifiers. The schematic is shown in figure 2.
The receiver was normally used with a quarter-wavelength fixed whip, though a
previously installed type R.1110 system antenna could be used. Power was obtained
from a separate 36 watt power unit operating off the 12- or 24-volt aircraft battery.
The dimensions of the receiver are approximately 11 in. by 10 in. by 6 in., and weighs
9 lb. The power unit weighs 13 lb. and its dimensions are the same as the receiver.
The weight of the auxiliary equipment, including one or two remote control positions,
is approximately 2 lb. or 4 lb. respectively. A general view of the equipment is shown
in fig. 1.
A Mark 5B stopwatch was included in the system package for timing the beacon signals.
The "Spitfire" R.1147 receiver is one of my treasured receivers. As one of the
museum officials wrote, I have the honor of being probably the only "expert" on the
R.1147 in the Western Hemisphere. That, and three bucks, will buy me lunch at McDonald's.
"There is no cannibalism in the British navy, absolutely
none, and when I say none, I mean there is a certain amount."
- Sir John Cunningham