Radio Location Goes to Sea - Chapter 2 of Radar and the Fighter Directors
By David L. Boslaugh, Capt USN, Retired
If it were not for a particular capability of the Royal Navy Signal School (RNSS) at HM Barracks, Portsmouth, it is hard to tell when the RN would have started a program to take radio location to sea. The school had engineers and shops that designed and manufactured electronic vacuum tubes (called “valves” by the British) that could be driven very hard at high voltages in shipboard radio transmitters. In April 1935, Watson-Watt’s engineers were running into difficulty with the valves available from industry. At the high pulsed voltages they were using in their experimental RDF equipment, the valve filaments were frequently burning out. That month engineer Arnold Wilkins of the RAF radio location project visited the school to learn more about their special valves that could sustain high driving voltages. He used the cover story that the RAF was running ionospheric transmission studies. [9, p.53] [29, p.8]
To get the needed engineering support from the school it was necessary to have Air Ministry issue a formal request to the Royal Navy. In the process the secret RDF project was revealed to the Admiralty’s Director of Scientific Research, Mr. Charles Wright. The Air Ministry also invited Wright to visit the RDF experiment station at Orfordness where, even though their equipment could only measure range at the time, he could see that the technology could be of use to the Royal Navy. Wright was allowed to discuss the experimental work at
Orfordness with CAPT G. W. Hallifax, head of the Admiralty Signal Department. Hallifax felt the technology might be extended to ship detection, and directed the commanding officer of the Signal School to start a shipboard radio location program. He indicated the school should request additional staff if required. The school responded with a modest request for one additional Scientific Officer and one Assistant II for the Experimental Department. The Admiralty gave approval on 29 August and the two additional staffers reported for duty in December. Development of a shipboard radio location transmitter was their first priority. In the meantime, one engineer from the Admiralty Research Laboratory and one from the Signal School had been sent on temporary duty to Orfordness to learn radio location. [29, p.10] [17, p.114]
In October, after a briefing on RAF progress at Orfordness, CAPT Hallifax and RNSS engineers established some goals for naval radio location. They were:
| Function | Airplanes | Ships |
|---|---|---|
| Warning of Approach | 60 miles | 10 miles |
| Precise Location | 10 miles | 5 miles |
The meaning of precise location was not defined because they did not yet know what would be possible. Some meeting notes indicate that ship location was more important than aircraft location. Chain Home had the luxury of a lot of real estate for antennas, whereas shipboard space and weight was definitely limited. Shipboard antennas would have to be considerably smaller than the CH antennas, and would therefore have to be driven at a higher frequency with a shorter wavelength to match their size. The RN had another concern that also called for higher frequencies. The low CH frequency generated radio waves that bounced off the ionosphere and thus could propagate for hundreds of miles following the earth’s curvature. For a ship, such emanations were a dead giveaway of its presence. With triangulation from two receiving sites the signal could be used to reveal a fairly precise location. The RN wanted to use higher frequencies that propagated “line of sight” waves straight out into the atmosphere rather than following earth’s curvature. [29, p.11]
Birth of the Type 79
The RN wanted valves that could generate powerful radio signals at wavelengths measured in centimeters rather than meters. This resulted in a contract with the University of Birmingham to investigate and to devise such a microwave generator. They would eventually succeed, but it would take some time. (More about this later.) In the meantime, RNSS had to resort to available valves that could generate useful power at wavelengths of a few meters, as compared to CH wavelengths around 25 meters. Scientific Officer R. A. Yeo of RNSS was assigned to the radio location project in September 1935 and decided on four meters for a start. He would be the only one working on the project until joined by Scientific Officer W. P. Andersen in December. The Signal School arranged to build a hut near the Royal Marine Barracks on the channel shore south of Portsmouth. Next to it were erected two towers rising to ship masthead height on which to mount a transmitting antenna and a receiving antenna. In mid July 1936, Yeo began installing his first RDF equipment in the hut. The equipment was designated radio equipment Type 79 because 79 happened to be the next available number in the school’s series of radio equipment. [29, pp.12-14]
By September 1936, Yeo’s shore-based equipment was able to detect and measure the range of close-in aircraft. Bearing could not yet be measured, and ships had not yet been detected. RNSS had an old coal burning former minesweeper named HMS Saltburn which was used for navigation training. The Admiralty and school representatives decided next to build a second Type 79 set to be installed in Saltburn. Non-directional antennas were hung 50 feet apart on a cable strung between Saltburn’s masts at a height of 75 feet above the water line. Two antennas were needed. One for transmitting and one for receiving. If the equipment shared the same antenna, the powerful outgoing pulse from the transmitter would disable sensitive circuits in the receiver. On 15 December 1936 the ship went to sea east of the Isle of Wight, and a test aircraft passed to and from the ship. Once detected, the plane could be tracked out to eighteen miles, and on the inward track the plane was picked up at fifteen miles. Even better, they detected a light cruiser at four miles. [29, pp.15-16]
The next technical challenge for Saltburn’s radio location equipment, now called Type 79X, was to measure target bearing, and directional antennas rotating in synchronism were needed for this. Yeo made up preliminary antenna drawings, and the Admiralty arranged for Portsmouth Dockyard to produce engineering drawings, build the antennas, and fit them atop Saltburn’s two masts. Portsmouth finished installing the antennas just a few hours before Saltburn was scheduled to go to sea for the test program, and the RNSS staff had no chance to calibrate the system on pre-measured targets. The July 1937 sea trials did not go well. Airplanes were detected only out to seven and a half miles, and the Eddystone Lighthouse was detected at seven miles. The school complained to the Admiralty that a contributing factor for the disappointing performance was lack of priority given for building and installing the antennas. The school was giving the project high priority, why couldn’t the Admiralty? [29, pp.17-18]
A Higher Priority
The Admiralty responded in early November by directing the school to move more resources to the radio location project. Also more RN support in general would be given, to include a cruiser available for sea trials along with more huts and tools for the school. The school established a new RDF Division in the Experimental Department to be headed by engineer C. E. Horton, who was at the time in charge of the Direction Finding Section. More scientific staff would also be provided. The new staff reviewed progress so far and realized that another reason for the disappointing performance was that available valves could not be made to operate efficiently at the chosen four-meter wavelength. They made the decision to rebuild the Type 79X to operate at a longer wavelength of seven and one half meters, for which there were proven valves available. They also redesigned the antennas to work with the longer wavelength. [29, pp.19-21]
By early 1938, the redesigned Type 79X was working well enough in Saltburn to convince the Admiralty to authorize the school to build two more copies, to be designated Type 79Y. The Admiralty directed that the two sets would be installed in the battleship HMS Rodney and the Cruiser HMS Sheffield for at-sea testing. In this case, heightened priority resulted in the two sets being built, installed, and operational within six months. While the two Type 79Y sets were being built as “Chinese” copies of the Type 79X, RNSS compiled all the lessons learned so far to be incorporated into two more prototype sets to be designated Type 79Z. This design was to be mechanically “ruggedized” for shipboard use, and they were to serve as models for manufacturing production drawings so that production could be moved from school shops to industry.[29, pp.21-22, p.26]
First Production
In October 1938 Sheffield reported on Type 79Y sea trials; she was getting detection ranges of thirty miles on airplanes at 3,000 feet, forty-eight miles on planes at 7,000 feet, and fifty-three miles at 10,000 feet. At the same time, the RNSS Valve Section was producing new valves that could produce considerably more power at the seven point five-meter wavelength, and many lessons had been learned from Rodney and Sheffield trials. The new valves and lessons learned were factored into the design of yet another version of the Type 79. This one could output seventy kW as compared to twenty kW for the Type 79Y, and when tested in May 1939, it detected airplanes at 3,000 feet out to forty miles and planes at 10,000 feet were seen at seventy miles. The Admiralty decided the new Type 79 was ready to go to sea, and thirty-nine battleships, battlecruisers, and cruisers were selected for installation. Flagships were excluded, the official reason given was a concern that the RDF equipment would interfere with communications. Others surmised that flag officers did not like the idea of radio location antennas taking top spot on the mast in place of their flags. Aircraft carriers were exempt also, no reason given. It is probable their deployment schedules kept them too busy to come into a yard for fitting. Under tight secrecy, a contract for forty Type 79 sets was awarded to a small London firm, Aeronautical and General Instruments, Ltd (AGI). [29, p.26]
In March 1939, Rodney and Sheffield took part in fleet exercises off Gibraltar where they had a chance to use their new radio location sets under realistic conditions. At one time, Rodney, with her Type 79Y transmitter turned off, detected Sheffield’s RDF transmission at a range of 100 miles, and with bearing accuracy of two degrees. This was not good. Senior officers saw that RDF could reveal a ships location; and it would have a dampening effect on future use of radio location. Meanwhile, RNSS had been working on even more design improvements, and in May 1939 completed two more sets, designated Type 79Z. These two were installed in the heavy cruiser HMS Suffolk and the anti-aircraft cruiser HMS Curlew in September. Also in this month AGI commenced deliveries of the first of the forty production sets. [29, p.26] [62, p.3]
Radio Location Goes to War
On 10 August 1939 the Admiralty decreed that all major combatant ships were to be fitted with RDF, at highest priority. This included the new-construction aircraft carriers of the Illustrious Class, and all operational carriers at their next shipyard availability. As of Great Britain’s declaration of war on Germany on 3 September 1939, the Royal Navy had two ships fitted with RDF, not counting Saltburn. The first operational use of Royal Navy RDF in combat would be on 26 September 1939 when the battleships HMS Rodney and HMS Nelson in company with the carrier HMS Ark Royal were sent out to escort the submarine HMS Spearfish that had been damaged by German surface ships near Horns Reef in the North Sea, and could not dive. They rescued Spearfish, and while returning to Scapa Flow, Rodney’s RDF picked up a large group of closing German bombers at eighty miles. Rodney notified Ark Royal, but as the bombers seemed to be heading for another group of RN ships further away, the carrier did not launch fighters.
The new-construction carrier HMS Illustrious was commissioned in April 1940, and would be the first RN carrier to have RDF. Later, while exercising with her air squadrons off Bermuda, Illustrious tried using RDF to guide fighter intercepts. HMS Formidable would be the second RN carrier to have RDF, commissioning in October 1940. [29, pp31-32 p.63]
Click here to go to Radio Location Takes to the Air - Chapter 3 of Radar and the Fighter Directors.
