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The Burning Blue

Research Notes, page 3

HF and VHF Radio in the Battle of Britain

Lee Brimmicombe-Wood: One vital issue I am trying to get to grips with in my research is how the RAF were able to control their fighters, particularly when the primary form of communication was the short-ranged High-Frequency (HF) radio telephone (R/T). Only late in the battle was this replaced with the longer-ranged VHF set. I asked Dave Thompson and Vic Ludlow of the RAF Signals Museum at Henlow in Bedfordshire for help. Here is the information they were able to supply me. I'd like to thank them very much for their help.

R/T Sets

LBW: In response to a question about the deployment of the TR.1133 VHF radio telephone. To date, my information on VHF deployment had been sketchy, with Wood and Dempster in The Narrow Margin stating that VHF deployment had been delayed after Dunkirk and that only some 16 squadrons had been fitted with the sets by the end of September 1940:

RAF Signals Museum: 'Service trials of TR.1133 took place at Duxford on 30th October 1939, with six Spitfires of No 66 Squadron'. It is reported that: 'An air-to-ground range of as much as 140 miles was obtained at 10,000 feet, and an air-to-air range of over 100 miles'. A few days later, the Chief of Air Staff approved the general introduction of VHF R/T in Fighter Command. The TR.1133, that had been 'hand made' up until then, was put into quantity production, since the TR.1143 was not yet ready.

From a provisioning aspect, no distinction was made between TR.1133 and TR.1143. Early in 1941, following continued delays in the TR.1143 reaching the production stage, a crystal-controlled receiver unit, R1225, was provided for use with the transmitter unit of TR.1133. The combination became known as TR.1133G and H (sic). 'By May 1940 installations of 'hand-made' TR.1133 had been completed in Nos 41, 54, 66 and 611 (Spitfire) Squadrons, and in Nos 17, 32, 56 and 213 (Hurricane) Squadrons'. However, during the month, many fighter aircraft were lost in air battles during the evacuation form Dunkirk, and on 26th May 1940 the squadrons were ordered to change back from VHF R/T to HF R/T ...'

In August 1940, HQ Fighter Command decided to restart the changeover to VHF R/T, beginning with Nos 19, 41 and 54 (Spitfire) Squadrons and Nos 17, 32, 56 and 229 (Hurricane) Squadrons. However, by the end of September 1940, the TR.1133 installation programme was not sufficiently advanced for Fighter Command to take full advantage of the 'superiority of VHF over HF R/T'.

TR.1143. Large-scale production and installation programmes for the TR.1143 were put in hand in 1942.

TR.1430. There is also mention of the need for 8-channels of VHF R/T communication for night fighter aircraft equipped with Airborne Interception (AI) radar. This need was initially met by dual TR.1143 installations. A new, 8-channel equipment (TR.1430) was developed to meet the requirement. By March 1944, 100 TR.1430 equipments had been produced but the number of channels needed had risen to 12. This requirement was met on three night fighter squadrons by the replacement of one of the dual TR.1143s with a TR.1430. Later, Mosquito and Welkin night fighters were equipped with dual TR.1430 sets, giving them 16 channels.

R/T Performance

LBW: In response to questions on R/T performance figures:

RAF Signals Museum: At HF, ranges obtained were largely determined by the aerials used. On a fighter, the HF aerial was a relatively short, non-resonant, fixed wire type (from the top of the short mast to the top of the tailfin). These aerials were inefficient compared to the physically and electrically large aerials that could be used on the ground installations. Thus air-to-air ranges were not great; air-to-ground ranges were considerably larger; ground-to-air ranges could be restricted by poor receivers in the aircraft. At VHF, aerials on the aircraft were of the order of a quarter-wavelength long (at the centre frequency of the band) and were relatively efficient compared to the fixed wire aerials used at HF. TR.1464. Late in WW2, a light-weight, 8-channel, VHF R/T set, the TR.1464, was developed, saving some 50 pounds in weight compared with earlier sets. During flight trials with the TR.1464 in March and April 1944, air-to-air ranges of 175 miles were obtained at 400 feet! Using the T.1131/R.1132 combination on the ground, air-to-ground ranges obtained were 90 miles with the aircraft at 2,000 feet, and 150 miles with the aircraft at 8,000 feet.

References:

Air Publication (AP) 3237; The Second World War, 1939-1945, Royal Air Force; Signals, Volume II, Telecommunications; Issued by the Air Ministry (AHB) 1958.

Confidential Document (CD) 1136 (later AP 1136); The Second World War, 1939-1945, Royal Air Force; Signals, Volume III, Aircraft Radio; Issued by the Air Ministry (AHB) 1956, Chapter 23 pp 609

TR9 Radio

LBW: The RAF Signals Museum later came back with the following additional information:

RAF Signals Museum: The following was found in CD 1116 (later AP 1116); The Second World War, 1939-1945, Royal Air Force; Signals, Volume V, Fighter Control and Interception; Issued by the Air Ministry (AHB) 1952.

There is a report that the TR9 (HF) set fitted to aircraft during 1932 and 1933 gave increased range over that obtained previously - 35 miles air-to-ground and 5 miles air-to-air being obtained.

At the time in question, the sector commander or controller in the operarions room did not attempt to speak to the pilot of an aircraft by radio telephone. Messages to pilots were written on slips of paper and passed to R/T operators for transmission. The R/T operators were specially chosen for their clear enunciation and they sat in sound-proofed enclosures.

In 1934, trials were carried out in which controllers spoke directly to pilots - when the trials proved successful, the practice was adopted in all sectors.

There is a reference to the need to make interceptions at ranges greater than the 35 -40 miles R/T range of the TR9. In January 1937 a requirement for a range of 100 miles with aircraft flying at 5,000 feet had been set out. In the meantime, 'the range deficiency of the TR9 was overcome by using mobile ground R/T relay stations, sited 30 or 40 miles forward of the sector RT station'.

LBW: This last piece of information was the lead I was looking for. A delve in the national archive proved very profitable and the resulting finds clarified a mystery about how RAF Fighter Command were able to control their fighters with short-ranged HF radio.

The Signals Museum added:

RAF Signals Museum: There was a need for increased frequency stability for the TR9. Frequency drift resulted from vibration and temperature changes during flight. In April 1937, crystal control was applied to the TR9; that was then designated the TR9C. Later, to facilitate transmissions for Direction Finding (DF) purposes within a flight of aircraft while still allowing R/T contact to be maintained, a second frequency channel was provided on the TR9; that became the TR9D.

In the history there is some expansion on the problems of trying to get the change-over fron HF sets to VHF sets effected. The proposed VHF set was to be built such that it would be physically interchangeable with the TR9, so that the aircraft fit could be changed from VHF to HF, and vice versa, at short notice!

By 30 October 1939, six Spitfire aircraft of No 66 Squadron at Duxford had been fitted with the TR.1133 VHF set. 'The results exceeded expectations. An air-to-ground range of as much as 140 miles was obtained (with the aircraft) at 20,000 feet and an air-to-air range of over 100 miles.' The TR.1133 was supposed to be replaced in May 1940 by an improved VHF equipment, the TR.1143. The TR.1143 had crystal control of both the transmitter and the receiver (this implies that the TR.1133's receiver had not been crystal controlled).

By the end of 1940, 41 fighter squadrons were completely re-equipped with VHF equipment (the versions of the VHF sets are not given in the history). The AOC-in-C of Fighter Command gave instructions for the remaining squadrons in the Command to be 'changed over to VHF R/T' (sic) by 1 March 1941. There is no information as to whether the change over was completed on time.

Reinforcing Groups

Lee Brimmicombe-Wood: I followed up inquiries regarding radio-telephony with the RAF Museum at Hendon. The Department of Research & Information Services were very helpful and I'd like to thank them for their reply. The Museum provided copies of pages from two AHB histories: CD 1136: Signals Vol. III, Aircraft Radio, and CD 1116: Signals Vol. V, Fighter Control and Interception.

On the subject of Radio Telephony they said:

RAF Museum Hendon: Another AHB monograph "Fighter control and interception" (1952) mentions the need for reinforcements to be brought in from a neighbouring Group: "Special arrangements were made for such occasions to provide R/T communication on the appropriate frequency. The R/T station of each sector which might be reinforced always held quartz crystals for adjacent group frequencies in order that it could control reinforcing aircraft on the frequency to which their sets were normally tuned."

Lee Brimmicombe-Wood: This matches with a reference in Dilip Sarkar's book Bader's Duxford Fighters which on p116-117 includes a first-hand account by Flt Lt. Gordon Batt of 238 Squadron at Middle Wallop being called to reinforce 'the West of London area' in August and September. It's clear from the account that 238 was under the control of an 11 Group controller on these occasions. No details were given as to which controller was being used but my guess would be that Northolt sector control handled this role. This assumes that Kenley and Biggin usually had their hands full and that Tangmere would be too far away to control the fighters.

However the system worked, this is a clear example of the excellent 10 and 11 Group method of close cooperation. However, I'm less convinced that this system was used to control aircraft from adjacent sectors. The key evidence is that the Batt quote is, as yet, the only example I've come across of a squadron under command of a controller that was not their own. I've found no other quotes or references to squadrons operating with controllers from other sectors.

R/T Tenders

LBW: Records in the National Archive finally shed light on the radio network briefly described in CD 1116. The following is the summary of my research.

LBW: One of the problems with Fighter Command's fighter control system in the run-up to the Battle of Britain was the lack of range of the High Frequency (HF) radio-telephone (R/T) system. As sector airfields only commanded squadrons in their own sector this restricted the ability of squadrons to operate. With a range of just 40 miles from ground stations to aircraft, most squadrons would pass out of control as soon as they flew past the coastline. A method for extending the range of fighter control was vital to Fighter Command. (1)

The long-term solution was the manufacture and deployment of Very High Frequency (VHF) communications. This would offer longer range (100-140 miles), greater fidelity and better resistance to jamming. However, problems with the supply of sets meant that VHF was withdrawn from service just before the battle and then reintroduced piecemeal part way through.

Another solution, intended to help control fighters toward bombers attacking convoys offshore, was the deployment of ‘sub-controllers' to Chain Home Low (CHL) radar stations. Through the summer of 1940, experiments at Foreness (and also the Chain Home (CH) stations at Bawdsey, Canewdon and Dunkirk) were conducted in which a controller would track enemy raids directly on a CHL radar and guide squadrons to intercept via a direct R/T link. This met with some success but the effort to equip CHL stations with the telephone landlines needed for control didn't get under way until September and October 1940, as the battle began to wane. (2)

The interim solution was to extend the range of fighter control through a system of radio relays for the HF system, and it was this that saw sterling service during the Battle of Britain, ensuring sufficient radio coverage of the battle zone. In 1934 a system of radio relays was proposed that would link sector airfields with mobile transmitter-receivers. Initial experiments were successful and an order were placed for the system.

The mobile transmitter-receivers were known as ‘R/T Tenders' and comprised a truck chassis mounting the radio equipment deployed some 30-40 miles from the sector airfield. The radio would be linked by high-quality telephone landline to the sector control room, permitting two-way communication between the air and ground. Because the infrastructure was already in place, most R/T Tenders were deployed close to the sector's Direction Finding (D/F) stations. (3)

The original order was for 16 R/T Tenders. A minute of 15/11/38 notes the intent to supply a minimum of one tender per sector with 12 Group receiving 2 per sector (an allocation questioned by Sholto Douglas). Eventually, a figure of 2 R/T tenders per sector was settled upon, with some additional tenders to be purchased for reserve and training. In addition, some sectors used telephone lines to their satellite airfields to carry R/T relay instructions, though this tended to jam-up the airfield's regular operation.

A letter of 13 Feb 1939 outlines the deployments of tenders to existing D/F sites. The following is a list of deployments in southern England, by sector. (4)

TANGMERE – Cuckfield

BIGGIN HILL – Great Wakering, Wittersham

KENLEY – Marden, Lewes

HORNCHURCH – Teynham, Tolleshunt D'Arcy

NORTH WEALD – Steeple, Sudbury

DEBDEN – Debenham

DUXFORD – Watton, Stow-Upland

NORTHOLT – Chalgrove

FILTON – Cucklington (also listed as Penselwood)

The Operations Records Book (ORB, Form 504) for Debden notes on 3/10/39 that they arranged a tie-line link to the Duxford R/T Tender at Watton. They also had a landline to Martlesham aerodrome for control of the squadron there. One R/T tender was, prior to the Battle, moved around between Martlesham, Oulton Broad near Lowestoft, Wattisham and Kirton-in-Lindsey. Eventually a line for this tender was put in at Carlton Colville. (5)

According to the file of requests for landlines Kenley was actively using Hawkinge airfield as an R/T relay, though the overload this put on the telephone lines severely restricted other telephone traffic. There's also a suggestion that Biggin Hill may have also used Hawkinge to relay R/T traffic. (6)

Following a request of 21/4/40 Tangmere was connected to the RDF station at Poling and used its R/T to relay commands. Tangmere also used the R/T tender at Newlands Corner near Guildford, but the use of this was transferred over to Northolt sector following a request of 22/6/40.

Middle Wallop firstly used the telephone link to Warmwell to relay R/T but because this overloaded the line, eventually a dedicated tender was set up a quarter-mile from the airfield. On 9/7/40 requests for circuits to R/T tenders at Baydon and Stoner Hill are made.

Filton was due to have a relay fitted to South Cerney airfield, but this was eventually moved to nearby Bibury, which was supposed to be working by 3/8/40. An R/T relay at “Blumsdon” (possibly Blunsdon near Swindon?) was shut down on 12/7/40.

On 23/7/40 an R/T relay line was requested from North Weald to Rochford. However, on 16/8/40 the relay line to Sudbury was asked to be ceased to make circuits available for teleprinter and other lines into Martlesham. Presumably the Sudbury location was not in use.

On 25/7/40 the relay line from Kenley to Lewes was asked to be ceased since the R/T tender had established a new location at Shoreham.

Notes:

(1) The range of 40 miles is the oft-quoted ground to air range with the TR9D High Frequency R/T set. However, range could vary with conditions and atmospherics and ranges up to 70 miles were occasionally recorded.

(2) The experiments with R/T control of squadrons from RDF stations is described in AIR 16/186 ‘R/T RDF Interception'.

(3) The description of the R/T Tender and its deployment can be found in AIR 2/1379 ‘Home Defence Review 1934 – Extension of the range of R/T communication with fighters'.

(4) These deployments are confirmed as completed in AIR 20/222 ‘R/T and RDF Station: provision'. The full list, including the Scottish deployments, is in AIR 2/1379.

(5) ORB RAF Station Debden: AIR 28/187

(6) The files AIR 2/7128 and AIR 2/7129, ‘Landline Requirements Fighter Command' are invaluable. Through requests for telephone lines and correspondence it is possible to trace the hurried installation of new lines for R/T relay traffic up to and into the Battle. The impression given is that except where new cable needed to be laid, new circuits could be implemented in days, and on a few occasions within hours.

VHF R/T

LBW: The National Archives also unravelled how the VHF changeover was effected.

LBW: As many histories of the Battle of Britain note, the RAF's fighter squadrons transferred over from High Frequency (HF) radio-telephones  to Very High Frequency (VHF) sets over the summer of 1940. However, there has been little detail published on how the changeover was effected. (1)

The original plan was to have all squadrons completely equipped with VHF by September 1940. A return for 30/4/40, prior to the Battle for France, shows the following squadrons had converted:

66 (Spit); 56 (Hurr); 54 (Spit); 213 (Hurr); 611 (Spit); 17 (Hurr); 32 (Hurr); 41 (Hurr).

The following were next in line to be converted:

222 (Hurr); 151 (Hurr); 65 (Spit); 23 (Blen); 229 (Hurr); 504 (Hurr).

The TR.1133 VHF set was designed to plug into the same space as the TR9D HF set so a squadron could in theory be converted quite quickly. On 17/5/40 17 Sqn. was converted from VHF back to HF in around five and three-quarter hours. (2)

But conversion was not simply a case of changing around the radios in the aircraft. An infrastructure of ground stations and radio relays had to be constructed. As outlined on 23/2/40, the plan was to have a transmitter-receiver at each sector airfield. Permanent ‘fixed' outlying relays would also be built to provide maximum coverage for each sector. Each sector would then be equipped with a single ‘mobile' R/T station (much like the HF R/T Tender) for deployment at the sector's forward airfield. In addition there would be ‘backward stations' to cover the wilds of Scotland and the west of England, special stations for Wick, the Orkneys and Shetlands, a reserve for each sector and additional stations for training.

The original list of fixed relay sites of 2/2/40 is as follows:

FILTON – (Quantock Hills) Exmoor

MIDDLE WALLOP – St. Alban's Head

TANGMERE – Beachy Head

KENLEY/BIGGIN HILL – Heathfield

BIGGIN HILL/WEST MALLING – (Manston) Hawkinge

HORNCHURCH – (Southminster) Manston

NORTH WEALD – (Walton-on-Naze) Martlesham

NORTHOLT – (Southend) Southminster

DEBDEN – (Martlesham) Halesworth

DUXFORD – Stalham

(Where a location is parenthesized, that is the originally planned location. This was changed on 12/4/40 to the second listed location.)

In spite of these plans, the early VHF sets were hand built and there were problems getting the set into mass production. The result was a shortage of equipment. In one personal note Lord Beaverbrook states that manufacture of sets had been affected by air raid warnings and bomb damage to stocks had also contributed to shortages. A memo of 9/5/40 states that it is doubtful whether the mobile R/T stations would be available by the end of September.

Then on 1/6/40 Air Chief Marshal Hugh Dowding issued a letter withdrawing the VHF sets from service and ordering all squadrons to convert back to HF. The reasoning was that casualties during the Dunkirk evacuation had left Fighter Command perilously short of VHF sets. There were barely enough spares to equip and maintain 4 squadrons. So in all it was best to fall back on the tried and tested HF system until sufficient sets, spares and mobile stations were available.

This was a blow, because the VHF sets, with their range, anti-jam features and fidelity were far superior to HF. They were particularly important for night fighting in which control over wide areas of airspace were necessary.

The change in policy comes in a memo of 18/8/40 which recommends a number of squadrons re-equip to VHF as soon as possible. It notes that the change-over assumes that sufficient personnel are available so that “HF and VHF can be operated in conjunction”. HF was to be maintained in order that dummy messages could be passed on HF frequencies. Also Squadrons should remain ready to change back to HF sets at short notice (presumably if the VHF net failed for some reason).

By 8/9/40 the following squadrons are said to have re-equipped:

CATTERICK: 219 (Blen), 54 (Spit); DIGBY: 29 (Blen); WITTERING: 23 (Blen), 229 (Hurr); DEBDEN: 17 (Hurr); NORTH WEALD: 25 (Blen), 46 (Hurr), 249 (Hurr); HORNCHURCH: 600 (Blen); MIDDLE WALLOP: 604 (Blen)

The following are in the process of re-equipping:

DEBDEN: 73 (Hurr); HORNCHURCH: 41 (Spit); BIGGIN HILL: 79 (Hurr); DUXFORD: 19 (Spit)

The reference to 229 Sqn is odd here, since it disappears off the roster for a while and doesn't reappear as equipping with VHF until a letter of 13/11/40. A return for ‘October 1940' which appears to be for the beginning of the month lists the following single-seat squadrons that have converted:

CATTERICK: 54 (Spit); WITTERING: 1 (Hurr); DUXFORD: 19 (Spit); DEBDEN: 17 (Hurr); NORTH WEALD: 46 (Hurr), 249 (Hurr); HORNCHURCH: 41 (Spit)

The following are on the ‘to be converted' list:

DUXFORD: 242 (Hurr); DEBDEN: 73 (Hurr); HORNCHURCH: 603 (Spit); BIGGIN HILL: 92 (Spit); KENLEY: 501 (Hurr), 253 (Hurr); NORTHOLT: 1 RCAF; TANGMERE: 213 (Hurr), 602 (Spit); MIDDLE WALLOP: 609 (Spit)

A damaged file fragment presumably from later in October tells us that 213 and 609 Squadrons completed conversion. The above figures don't match entirely with a letter of 7/10/40 listing commencement dates for conversion of 10/10 for 92, 242, 238 and 73; 17/10 for 603, 501, 253 and 602; and 24/10 for 1 RCAF, 303 and 87 Squadrons. This is ignoring the ongoing desperate pleas for re-equipment of the night-fighting Defiants.

The most definitive statement for VHF in Fighter Command for October 1940 comes from a memo of 22/10/40 which lists the following:

CATTERICK: 54 (Spit), 600 (Blen); DIGBY: 29 (Blen); WITTERING: 1 (Hurr), 266 (Spit); DUXFORD: 19 (Spit), 242 (Hurr); DEBDEN: 17 (Hurr), 73 (Hurr), 25 (Blen); NORTH WEALD: 46 (Hurr), 249 (Hurr); HORNCHURCH: 41 (Spit), 603 (Spit); BIGGIN HILL: 92 (Spit), 66 (Spit); TANGMERE: 23 (Blen), 213 (Hurr), 602 (Spit); MIDDLE WALLOP: 238 (Hurr), 604 (Blen), 609 (Spit)

(In an almost identical return of 29/10/40, 611 (Spit) and 151 (Hurr) at Digby are said to have been converted.) The 22/10 memo also states the following are being fitted with VHF:

BIGGIN HILL: 141 (Defiant); KENLEY: 501 (Hurr), 253 (Hurr)

The following squadrons are ‘earmarked' for conversion:

NORTHOLT: 302 (Hurr), 615 (Hurr)

By the letter of 13/11/40 almost all fighter squadrons have been converted. The letter notes the following are about to be fitted: 145 (Hurr), 605 (Hurr) and 229 (Hurr). This leaves only 74 (Spit), 222 (Spit) and 264 (Defiant) unconverted by the end of the Battle of Britain.

Notes:

(1) Much of this account was drawn from the files AIR 2/2946 ‘COMMUNICATIONS: VHF in the Royal Air Force 1938-1940' and AIR 16/185 ‘VHF Equipment Policy'.

(2) Operational Records Book, RAF Station Debden: AIR 28/187


The Burning Blue copyright © Lee Brimmicombe-Wood, 2006. All rights reserved.