the Battle of the Ruhr, opening with a raid on Essen on 5 March that produced rather poor results in spite of proper marking. The next major raid against the Krupp factory in Essen on 12/13 March was somewhat more successful, followed by a mix of raids that met with very different results. By May, however, the technique was tuned and a series of very large raids, typically with 500 to 800 bombers, demonstrated increasingly successful results. Among these was a late-May raid on Dortmund that caused the Hoesch steelworks to cease production, and a late July raid on Krupps that Goebbels stated had caused "complete stoppage of production in the Krupps works". Analysis of the results demonstrated that the number of bombs that fell on their targets doubled from the pre-Oboe era. German countermeasures Oboe missions were clearly identifiable to German radar operators; the aircraft would start some distance north or south of the target and then approach it on an arcing path they referred to as "Boomerang". Although the operators quickly became accustomed to these aircraft, actually arranging an interception of the high-flying and high-speed aircraft proved extremely difficult. It took the Germans more than a year to decipher the operation of the system, led by the engineer H. Widdra, who had detected the British "Pip-squeak" Identification friend or foe [IFF] system in 1940. The first attempt to jam Oboe took place at the end of August 1943 during an attack on the Bochumer Verein steelworks in Essen. A system set up at the Maibaum tracking station in Kettwig broadcast false dot and dash signals on the 1.5 m band, hoping to make it impossible for the pilot to figure out if they were at the right position. This was the same technique that the British had used against German systems during The Blitz. Unknown to the Germans, the Oboe system had already moved to the microwave-frequency 10 cm Oboe Mk. II, but the British kept broadcasting the older signals as a ruse. The failure to jam Oboe remained a mystery until July 1944, when the older signal was incorrectly set to mark one target while a pathfinder perfectly marked another. The Germans quickly surmised that there was another signal or system in use. The Germans were well acquainted with the British microwave systems in the 10 cm area, but in April 1944 the RAF had already introduced