The FuG 240 "Berlin" was an airborne interception radar introduced very late in World War II by the German Luftwaffe. It was the first German radar to be based on the cavity magnetron, eliminating the need for the large antenna arrays seen on earlier radars, and thereby greatly increasing the performance of the night fighters. Introduced by Telefunken in April 1945, only about 25 units saw service.
The German Luftwaffe was one of the first forces to introduce a airborne interception radar, in the form of the FuG 202 "Lichtenstein" which first appeared operationally in 1942. These units operated at 490 MHz, in the low UHF band with a wavelength of 75 cm. Generally speaking, radio antennas have to be sized roughly to the wavelength, or a fraction of it, so the FuG 202 required large, 32-dipole Matratze (mattress) antenna arrays that projected in front of the aircraft and caused considerable drag. The slightly updated FuG 212 model also operated in the same frequency ranges, and featured the large antenna array.
By 1943 a series of efforts and lucky intercepts had allowed the Royal Air Force to introduce jammers that rendered the early UHF-band Lichtenstein systems effectively useless. Worse, they were in the process of introducing the Serrate radar detector which allowed their own night fighters to home in on the Lichtenstein radars. Over the summer and fall of 1943, the RAF racked up an impressive record shooting down German night fighters.
The Luftwaffe responded by introducing the FuG 220 Lichtenstein SN-2 in late 1943. To avoid RAF jamming, the SN-2 operated on entirely different frequencies, only 90 MHz, or about 3 m wavelength. The SN-2's lower frequency range required enormous eight-dipole Hirschgeweih (stag's antlers) antennas that created so much drag that night fighters equipped with the SN-2 suffered a speed loss on the order of 50 km/h. For some aircraft with already marginal performance, like the Messerschmitt Bf 110, this made them easy targets for British night fighters.
The Lichtensteins was eventually supplanted by the Neptun radar. Based on the same basic technology as the Lichtensteins, the Neptun operated on six frequencies between 158 and 187 MHz, although this unit was only a stop-gap solution.
The Royal Air Force's first Airborne Intercept radars operated in the 1.5 m band and featured similar antenna systems as their later German counterparts. However, the introduction of the cavity magnetron in 1940 changed things dramatically. The magnetron efficiently generated microwaves from a device the size of a coffee tin, lowering operational wavelengths from the meter range to 9.8 cm. This likewise reduced the size of the required antenna system to something on the order of a few centimetres. Instead of simply using a smaller Yagi antenna, the system was paired with a new parabolic dish which allowed for conical scanning. The result was a small, light weight, extremely powerful, long range and easy to read radar. This lead in radar technology was never caught up by the Luftwaffe.
The magnetron was initially limited to aircraft operating over the UK or sea, so that if the aircraft was lost the magnetron would not fall into German hands. However, as the war progressed several new uses for the magnetron were developed, notably ground-mapping systems like the H2S radar. These allowed aircraft to navigate and bomb accurately in any weather. This was obviously of great use to RAF Bomber Command's efforts, and an intense debate broke out over whether or not to deploy it, given the security concerns. In the end the decision was taken to install H2S units, starting with the Pathfinder Force.
The inevitable occurred on 2 February 1943, when a Short Stirling Pathfinder was shot down near Rotterdam. Examining the wreckage, they found an interesting device they called the "Rotterdam Gerät" (Rotterdam Device). They quickly determined it to be a centimetre wavelength radar system, although its exact purpose was unclear. Wolfgang Martini immediately set up a team to understand the new system and devise countermeasures. This work led to the FuG 351 "Korfu" device, a radio receiver tuned to the H2S frequencies that was used to track the Pathfinders in flight.
The magnetron itself was sent to Telefunken, who started work on their own versions of the device and an AI radar based on it. The system they developed was almost identical to its British counterpart, differing largely in the display system. Given the limited number of changes, it is not well explained why it took so long to get into production, over two years. Production units were not ready until the spring of 1945, and were not fit until April, just before the war ended.
The Berlin N-2 model was fit primarily to Junkers Ju 88G-6 night-fighters, behind a plywood dome. This so greatly reduced drag compared to the late-model Lichtensteins and Neptun that the fighters gained back 50 km/h in performance. The radar output 15 kW and was effective against bomber-sized targets at up to 9 km. It's short-distance cutoff at 500 m eliminated the need for a second short-range radar system. The N-3 version used an updated display system that featured a C-scope output that greatly simplified the intercept.
The N-4 was a re-developed version of the Berlin that rotated the antenna in the horizontal plane under a teardrop housing on the top of the aircraft. The result was a 360-degree image of the sky around the aircraft that was presented on a plan position indicator (PPI). This version was later renamed the FuG 244 "Bremen", but never saw production. The Bremen was the first true airborne early warning (AEW) system to be developed, although no production units were produced.
- Power: 15 kW
- Search angle: +/− 55°
- Antenna diameter: 70 cm
- Frequency range: 3,250–3,330 MHz
- Range: 500–9,000m