The Type 277 was a surface search and secondary aircraft early warning radar used by the Royal Navy and allies during World War II and the post-war era.

It was a major update of the earlier Type 271 radar, offering much more power, better signal processing, new displays, and new antennas with greatly improved performance and much simpler mounting requirements.

While it was still being fitted to escort ships during 1941 and 1942, great strides in technique were being made in cavity magnetron, waveguide, antenna design and general electronics.

While the 271 offered performance to about 3 miles (4.8 km) against a U-boat and had to communicate with the commanders by voice tube, the 277 was limited only by the radar horizon and its display could be read directly on the bridge on larger ships.

At this time, the entire field of microwaves was in its infancy, the requisite cavity magnetron and sealed crystal detectors having been developed only a year earlier.

[4] During the same period, the art of waveguide and feed horn design was improving rapidly, replacing the older coaxial cables and dipole antennas that had significant losses at microwave frequencies.

[10] In tests between December 1942 and February 1943 it demonstrated its ability to detect a Bristol Beaufighter at 80 miles (130 km) while flying at 8,000 ft altitude.

[7][b] The performance at Orne was so impressive that the 277T was ordered into production at Allen West & Co., Metrovick and the Marconi Company for Army and Royal Air Force (RAF) use as Chain Home Extra Low (CHEL).

The waveguide was attached at the bearing opposite the vertical motor, running up and over the top of the reflector and then straight down the front of it with a hole in the middle to provide the feedhorn.

Having the waveguide run down the entire face meant it could be connected to the reflector at the top and bottom, providing excellent mechanical support.

The later, developed by the Air Ministry's Telecommunications Research Establishment (TRE) used a cursor[c] that allowed the operator to pick out one "blip" on the display and read the elevation and use the range measurement to solve for the altitude.

There was no concern about performance; the new transmitter and greatly improved antenna gain meant the system was providing about 25 times the beam energy.

When combined with the new magnetron from the 277 and the use of a waveguide, it was calculated that it would offer almost exactly the same performance as the much larger 273Q parabolic antennas, and thus provide an excellent replacement for the failed 272.

It was noted that the original 4 foot (1.2 m) by 10 inches (250 mm) cheese antenna would have limited vertical coverage, which meant it would have difficulty tracking aircraft at higher altitudes.

[20] A further change, seemingly trivial in retrospect, was to eliminate the cylindrical radome and replace it with a single sheet of perspex sealing the front of the cheese.

The waveguide on Tuscan was slightly longer than normal at 70 feet (21 m), but any loss in performance was offset by a switch from brass to low-loss copper.

To improve their performance, the Navy grew increasingly interested in a radar that was dedicated to the continual plotting of the location of aircraft at close range, when their relative position to the ship was changing rapidly.

[20] The main problem with using the existing systems for aircraft tracking was that the new parabolic antennas had very narrow beams that were not suitable for continually scanning to the high angles that the TI role desired, up to 70 degrees above the ship.

This left a problem for destroyers and small cruisers; these ships lacked the room to mount both the AUJ for surface search and AUR for TI, and would have to make do with one or the other.

In comparison, the 273Q had demonstrated the ability to detect a destroyer when only the top parts of its mast were above the horizon, and the 276 was expected to provide similar range.

This was improved on by a small but important modification; instead of placing the antenna in a radome, the system was protected simply by covering the front of the cheese with a perspex plate, as in the case of the 293.

In tests in Firth of Clyde and the North Channel, Copper Cliff was able to reliably detect a surfaced submarine at 11 miles (18 km) when only part of the conning tower was visible.

Against a Schnorkel, which was only 3 feet (0.91 m) high, the range was reduced to about 5.5 miles (8.9 km), and when closer to the surface it became lost in the radar clutter of the waves.

[27] Tests of the 277T in late 1942 were extremely promising, and later that year a Staff Requirement was issued for a dedicated Fighter Direction radar, or FD.

[28] Initial calculations showed that a cheese-type antenna 12 feet (3.7 m) wide with a 17 inches (430 mm) gap between the top and bottom could provide the same gain as the 277's parabolic dish.

To fill the performance gap, plans were made to use a magnetron running at 2 MW and having a longer 5 us pulse length, thus producing ten times as much power as the existing systems.

[32] As testing of the 277T began to turn in superb results, the British Army adopted it, almost without change, as Radar, Coast Defence, Number 1, Mark VI, or CD No.

[33][i] When the Army introduced its first Coast Defense radars, the Air Ministry took a number of them and placed them on high towers so they could see low-flying aircraft.

In service, it became clear that the accuracy of the Type 7's lobe switching system for height measurement was not optimal, and using it also required the power of the transmitter to be split in half and thus lowered range when used.

[42] The entire equipment system was packaged similar to the later 271P and Q models, consisting of a single cabinet with two large boxes on the bottom and middle, and a much smaller unit on top.