Light Weight Air Warning Radar
The Light Weight Air Warning Radar, or LW/AW was a portable early warning radar produced in Australia during the Second World War. It was designed by the Council for Scientific and Industrial Research, today's CSIRO, to provide field troops with air attack warning in the northern Australia and New Guinea theatres.
From 1940, CSIR had been designing a large system known as Air Warning, Mark I for fixed emplacements, similar in concept to the UK's Chain Home. The first Japanese air attacks on Darwin were carried out before it was in place, and it was some time before this larger unit could be put into action. This led to a December 1941 request for a smaller version that could be held in reserve and then moved into location if the AW system was attacked. Shortly thereafter, field units requested a portable system that could be quickly delivered to forward airfields. In the summer of 1942, CSIR began work on a system that would fill both needs.
The LW/AW emerged as a system designed to be carried in rough conditions and small and light enough to fit in the Douglas DC-2 and Douglas Dakota. Otherwise similar to the AW, a new antenna design based on the UK's Chain Home Low provided the desired range in a robust multi-unit array that could be easily assembled in the field. The prototype entered testing in September 1942, and the first two production models were moved to New Guinea in mid-October and entered full operation on 8 November. The Mark I and slightly modified Mark IA served until the end of the war with about 260 produced.
The Mark II had a larger cabin and modified antenna to support IFF Mark III but was otherwise similar. Minor improvements led to the Mark III that was introduced in 1944, but the earlier versions were so successful it was not put into production. A modification of the Mk. III was used on ships as the A286Q and about 120 were produced.
History
Earlier developments
In early 1939, the British government invited teams from Canada, Australia, New Zealand and South Africa to visit the UK to be briefed on the then highly secret developments in radar. In September, with the opening of World War II, the Radio Physics Laboratory of the Council for Scientific and Industrial Research began development of a surface-search radar, known as Shore Defence, or SHD. SHD was similar to the UK's own Coast Defense system, but added a switch that allowed a single antenna to be used both for transmission and reception, simplifying the system compared to the two-antenna CD. The first SHD was installed in Dover Heights near the entrance to Sydney Harbour.As attention turned to the threat of air attack, the SHD system was modified with a different antenna layout to allow it to scan to higher angles. This produced the Air Warning radar, or AW Mark I. Although this was a powerful system, with detection on bomber-sized targets out to the range of, it was also relatively large and could only be delivered by ship. Shipping the system took time, and assembly even longer. The system intended for Darwin, which was in range of Japanese bombers from New Guinea, was still being prepared when the first attack took place on 19 February 1942. Darwin was subject to repeated attack, yet it still took another seven weeks before the first set was operational.
Smaller systems
In December 1941, Wing Commander A.G. Pitcher suggested the Royal Australian Air Force produce a number of smaller mobile radar systems to act as backup in case the AW sites were attacked. These would be held off-site and moved to the original AW location on demand. Doing so would require a smaller and more mobile system. In January 1942, Flying Officer B.F.N. Israel returned from working with the Royal Air Force in Singapore and stressed that there was a need for a truly mobile system that could be used in the field. Israel was posted to Sydney as the radar liaison officer between the RPL and HMV, the manufacturer.By the summer of 1942, the Australian Army was advancing and setting up new airfields. The need for early warning sets at these advanced bases was a serious concern as they were within easy attack range from several Japanese airfields. Pither sent a memo to John Worledge of the NSW Railways who was leading a group that produced the mechanical structures for the AW and SHD sets. Pither suggested that the electronics from the AW were suitable for mobile use if packaged correctly, but the antenna system needed to be produced in a version that could be flown into these airfields using Douglas DC-2 aircraft, which formed the basis of the Australian Transport Command. Pither also copied Israel and Squadron Leader Mitchell, Commander of the Radio School at Richmond.
Pither suggested one way to reduce the size of the antenna would be to remove one horizontal row of elements. But this would reduce the power by and greatly reduce range. Israel, who had seen the Chain Home Low systems in Singapore, was aware of their layout. He liaised with J.L. Pawsey, an expert in antenna design at the RPL, who produced a new design of four rows of eight dipoles, which maintained 89% of the original antenna gain and improved electronics regained another 5%. But the main benefit was that the antenna was now made up of eight 2 by 2 element cubes that were small enough for air transport and could be connected together in the field. The system was later referred to as the "Worledge aerial system".
Into service
The prototype system was completed in September 1942, and shipped to Dover Heights for testing. The first two production units were flown to Papua New Guinea some time in October. The first was set up at Tufi Airport and went operational on 8 November. Getting the system in place was a difficult job:The second unit was operational at Dobodura Airfield on 3 December. Dobodura was a forward location near Buna Airfield, which was itself captured on 14 December 1942. Further units continued to arrive in the theatre, at which point several problems were first noticed. One was that when the radar was turned off for maintenance, the cooling electronics would lead to significant condensation inside the system, which in turn led to the development of fungi inside the units. The problem was considered so severe that in late 1942, General Thomas Blamey was put in charge of getting the problems solved. In early 1943 field units were sent a 205 W electrical heater that automatically turned on when the electronics turned off. This maintained the temperature within the two large equipment chassis.
Other changes were more minor. The packaging continued to be improved through this period, and its ability to float if dropped in the water was tested in Sydney Harbour and the surf at Manly Beach. Another change was to raise the height of the turntable over the ground, as it was noticed airflow under the equipment was too low to properly cool the bottom of the chassis. These changes were formalized as LW/AW Mark IA. Over 260 Mark I and Mark IAs were produced.
Mark II
In April 1942, the supply of the US built VT90 micropup vacuum tubes that powered the transmitter ran low. An effort began to use an alternative design, the 100TH Eimac. These proved to be able to generate much greater output than the VT90s of the Mark I, and would offer significant gains in effective range. This was not so important for the LW role, but too-short range had proven to be a significant problem for the fixed-place AW units which were typically protecting larger areas like cities. An effort began to replace these with the new units as AW Mark II, which used four of the Eimac tubes in a ring to produce peak power of 50 kW. This was also adapted with a smaller antenna as the A79, for small ships. The first production examples arrived in December 1942, but these proved difficult to keep operational in the field, Supplies of the VT90 improved and the AW Mark II was dropped after only 15 were produced.By 1943, IFF Mark III was being widely introduced in the US forces, and ad-hoc additions to LW/AW were common. In the second half of the year, a rush effort began to build a more formal solution to connecting the US-built BL3 IFF interrogator units and make them part of the standard equipment list. The IFF antennas were placed on top of the existing antenna array. At the same time, operator complaints about the existing cramped tent leading to poor airflow and resulting high temperatures and humidity led to an effort to design a larger enclosure. This new system became the LW/AW Mark II.
One of the first Mark II units was sent to Bipi Island in early 1944 as part of the Admiralty Islands campaign. It arrived on 5 April and had to be ferried ashore on the local's canoes. The site proved to be poor and it was moved to Los Negros Island in May. before moving again to Manus Island on 26 July. But the time it was operational again it had been moved eighteen times and yet was only non-operational for five hours in nineteen months of operations.
Mark III
An improved version of the micropup, the NT99, began to arrive around the same time. When applied to the AW electronics, it allowed the power to be increased to 150 kW. An experimental version was operational in July 1943, which revealed that the power was so much that the antenna feeder lines of the LW/AW set would have to be modified. A prototype of the resulting LW/AW Mark III was sent to Los Negros in February 1944. The next month, Bruce Alexander who had worked on the transmitter design, flew to test the unit and related that just before arrived the unit had picked up an aircraft up to which was invisible to the Mark I. This turned out to be a C-47 that was lost, and fighters were sent to escort it to the airfield.Although the Mark III proved to be a more powerful unit, it was not put into widespread production due to the earlier versions working well enough and the complications that the new parts would cause in logistics.