Producing Window’ in WWII

by Peter Greenhill and Pat Valentine, with input from Alan Hoare and Basil Boden

The Idea

‘Window’ was, on the Allied side, the brainchild of British physicist R.V. Jones, whose responsibility it was to anticipate how the Germans would put science to use in time of war. His work was partly in the area of radar and infra-red aircraft detection – finding ways to confound enemy radar. Radar could be jammed, but jamming had a down-side. The aircraft carrying the jamming equipment could be detected and tracked down by properly equipped enemy fighters. The creation of spurious, misleading echoes, on the other hand, did not involve such risk, so Jones decided to pursue this idea.

He worked out that a small strip of wire or metal, of a length equal to half the wavelength used by the radar station, would resonate to the incoming radio waves and create echoes equivalent to a square sheet of metal, its sides equal in length to half a wavelength. In short, a few hundred strips, released in a bunch and allowed to scatter, would give off echoes equivalent to those of an entire Lancaster bomber. A dense cloud of the strips could act as a kind of smoke screen, effectively masking the presence of actual aircraft on a raid or, even better, leading the enemy’s radar operators to believe they were seeing evidence of major attacks where none existed. Most of the German radar wavelengths that had to be dealt with were about 50 cm (20 inches), so the strips needed to be just 25 cm (10 inches) long, and light enough to fall slowly through the air.

The idea was simple but ingenious – and a test proved that it worked well. It was calculated that, to be effective, some 12 tons of ‘Window’ should be released for raids of a particular magnitude. Huge amounts of the material would have to be produced – and this was where Sun Engraving came in.

The Production

In the fourth volume of The ‘Sun’ at War (published in October 1945), Noel Hunter recounts that, after undertaking various experiments at the request of the Air Ministry, he, along with Charles Cook and Cyril Greenhill, attended a meeting at the Ministry of Aircraft Production during which they were asked to start immediate production of material that Sun code-named ‘Mixture’ (and called ‘Mixture’ in all messages and correspondence thereafter) – but that was, in fact, ‘Window.’ The work was on the Secret List, and in October 1945, details could still not be divulged.

Here are some of them now.

Sun employee Alan Hoare, who was in the RAF during the war, was recalled to the Sun and became closely involved with the production of ‘Window.’ Alan became part of a group that devised the production method, and he then ran a press with a team of six others (between 1943 and 1945) to produce the material. He recalls no special instructions to the staff regarding secrecy, but by then, the Sun was so heavily involved in classified war work that maintaining total secrecy on projects must have been second nature to everyone.

Alan says that a Baker-Perkins rotary gravure press (58-inch) was allocated for the job. In place of the magazine paper normally printed, a web of metal foil was fed in, reels of which weighed about a ton. The foil was printed black on both sides in a 24-page ‘magazine’ format, at a rate of about 10,000 ‘books’ per hour. Drying was carried out by steam-heated drums, with additional air blowing on the web. The ‘books’ were sent to the trimmers in bundles, the fold was trimmed off, and the bundles were cut into 6-inch x 1-inch strips – a format somewhat akin to the strips of coloured paper used in those days to make Christmas paper chains (the final size must have been a refinement of early calculations). The strips were packed into bundles about 2 inches thick by ladies in the finishing department, and the bundles were secured at one end with elastic bands.

Alan believes that Sun was the only firm producing ‘Window’ for the Air Ministry. The firm was notified each time a new batch of ‘Window’ was needed. Final production of the black-inked foil totalled several hundred tons, not counting the many tons of waste material that had to be disposed of after trimming.

The Delivery

Bomber crews were lectured on the proper use of ‘Window’ in late spring of 1943, and Churchill agreed to its use from July 23 on. The first raid using ‘Window’ took place on Hamburg on the night of July 24, with an aircraft loss rate of 1.5 per cent. Previous raids on Hamburg had resulted in a loss rate of 6.1 per cent, so in one night alone, ‘Window’ saved between 70 and 80 aircraft and their crews. After four raids using ‘Window,’ total losses were calculated as slightly under half those of previous, ‘Window’-less, raids on the same targets. In intercepted radio communications from German radar stations, agitated operators could be heard reporting that they had lost sight of their targets, or could make no sense of what they were seeing on their screens.

A fundamental part of the plans for D-Day involved tricking the Germans into thinking that the Allied landings would be taking place east of the Seine. ‘Window’ made this subterfuge possible. Lancaster bombers, flying in rectangular orbits of eight miles by two and very slowly moving the orbits south-easterly while dropping ‘Window’ continuously, caused the enemy’s radar to pick up what looked like a massive sea-borne convoy advancing on Fécamp. (A second ‘convoy’ was similarly sent towards Boulogne-sur-Mer.) The stratagem worked, and the Germans prepared to counter what was in fact a bogus armada, while the Allies made their landings further south, on the beaches of Normandy. In his Official Despatch (in the London Gazette, January 2, 1947), Allied Expeditionary Air Force Commander Sir Trafford Leigh Mallory stated: “ fighter aircraft hindered our airborne operations; the enemy was confused and his troop movements were delayed.”

In a subsequent letter to the Sun, Air Vice-Marshal Sir Victor Tait, K.B.E., C.B., wrote:


Dear Mr Hunter,
I would like to express on behalf of the Royal Air Force our appreciation of the great work done by the Sun Engraving Co., Ltd., in the production of the material which I believe you originally called “Mixture,” and was later known as “Window.”
We set you many difficult and urgent problems in the production of this equipment, and we all have the greatest admiration of the grand way in which all the personnel concerned in your Firm understood it, gave us what we needed, and gave it to us in such a very short time.
This part of the work done by your Company was a valuable contribution to the war effort, and we are very appreciative of the grand job done by all the members of the firm in meeting our needs.
Yours sincerely,
(Signed) V.H. Tait

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Basil Boden has passed on a humorous footnote to this story. A Sun colleague, Violet Withey (Beckley), turned twenty-one on a day when she and her mates were working overtime on what the workers called the “black job.” At some point, she was grabbed by the men working with her, and the “black job” was stuffed down and up her clothes. She was bundled into a pram – a three-sided, two-wheeled cart used to carry books or sections – and was paraded around the firm under a banner announcing to everyone that she was twenty-one that day. She was showered with “black job.” Violet later told Basil that the black had come off, and she was surprised at how black she was – and where!!!

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Most Secret War: British Scientific Intelligence, 1939-1945, by R.V. Jones (Hamish Hamilton, 1978) gives a detailed account of the genesis and use of ‘Window’ during the Second World War, and was used as a reference in the preparation of this article.