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 3 miles (4.8 km) and its periscope alone at 900 yards (820 m).

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.

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.

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 5 centimetres (2.0 in) 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.