The Type 271 was a surface search radar used by the Royal Navy and allies during World War II. The first widely used naval microwave-frequency system, it was equipped with an antenna small enough to allow it to be mounted on small ships like corvettes and frigates, while its improved resolution over earlier radars allowed it to pick up a surfaced U-boat at around and its periscope alone at .
The prototype, 271X, was fitted to HMS Orchis in March 1941 and declared operational in May. Small numbers became available during the year, with about thirty sets in operation by October. The design spawned two larger versions, Type 272 for destroyers and small cruisers, and Type 273 for larger cruisers and battleships. The 272 was not considered successful and not widely used. The 273 differed in having larger and more focused antennas, providing higher gain and thus longer range. This proved very successful and was widely used.
Improved versions, known alternately as Q models or Mark IV, were introduced in early 1943. These had a more powerful 70 kW magnetron for greater range and added a plan position indicator (PPI) display which eased the task of arranging interceptions. The near-simultaneous arrival of ASV Mark III radar, huff-duff, Type 271 and new breaks into the German's Naval Enigma codes swung the Battle of the Atlantic decidedly in favour of the Royal Navy. Later that year, the 273Q aboard HMS Duke of York found the German battleship Scharnhorst at night, leading to its destruction during the Battle of the North Cape.
By the late-war period, improved versions of all of these designs were introduced. Originally known as the Mark V models, in March 1943 these were renamed Type 277, 276 and 293. These new models were retrofitted as ships came in for servicing and were widespread by late 1944. Type 271Q models remained in service on a number of ships in the post-war period, generally passing out of service with the ships that carried them.
History Background The Royal Navy learned of Robert Watson-Watt's radar experiments in 1935 and began exploring the use of radar for naval uses very quickly. In contrast to the Air Ministry, which had no formal electronics establishment at the time, the Navy's Experimental Department in Portsmouth was a powerhouse in electronics design and was able to quickly develop a series of radars for naval use. In 1938, their Type 79 radar was the first naval radar to enter service.
At the time, the only high-power radio frequency electronics operated in the shortwave bands, with wavelengths measured in metres. Existing valves (vacuum tubes) could operate at an absolute maximum of 600 MHz (50 cm wavelength), but operation anywhere near this range resulted in very low efficiency and output power. Most efforts worked on much longer wavelengths, several metres or more, where commercial electronics for shortwave broadcasts already existed.
For a variety of reasons, antennas have to be a certain size relative to the wavelength of their signals, with the half-wave dipole being a common design. This meant that the radar antennas of this era had to be metres across to have reasonable performance. The prototype Type 79X, which was fitted experimentally to the minesweeper HMS Saltburn in October 1936, used a 4 m wavelength that required the antennas to be strung between the ship's masts. It could only be aimed by turning the entire ship. To improve power, a version with an even longer 7 m wavelength was developed for HMS Sheffield that provided between 15 and 20 kW of power. Its antenna could be rotated, but was enormous and heavy.
Magnetron
In February 1940, John Randall and Harry Boot built a working cavity magnetron, which soon produced 1 kW of power at a wavelength of only 10 cm from a device about the size of a shoe box. A half-wave dipole for this wavelength was only long, and could easily be fitted to almost any ship or aircraft. It represented an enormous leap in performance, and microwave radar development by all of the forces began immediately. While the magnetron solved the problem of generating short-wavelength signals with high power, that alone does not make a complete radar system. One also needs a radio signal detector that can operate at equally high frequencies, cables capable of carrying that signal to the antenna efficiently, and a host of other developments.
The Navy was in a particularly good position to take advantage of the magnetron, as part of their Experimental Department was the Valve Laboratory. In 1939, the Valve Laboratory was put in charge of the Committee for the Coordination of Valve Development (CDV), leading the development of new valve technology for all of the UK's armed forces. The Valve Laboratory led development of the tunable reflex klystron that provided the needed intermediate frequency signal for a superheterodyne receiver that operated at microwave frequencies, while the Telecommunications Research Establishment (TRE), the Air Ministry's research arm, introduced a silicon-tungsten crystal detector that generated the appropriate high-frequency signals for the reflex klystron.
By July 1940, samples of all of these devices had arrived at the TRE's experimental shops, along with more powerful magnetrons working between 5 and 10 kW. Herbert Skinner cobbled together the various parts on a table to produce the first working microwave radar. This used the updated NT98 magnetron that produced 5 kW of power. They demonstrated it by having someone ride a bicycle in front of the device while holding a metal plate. This test demonstrated the new radar's ability to pick up targets even when they were very close to the horizon, something previous designs could not do due to reflections off the ground that the tightly focused microwave signal avoided. Development of production radars using this basic design was undertaken immediately by the Air Ministry and Admiralty.
From 1940 the possibility of air attack on the Experimental Department was considered serious, but it was not until late that year that a move was initiated. In March 1941, the Experimental Department became the Admiralty Signal Establishment (ASE), a name it retained for the rest of the war period. In August, the ASE moved to Lythe Hill House in Haslemere, closer to London.
Development While the Navy's radar development was making great strides, operational use in the Battle of the Atlantic only began in 1941. Through the summer and autumn of 1940, a period after the Fall of France later called the First Happy Time by Germans, British losses in the North Atlantic rose to unsustainable levels.
A report on the state of the ongoing U-boat war in September noted that 70% of all successful attacks by U-boats were made at night and on the surface. This was possible because Asdic could not detect a surfaced submarine, and the radars available at that time were too large to be fitted to the most common escorts. During this wide-ranging "Trade Protection Meeting", the call for "An efficient radar set for anti-submarine surface and air escorts must be developed" was approved by the Prime Minister and given the highest national priority.
In October 1940 a team from the ASE under the direction of Stenhard Landale was sent to the TRE's labs in Swanage to study their lash-up devices. By this time the TRE had two systems in operation and the second could be mounted on a trailer. When tested on the sea-side cliff near the labs, this radar successfully detected small ships in Swanage Bay. The Navy team developed their own version of the prototype, known only as "Apparatus C", which was tested for the first time on the trailer on 8 December 1940.
The antenna, consisting of two parabolic reflectors, worked well on the top of a cliff, but would not work well nearer to the surface as it would be in the case of a mounting on a ship. In this case the low angle between the antenna and waves on the sea would cause spurious returns, or "clutter", that could hide a target. Herbert Skinner, who was leading antenna development at the TRE, took it upon himself to test the existing designs at various altitudes. During a series of tests between 15 and 17 December, Skinner used the TRE's "Apparatus B" against the small ship Titlark and demonstrated tracking at at the top of the cliff, at the Peveril Point, and at .
Another problem would be keeping the signal from the tightly focused pencil beams on the target as the ship rolled and pitched in the sea. Bernard Lovell suggested the solution would be to use an antenna that had a narrow horizontal beam like the parabolas they were using, but little vertical focussing. This would create a fan-shaped beam spread over about 80 degrees vertically that would continue to paint the target as the radar-carrying ship moved in the waves. The resulting design became known as a "cheese antenna" due to it looking like a section cut from a cheese wheel. A prototype was mounted to the admiralty trailer on 19 December and towed to the beach for testing.
271X Some sense of the urgency of the development program can be seen in the fact that an order for 12 sets had already been placed with the Admiralty's communications laboratory at Eastney (outside Portsmouth). Initially known simply as the Type 271, these models were later referred to as 271X to indicate their prototype status.
The coaxial cables used to carry the signal from the radar to the receiver lost about 22 dB per of length at microwave frequencies. Even at short distances this would result in unacceptable losses. The solution was to place the transmitter and the initial stages of the receiver on the back of the antennas in a metal box, reducing the wiring length to about . However, the local oscillator, a reflex klystron, had to be mounted at the operator's station because it required periodic manual tuning. This limited the maximum distance between the main receiver and the antennas to about . This problem was solved by placing the operator's cabin directly below the antenna.
The only other significant change between the Apparatus C and 271X was a minor change to the antenna, clipping off the outside edges to reduce its width, and slightly increasing the distance between the upper and lower plates from to offset the slight loss of performance from the clipping. This new antenna design was known as Outfit ANA. The antenna was placed on a rotating platform that was manually turned around the vertical axis using a drive shaft that passed through the roof of the radar operator's cabin and ended in a steering wheel taken from an automobile. Because the coaxial cables carrying the signal to the cabin had only so much play, the antennas were limited to about 200 degrees of rotation, unable to point to the rear.
To protect the system from the elements, a cylindrical radome was constructed using perspex, which was at that time the only known microwave-transparent material with sufficient mechanical strength. The system used a number of flat panels held together in teak framing. The resulting arrangement had a very strong resemblance to a lantern, which quickly became its nickname.
Initial trials
At a meeting on 11 February 1941, the prototype order at Eastney was increased with another 12 units, and an order for 100 production sets was placed with Allen West and Company. At the same meeting, the newly completed Flower-class corvette HMS Orchis was set aside for sea trials. The first batch of prototypes had been completed by the end of February 1941, which led to a follow-on order for another 150 sets.