had predicted, it took the Germans two years to complete the development of magnetron based radars. The first to reach operation in early 1945 was the FuG 240 Berlin, an AI radar very similar to the British AI Mk. VIII. By this time the country was on the brink of defeat and Berlin never entered service. A small number were fitted experimentally, one of which was captured by the RAF in a shot-down Ju 88. Several other radars developed from the same basic systems were also introduced but saw limited or no service. One advancement made by the Germans during this period was a new type of antenna using a dielectric to shape the output, known in the UK as a polyrod. Continued developments Improved computers In a separate line of development, the RAF was working on a pair of mechanical computers known as the Air Mileage Unit (AMU) and Air Position Indicator (API), which continually performed dead reckoning calculations, greatly reducing navigator workload. This was fed by inputs similar to those for the Mk. XIV bomb sight, namely the estimated wind direction and speed, with the aircraft heading and speed fed in automatically from the aircraft instruments. The system output was a varying voltage that could be used to drive the Mk. XIV bomb sight. In a development known as Mark IV, H2S was modified to also read these voltages, which offset the centre of the display by an amount proportional to the signals. This would counteract the motion of the aircraft, and "freeze" the display. When initially set up these calculations were never perfect, so some residual drift on the display was normally encountered. The navigator could then fine tune these settings with controls on the display, adjusting them until the image was perfectly still. These values then fed back into the AMU and API, producing highly accurate measurements of the winds aloft. The Mk. IVA used the larger whirligig scanner. None were available by the time the war ended. K band Further improvements in magnetron and receiver design during the war led to the ability to use even shorter wavelengths, and in the summer of 1943 the decision was made to begin development of versions operating in the K band at 1.25 cm. This would improve the resolution by more than a factor of two over the X band versions, and was especially interesting as a system