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ALLIED SEARCH RADARS
(By Dave Saxton)

Royal Navy Warning Radars.

Type 279. The first radar used on Royal Navy warships was Type 79 with the first sea trials conducted in early 1939. It operated on a wave length of 7.5 meters. Type 279 was an improved version. The antennas consisted of a send antenna, and receive antenna, each placed at the mast head of the foremast and the mainmast, and rotated together.

Although Type-279 was originally designed primarily for both surface surveilence and air warning, in practice it was used mainly for air warning, and became in effect an air warning radar. In the Pacific, late war, the Type-279 was still highly prized for extra long range air warning.

Type 281. The 281 was designed as follow on to the 279 operating instead on a wave length 3.3 meters in late 1940. It was decided to use the 281 as a long range air warning system, and keep the 279 as a long range surface search set. Meters wave length radars have a unique ability to propagate over the horizon. The antenna arrays were made duplexing, or able to both send and receive, so the 279 antenna was the one on the head of the foremast and the 281 antenna was at the head of the mainmast. Range resolution was 450 meters.

According to Derek Howse, some British warships retained the Type-279 without duplexing, and did not have duplexing Type- 279 and Type-281 at the same time. To the trained eye the indvidual ships of the KGV class could be identified from the radar antennas on the mast heads. KGV had Type-279, but others had Type-281 instead.

Type 286. There was need for providing radar to smaller warships, so the Air to Surface Vessel radar operating on a wave length 1.5 meters was adopted for surface ships and became Type 286. The early models did not have a movable antenna. Nonetheless, the 286 was used to attack a few U-boats in late 1940, although it was not very useful system to use vs. U-boats.

The above metric radars were virtually useless in the war against the U-boat for two primary reasons. The first reason was due to the fact of the very long wave lengths; narrow beam widths could not be obtained without absolutely huge antenna arrays. Beam widths on the order of 20* were the order of the day. The second problem was sea clutter. Ocean swells can reach heights of about 1/2 the wave length of metric radars, so that the swells act as relecting dipoles. This makes it virtually impossible to detect a surfaced sub in ocean swells. Decimetric, and centimetric wave lenghts don't have this problem. It's likely that had a centimetric alternative not been developed during 1940, that a surface search set based on the 50cm gunnery radars would have been developed.

Types 271 & 273. The 271 was the first centimetric radar operating on 10cm. It was rushed to service with sea trials conducted in March and April 1941, and about 30 sets in service by Oct 1941 mainly on convoy escorts. It could not only detect surfaced subs to range of 3,500 yards, it could detect just the periscopes. The antenna was two 1/2 parabolic wave guide windows stacked on top of each other (called cheese because they looked like a cut open rounds of cheese) and placed inside of a plastic cylinder called a lantern. The beam width was about 6* (or about the same as a contemporary Seetakt ). Range resolution was 225 meters. A few capital ships were also equipped with the 271 by late 1941, because the 273 for cruisers and battleships was later coming. The 273 differed in the antenna array. The antenna was two 90cm dishes placed side by side on a common stabilized mounting. The greater antenna gain resulted in greater range than the 271 (26,000 yards vs. a BB for the 271). Both the 271 and the 273 had to make due with A scope displays until mid 1943 when PPI displays were retrofitted. The late model 273Q on DoY detected the Scharnhorst at a range of 23 nm.

Types 277 and 293. In 1944 large warships began to be equipped with the 277. The 277 had a single dish on a stabilized mounting that could tilt the dish to find the height of aircraft. The 277 also used a special PPI called a Skiatron. It was for use in tracking aircraft and surface search, but was biased to the air detection role. In 1945, the 279 was finally replaced by the centimetric 293. The 293 was therefore used in tandem with the 277, with a surface search bias. The 293 used a small blade reflective antenna similar to the American SG. It could be sited on the head of the foremast displacing the 279. A KGV class battleship in 1945 typically had the 293 on the head of the foremast and the 277 antenna on the starfish of the foremast, with an improved 281 still at the head of the mainmast.

The use of Types 277 and 293 together as a team came from the intention of having a new "warning" radar that would be useful vs surface targets and air targets all in one compact unit. A new generation 10cm warning radar to replace Types 273 and 271 came as Type 276 in late 1944 on a few destroyers. The problem with the 276 was that it didn't pickup and track high flying aircraft well, although it's surface performance was very good. It was then decided by committe to replace (or rather convert) the 276 to Type 293 with a new tilted antenna design. The 293 eventually became very good at tracking high flying aircraft, but it wasn't good at tracking low flying aircraft.

The 277 was good vs both low flying aircraft and surface targets, but recieved much critism, because of the need to stop continous rotation in order to find the height. With the antenna stopped, and focusing on one target, the panoramic view on the PPI was temporarily lost. Moreover, pressed into the role of fighter direction late in the war on carriers, it couldn't do that additional job as well as radar designed just for that role.

USN Search Radars.

CXAM, SC and SK. These are all based on the same basic 1.5 meter wave length radar system design. These radars were built by RCA based on a prototype developed by the Naval Research Lab called the XAF. The XAF was first tested in early 1939, and the Navy was jazzed that it could track shells in flight and spot the fall of shot. Max range at that time was 16,000 yards. CXAM was the first production version, with the first sets going on Pacific Fleet carriers during 1941. It proved valuable during the early carrier battles, and was a major player. The SC utilized a small antenna so it could be mounted on small warships. Of course the use of a small antenna caused it to have poor bearing resolution. The SC was nonetheless used though out the war. The SK used a large antenna array of either a mattress dipole array (the flying bed spring) or a large round dish. The larger antennas and their greater gain produced a significant boost in range compared to the SC. The SC could detect a battleship to 37,800 yards, and the SK extended this to about 50,000 yards. The SK used a PPI display instead of an A-scope. The SK became the main air warning radar for the USN during WWII. Performance figures from USN sources are: bearing resolution 10*, range resolution 500 yards. Iím hesitant to list the range accuracy, because itís seems way out of line at +/- 1,000 yards.

SG. The SG, built by Raytheon, is often described as the most important naval radar of WWII. This is probably a correct assessment at least for the crucial Guadalcanal battles . The SG was a better utilization of the 10 cm wave length technology given by the British, than the Royal Navyís own Type 271, in large part because it used a PPI display from the start. This is the kind of display we see in the movies. The PPI was much easier to read than the abstract display of an A-scope. If one could read a map, one could make sense of the PPI display. A mistake of the first versions of the SG, was that the one PPI was in the radar office and could not be viewed by the shipís command on the bridge, or in a CIC (A mistake the Germans did not make with their Berlin radar systems). A PPI is not as accurate as an A-scope, but the SG was adequately accurate even with the PPI. Range accuracy was +/- 150 yards. The resolution for range was 500 yards (300 yards on the A-scope). The antenna, although small, could deliver a beam width of 5 degrees with a 10cm wave length. Average range was about 40,000 yards. The SG could detect a surfaced sub at 12,000 yards, and a periscope to 4,000 yards. The antenna was a reflective type shaped like a small curved blade, and was small enough that it could be mounted just about anywhere convenient, including the mast head of small warship.

In mid 1943 the SG-I came into service. This version had multiple slave PPI displays, and the power output was boosted to 50kw. I try to avoid placing much emphasis on power output, because by itself itís meaningless. For power output to be meaningful it must be correlated with other factors, such as wave length, band width, min signal to noise ratio, antenna gain, noise temperature, and so forth.

SJ. The SJ was an active radar for use of submarines designed by Bell Labs. It was a very important component of the American submarine campaign in the Pacific. The SJ was first put to sea in late 1942. The SJ worked on a wave length of 10cm and used a PPI display. The antenna was designed by the NRL. The antenna wave guide was designed to be water proof, so it could be left on the sail and it would not need to be stowed if the submarine dived. It resembled a scaled down SG antenna in appearance. The miniscule size of the SJ antenna meant that the horizontal beam width was about twice that of the SG using virtually the same wave length. The horizontal beam width was 9* The range for picking up surface ships was about 10,000 yards. This range performance might not seem good, but it was fairly typical for submarine based radar during WWII. Submarine radars need to use very compact antennas, and they typically can only be operated a few meters above the sea surface.

SD. The SD was a metric air warning radar for use on submarines. It operated on a wave length of 265cm. Accuracy and resolution were not good, but that wasn't required of it. It's mission was to detect approaching aircraft and allow the submarine to dive, before the aircraft could detect the submarine, either by the aircraft's own radar, or visually. The SD found little use in the Pacific, and often boat commanders just ignored it, because the Japanese rarely employed aviation effectively in their ASW activities.

French Efforts.

The French Navy began to be concerned about rumors of radio detection systems by 1939, and the French firm of SFR in Paris began experiments. SFR employed noted researcher H Gutton.

Using a 12 segment magnetron, the first devices produced power outputs of 6 watts at 10 cm wave length, and 10 watts at 16 cm wave length. Using a pulsing technique, the power was up to 50 watts operating on a wave length of 16cm, just before the fall of France in 1940. This set demonstrated an ability to detect a battleship to a max range of 10,000 meters. Gutton tried using an oxide cathode in place of a thoriated tungsten, and that increased the power ten fold to 0.5 kw. As France fell, these test results were transferred to Megaw in England, resulting in Megaw insisting on modifying the British cavity magnetron to include a large oxide cathode.

The Vichy Navy attempted continued clandestine development of the SFR 16cm equipment. Little is known, but some claim that the equipment was eventually able to detect a battleship to a max range of 25,000 meters by 1944. All prototypes were destroyed, along with the documentation, when the Vichy fleet was scuttled.


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