Following is an excerpt from the book “The Most Secret War”, with very reveling account of Doc. R.V. Jones, who headed the British Scientific Intelligence during the War World Two. I imagine the facts in this text will surprise a lot of people, just like they did with me.

            That morning (February 28th, 1942) I had agreed to go to the Headquarters of No. Eleven Group at Uxbridge, to advise them what they could do about the German radar, which was detecting the ‘Rhubarb’ fighter sweeps the Group was carrying out in the Pas de Calais. When I arrived I met a party headed by Air Commodore Harcourt Smith in the chair and including Wing Commander ‘Sunshine’ Wells, in peacetime a grammar school headmaster from Gravesend way, but now Chief Intelligence Officer of the Group. My hosts were adamant that the Germans must have developed a new form of radar that could detect bombs in aircraft. When I asked them for their evidence they said that they carried out two kinds of sweep. One included a few Blenheim bombers, which represented the ‘teeth’ while the other was exactly similar, except that it consisted purely of fighters. The main aim of these sweeps was to get the German fighters to come up and fight, in the hope of gradually wearing them down and establishing air superiority. This had at first been fairly successful but now the fighters would only come up if the bombers were present and the problem therefore was how the Germans could detect six or so bombers in the presence of fifty or a hundred fighters. The theory was that somehow the German radar could see the bombs in the aircraft.
    When I assured my hosts that as far as I knew about German radar, or indeed any other radar for that matter, it did not have the ability to achieve such a feat of detection, I was then challenged to say how the Germans could possibly know the difference between the one kind of sweep and the other. There was a silence while I thought, and then such an obvious solution occurred to me that I hardly dared to make it. Finally I said ‘Bombers have not got the speed of fighters. When the fighters are escorting the Blenheims, do they slow down?’ There was a stunned silence broken by Sunshine Wells exclaiming ‘Christ!’ It turned out that when there were no bombers the fighter sweeps were executed at standard fighter speeds, whereas the others were at the slower speeds of bombers so the Germans had a perfectly obvious clue. Thereafter we flew all sweeps at the same speed.
                        *My italics. "The Most Secret War - British Scientific Intelligence 1939-45", R.V. Jones, page 314.

Northolt. Early 1942. Servicing radio of PK-C, AB931.