AMES Type 6
AMES Type 6, also known as the Light Warning Set or L/W, was a portable early warning radar developed by the Air Ministry Experimental Station for use by the Royal Air Force in the field. Units in British Army service were officially known as Radar, Anti-Aircraft, Number 4, or AA. No. 4 for short, although this name was rarely used in practice. The system was also built in Canada for use by the US Army, who referred to them as SCR-602-A.
The antenna consisted of four Yagi antennas mounted on a central pole. Supports running from the pole to the corners of the antennas give it a distinctive hourglass-like shape. The pole sat in a bearing to provide rotation around the vertical axis, allowing it to scan the horizon. The Type 6 was based on the same 1.5 m wavelength electronics as the ASV Mark II. As this wavelength became increasingly congested, the Type 6 Mark III moved to 212 MHz and introduced a new truck mounting to improve mobility. A small number of Type 6 Mark IVs operating at 193 MHz were also produced for use in the presence of German jamming. In the field, all Type 6's were normally paired with an IFF Mark III transceiver, using an omnidirectional antenna.
A completely different version was introduced as the Type 6 Mark V. This version moved from the 1.5 m band to 50 cm, allowing it to operate from much smaller antennas. The antenna array was more complex, using eight Yagis for improved antenna gain. The US equivalent of the Mark V was the SCR-602-T8, but this version replaced the multiple Yagis with a parabolic reflector. Both models were intended as stop-gap systems that could be quickly set up during forward movements while awaiting the arrival of larger and more capable units. These saw use primarily in northern Europe after D-Day.
Several hundred Type 6's were produced, along with an unknown number of SCR-602's. They were widespread in the mid-to-late World War II period, seeing action in North Africa, Italy, and South East Asia. A similar system was developed in Australia, the Light Weight Air Warning Radar, and Canada, the Zone Position Indicator. Almost all of these were replaced in the post-war period by the US AN/TPS-1, which saw widespread use by many nations.
History
Higher frequencies
By early 1936, after less than a year of development, the Chain Home system was reaching the goals set for its initial operational requirements. This was possible because the system had deliberately been based on an existing commercial shortwave radio system, the 50 m BBC World Service. As this required enormous antennas, the team reduced the wavelength in steps to the final 10 to 13 m used during the war.In April 1937, Henry Tizard grew concerned that CH would be so effective at guiding RAF Fighter Command's aircraft onto their targets that the Luftwaffe would give up daylight bombing and turn to night bombing. A pilot's eyesight at night is too limited to effectively engage aircraft even with CH guidance. The solution was a radar set with limited range that was small and light enough to be mounted within an aircraft, closing the gap between the CH's accuracy and the approximate range of the pilot's eyesight.
"Taffy" Bowen led the development effort of what became known as Airborne Interception radar, or AI for short. To make the system practical, it had to use the smallest possible antennas, which in turn demanded the use of the highest possible frequencies. Given the state of the art, a suitable high-frequency receiver proved to be the single largest problem in AI development. This was finally solved when Bowen's former thesis advisor, Edward V. Appleton, told him that there were many unused experimental television receivers at Pye Electronics that might be suitable. Bowen's team found these were far and away the best receivers in Britain. This "Pye strip" receiver was soon being used by other teams for all sorts of uses.
As the threat of U-boat attacks grew, the AI unit was given much larger antennas to improve sensitivity, entering service in 1939 as Radar, Air-Surface Vessel, Mark I. This system used two Yagi antennas pointed slightly to the right and left of the centreline of the aircraft, and by comparing the relative strength of the returns from the two antennas, the radar operator could determine which side of the aircraft the target lay on.
Portability
As the Battle of Britain wound down, the RAF turned their attention to offensive operations in the North Africa campaign. With huge stretches of open land between the opposing forces, and few secure locations to use as forward reporting centers, Luftwaffe sorties were rarely intercepted. At the same time, the British Army found that while their new Gun Laying radars provided accurate aiming information for their anti-aircraft artillery, their highly directional antennas made them almost useless for detecting the aircraft and giving them time to prepare for action. As the campaign was marked by its mobile nature, sweeping hundreds of miles back and forth across the desert, there was a need for an equally mobile early warning radar system.The Air Ministry had realized the need for a mobile radar system and had developed the AMES Type 9 Mobile Radar Unit for this role. These were "transportable" versions of the original Chain Home systems. Thirteen heavy trucks were required to carry the system, which used two tall towers that could be collapsed vertically for transport. Despite a crew of 60 personnel, the system still took two days to be set up. While effective, with detection ranges up to, the system was far too unwieldy for use in the African theatre. The need for a much smaller and more portable system was obvious.
ASV, designed from the start to be robust and lightweight for use in aircraft, was a natural fit for this role. Two such systems were built by fitters who pulled the equipment out of anti-submarine Vickers Wellington aircraft and adapted them to the early warning role by modifying the antennas. For ASV, the antenna was designed for short to medium range and high angular accuracy with respect to the nose of the aircraft. For the early-warning role accuracy was less important than longer range, which was difficult due to the small size of its targets compared to a U-boat. Instead of the two antennas pointing in different directions, fitters attached them to a square metal frame so they were pointed in the same direction and exactly one wavelength apart, so that their signals added together along the line directly in front of them, the "line of shoot", doubling gain.
A second set of two antennas was added, mounted on the same pole but positioned about one and a half wavelength below the first. This took advantage of the fact that antennas located close to the ground will reflect some signal off it, and the combination of the direct wave and the reflected one causes a series of vertical "lobes" to be created due to the resulting interference pattern. The operator could switch between the upper and lower sets, which had different lobe patterns, which allowed some estimation of the altitude. For long-range detection the two pairs were used at the same time, doubling gain again. With both pairs operational, detection range was on the order of, far short of the MRU systems but its small size and mobility allowed it to be moved as close as from the front and made up any difference in performance though improved siting.
The initial version was transportable, meaning it could be transported by truck and set up on-site. It could also be broken down into small loads for transport via aircraft or mule pack. The antenna array was mounted on a pole that sat on a horizontal turntable that was placed on the ground. The radio frequency equipment was placed on the same turntable, and rotated with the antennas. These parts were protected by tenting. A separate generator provided power, and if a radio was needed to maintain communications with other units, a transmitter truck was added.
Improved models
These early lash-ups, retroactively known as Mark I, were soon replaced by new production units, the Mark II. The only major difference, aside from minor mechanical details, was the addition of an input for an IFF Mark III interrogator. By 1944 only a few of these were still in use, mostly overseas.The Mk. IIs were mostly replaced by the Mark III, which operated on a slightly different frequency, 212 MHz, in order to avoid interference with the ever-growing list of systems operating on the 1.5 m band. The Mark III was designed to be operated from a tent, like earlier models, or mounted directly on the back of a truck, initially a Fordson WOT. The truck version could be set up simply by starting up the generator and connecting the display. Similar conversions were made using the Canadian Military Pattern truck, both the Chevrolet C15A and Ford F15. The system required a crew of only three, or ten to provide continual 24-hour operation, and it could set it up at a new site in as little as 30 minutes.
The system proved to be extremely useful in the field. On two occasions, LWRs were airlifted into recently captured enemy airfields and manpacked to nearby hills and set up within 30 minutes of arrival. This meant they were ready to provide early warning of counterattacks before the enemy even had time to arrange them. At Cape Serrat in Tunisia, two systems were carried through a forest and swampland to a lighthouse where they were able to track Luftwaffe aircraft making supply flights into Tunis. This allowed the RAF to attack them, including a disastrous mission in which 16 of 27 Messerschmitt Me 323 Gigants were shot down. The Germans were so angry that they subjected the site to 67 air attacks, forcing the units to be resupplied by foot at night and suffering from a continual lack of food and water.