Junkers Ju 52. By the next summer, the experimental set-up had been developed into a prototype known as the Darmstadt, which offered a range accuracy of at , not nearly enough for gun laying. Attitudes changed in late 1938, when a full development contract was received from the Luftwaffe. The resulting system, known as the FuMG 62, as well as the prototype system FuMG 39T Darmstadt were demonstrated to Hitler at Rechlin in July 1939. The Telefunken team developed an accurate system based on a klystron microwave tube operating in the range of —an extremely short wavelength for the time—with a pulse length of 2 microseconds, a peak power of and a pulse repetition frequency (PRF) of 3,750 Hz. It had a maximum range of about and was accurate to about in range. Würzburg used a paraboloid dish antenna, which could be "folded" along the horizontal midline for travel on a wheeled trailer. The system was first accepted into service in 1940 and this basic layout were delivered. Operational models Several versions of the basic Würzburg system were deployed over the course of the war. The first, Würzburg A, was operated manually and required the operators to pinpoint the target by maintaining a maximum signal on their oscilloscope display. Since the signal strength changed on its own for various reasons as well as being on or off target, this was inaccurate and generally required the use of a searchlight to spot the target once the radar had settled on an approximate position. Nevertheless, one of the first Würzburgs in service directly assisted in the shooting-down of an aircraft in May 1940 by orally relaying commands to a flak unit. An experimental Würzburg B added an infra-red detector for fine tuning, but in general these devices proved to be unusable and production was discontinued. Würzburg C featured lobe switching to improve aiming accuracy. It sent the signal out of one of two slightly off-center antennas in the middle of the reflector, with the signal being switched rapidly between the dipoles. After slightly delaying the signal from one of the dipoles, the returns were sent to an oscilloscope display. The result appeared as two closely separated blips which the operator attempted to keep at the same height on the display. This system offered much faster feedback on changes in the target position. Changes in signal strength, due to changes in the reflection