Allied search radars

[Dave Saxton]

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.

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 recieve, 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.

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 lenghs 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 stablized mounting. The greater antenna gain resulted in greater range than the 271 (26,00 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 retofitted. 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 stablized 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 simlar 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.

[Dave Saxton]

CXAM, SC and SKThese 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. I’m hesitant to list the range accuracy, because it’s 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 Navy’s 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 ship’s 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 it’s 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.

I’ll try and provide some information on the SJ and SD, and perhaps the French radar research when I find some time.

[Bgile]

Thanks for your research!

I'd add that a long range radar needs high power. Even with optimum antenna and all the other factors, you still need high power to get the pulse out there a hundred miles or in some cases much more, and get it's reflection back at a detectable strength. Air search radars are always also high power output radars. Surface search range is horizon limited, so power isn't as important.

The ships using radar in WWII were often designed before it existed. There was usually a big problem finding places to put the displays and dissipate the heat produced by them.

[dunmunro]

Don't forget the incomparable RCN CSC and SW1C series of naval Radars.

[Dave Saxton]

Hi dunmunro,

It's very difficult to find good information on those radars that is also based on solid sources. I tend to have little faith in internet sourced material as well. A good study could be done, but it hasn't been done that I'm aware of. It should be done. From the few words given by a few trusted scholars, the RCN radars were given the code name Night Watchman, and would appear to be rather similar to the USN 1.5 meter equipment. Often superficial examination can be very misleading though. One source mentioned that they used commercially available componants, but so did the CXAM.

Commonwealth nations came up with some rather good radars on their own. Australia came up the portable LW/AW (light weight air warning) 1.5 meter radar that proved very handy in the Pacific, especially where the US Army SCR268 was unavailable or too heavy to move to a remote location. South Africa produced the JB air warning radar working on 3.5 meters.

[Dave Saxton]

...I'd add that a long range radar needs high power. Even with optimum antenna and all the other factors, you still need high power to get the pulse out there a hundred miles or in some cases much more, and get it's reflection back at a detectable strength. Air search radars are always also high power output radars. Surface search range is horizon limited, so power isn't as important.

....

Hi Steve,

No radar engineer would turn down more power if it did not cause other unwanted trade offs, and more and more power is essential as the wave length becomes smaller. In the radar equation, wave length squared is found in the numerator, or if frequency is used, it is found in the denominator. It's easy to see that 3 squared hardly compares with 50 or hundreds squared as a factor. An X band radar will probably have greater antenna gain than a UHF or a VHF radar, but this hardly offsets the effect of wave length numerically. Also in the case of a very short pulse width, it's even more difficult to obtain reflections of detectable strength, as the illumination energy is the power multiplied by the pulse width. For example, if the pulse width is 0.4 us and the power is 50kw, then the energy is 20. This is essentially not much more energy than a radar with 8kw and a pulse width of 2 us, especially when one takes into account that everything else being equal, it takes 16 times more power to effect a doubling in range. A very short pulse width also requires more power, because it requires more bandwidth and bandwidth is found in the denominator. Other factors interact as well. Thus a centimetric radar, especially with a very short pulse width, will require expotentially more power.

During the vacuum tube era it was much easier to obtain more power at longer wave lengths. For example, the Western Electric 316A trasmitting triode could give an max output of 1000 watts (two tubes in push/pull) when operated at 40 cm. At 60 cm the output was doubled, and at 150 cm it was 8 times greater. Much larger vacuum tubes could be used at lower frequencies as well. Meters wave length radars used huge vacuum tubes. All these factors meant that long range air warning radars were usually long wave radars, in addition to the fact that the longer wave radars proved adept at long range aircraft detection in any case.

[dunmunro]

Hi dunmunro,

It's very difficult to find good information on those radars that is also based on solid sources. I tend to have little faith in internet sourced material as well. A good study could be done, but it hasn't been done that I'm aware of. It should be done. From the few words given by a few trusted scholars, the RCN radars were given the code name Night Watchman, and would appear to be rather similar to the USN 1.5 meter equipment. Often superficial examination can be very misleading though. One source mentioned that they used commercially available componants, but so did the CXAM.

Commonwealth nations came up with some rather good radars on their own. Australia came up the portable LW/AW (light weight air warning) 1.5 meter radar that proved very handy in the Pacific, especially where the US Army SCR268 was unavailable or too heavy to move to a remote location. South Africa produced the JB air warning radar working on 3.5 meters.

Well the early RCN radars were not that great, but the amazing thing is that Canada was deploying indigenously designed and built naval radars in fair numbers prior to Pearl Harbour, at a time when radar was rare, even in the USN. Here's a website with a fair bit of info on early RCN radar:
http://www.jproc.ca/sari/

[Dave Saxton]

Another crucial factor, and more specifically for the required power of centimetric radars, is the sensitivity of the reciever. According to Bryan Callick (Noted British radar scientist and a Tech Mission team leader) a reciever for 10cm wave lengths in 1941 required at least 10 times more reflected energy to obtain detection, than a 10cm reciever designed after 1943.

[Dave Saxton]

Well the early RCN radars were not that great, but the amazing thing is that Canada was deploying indigenously designed and built naval radars in fair numbers prior to Pearl Harbour, at a time when radar was rare, even in the USN. Here's a website with a fair bit of info on early RCN radar:
http://www.jproc.ca/sari/

Thank's for the link.

[Bgile]

I believe that in the 60s and 70s when I was in the US Navy, air search radars were generally S-band, with a pulse width of several us length. They were very powerful and weren't generally allowed to transmit in harbor. Most of my experience was in a submarine though, analyzing someone else's transmissions. We could detect air search radars a very long ways away, and surface search quite a bit closer. Our main periscope incorporated a number of antennas, covering the entire radar and communications spectrum.

[RF]

As an observation to this one other aspect of course is that these radars are only as effective as to the use they are put.

I recall that for example that the hilfskreuzer Michel, in the south Pacific, was picked up by the radar of a US cruiser, possibly USS Trenton or USS Pensacola (the Germans picked up the radar transmissions), but the cruiser ignored the Michel and failed to investigate the presence of an unidentified vessel.
In the outcome it didn't really matter (except to the crew of the Norwegian merchant ship India, Michel's last victim, who were all killed) as Michel was sunk a few weeks later.

[Bgile]

RF,

Are you sure the cruiser picked up Michael on her radar? You can detect radar at a much greater distance than that at which it can detect you.

[RF]

The Germans did sight the ship at long range but coudn't identify the vessel other than it was a light cruiser. One surviving officer of Michel, the adjutant Jurgen Herr, was sure the Americans must have seen Michel.

One other source of information on this, I can't recall exactly where, was in an attempt to reconstruct the exact track of Michel on its second cruise (as the war diary was lost with the ship and apparently only two officers survived) which involved tracing the US warship involved. The ship was believed to be the USS Trenton whose captain, S C Norton, did make an entry in his log about a momentary radar trace on the day in question, but offers no explanation as to why no investigative action was taken. The author of the article hypothesises that no action was taken due to the area being beyond the range of Japanese military activity.

[Dave Saxton]

Being too proactive can also get you into trouble. In April 1940 the Lutzow's Seetakt got a contact at 15 km, but as they got closer the contact disappeared. They realized that it may have been a surfaced submarine that had since dived, so they quickly turned away. It was too late, as just then a torpedo hit them in the .....

[Dave Saxton]

SJ The SJ was an active radar for use of submarines designed by Bell Labs. It was a very important componant 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.