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Introduction

Colin Latham's work-in-progress

DRAFT -   9RS                                                       JAN 2011

              No. 9 RADIO SCHOOL, RAF YATESBURY  (Chap. 2)

By now, at the close of the first decade of the 21^st century, much has been written and shown on television about Britain’s achievements in the science and engineering of radar systems before and during the Second World War, 1939 - 45.   It has emerged that without our long-range early warning capability the RAF could not have achieved their victory over the Luftwaffe in 1940 and Hitler’s plan to invade us would have gone ahead, with unthinkable consequences.

At the time our radar work was necessarily secret but later, after the war, much of the story was revealed publicly.  As a result, the impression was gained in some quarters that we alone had invented radar.  Not so, the idea of detecting distant objects such as ships and aircraft, by means of reflected radio waves, had arisen in several countries including the USA and Germany itself.  Indeed, some articles and documentary programmes have indicated that German equipment was more advanced than ours. This  assertion,  while true in part,  needs to be considered  in the light of the circumstances at the time and the different aims of the two countries.

Germany may claim, with some justification, to have been ahead of us initially by virtue of Christian Hulsmeyer’s Telemobiloscope, granted a patent (in London) as long ago as 1904.  Unfortunately for him his ship-mounted equipment failed to convince marine authorities in Germany of its effectiveness in locating other vessels. That is hardly surprising in view of the primitive state - at that time - of the enabling technologies of adio transmission and reception.  Nevertheless, the basic idea was sound even though far ahead of its time.

Turning now to the 1930s and the preparations for the anticipated war, the main difference between German and British radar research lay in their applications and choices of wavelengths.  During that decade radio - more generally known as ‘wireless’ - had become increasingly popular and most people were familiar with the terms ‘long waves’ and ‘medium waves‘ to which their domestic receivers could be switched for different programmes. Long waves were from 1000 - 2000 metres and medium from 200-500 m. 

The gap between long and medium was used for non-entertainment purposes as were wavelengths down to 10m, known as ‘short waves’ in which certain defined channels were allocated to licensed amateur transmitting/receiving stations (‘Hams’).  At first wavelengths below 10m called ‘ultra short waves’, were largely ignored due to the difficulty of obtaining  radio valves to work at such high frequencies. (For the relationship between wavelength and frequency see p….).  The opening of the British 405-line television service on 7m from Alexandra Palace in 1936 was regarded as a novel and major technical achievement in the use of ultra short waves.

For the pioneering scientists in both Britain and Germany the advantages of very short waves for radar were obvious in the interests of positional accuracy in the detection of targets and the compactness of aerial systems. However, the state of transmitting valve development in the mid 1930s was such that every downward step in wavelength was accompanied, inevitably, by a severe reduction in power output. 

It is well known that the strength of a received radio signal varies inversely as the square of the range from transmitter to receiver.  Thus, to double the range of a communication system  by transmitter power it must be increased by a factor of four.  But for radar this happens in both directions, first to the signal transmitted to the target and then to the reflected echo.  Thus to double the range of a radar, by transmitter power alone, an increase of sixteen times is needed.   It appears that Germany’s initial interest in radar was to improve the gun-laying capability of its warships and ground-based anti-aircraft batteries for both of which accuracy of position-finding was more important than long range.  They therefore concentrated on very short wavelengths despite the inevitable power limitations.

Anyone who has studied David Pritchard’s,The Radar War, published 1989, and sub-titled Germany’s Pioneering Achievement 1904-45, will see how that country produced numerous excellent equipments for different applications from the 1930s using wavelengths of a metre or so, and even down to 50cm,  from the early experimental days of radar in the 1930s and onwards throughout the war.  

By contrast we in Britain, from the mid-1930s, concentrated initially on producing a radar system with the longest possible range to give us the earliest warning of intruding aircraft. The plan was that, given sufficient warning, our numerically limited resources of fighter aircraft could avoid continuous standing patrols of our coastlines but be scrambled in time to meet and engage an approaching enemy. The aim was for detection ranges of 100 miles or more and we therefore used much longer wavelengths at which the essential high transmitter powers could be obtained reliably from the valve technology then available. 

 Our first radar system, Chain Home (CH), working at wavelengths of some 10-15 metres, and using static aerials, was somewhat less capable, in theory,  of accurate bearing measurements compared with those at shorter wavelengths:  nevertheless it did provide reliable and adequate long-range detection of aircraft.  CH displays were calibrated accurately to 200 miles range, some echoes being seen well beyond 100 miles. In practice many incoming raids were plotted simultaneously by closely-spaced CH stations so that a higher degree of positional accuracy could be assessed at filter rooms by range cuts if required (but rarely needed for moving targets at long ranges).

The value of CH, using transmitters from some 300-600 kW pulse power, which required special development, lay not only in the radar stations themselves but in that so many were installed around our coasts all reporting, via centralised filter rooms, to fighter squadrons for immediate take-off to intercept intruders.  In that overall organisational respect, with its vast  network of communications, we were far ahead. Again, by c.1942 we overtook Germany dramatically in the shorter wavelength race by producing radars using magnetron transmitters with unprecedented powers of hundreds of kilowatts of pulsed output at wavelengths of less than 10cm.  Such radars could use scanning aerials even more compact than the then current German types, ideally suitable for airborne radars as well as new generations of  long range microwave sets, thereby setting the scene for the majority of surveillance radars after the war.  

The early use of a long wavelength (in radar terms) for British long-range radars demanded large aerials - so large that it was impractical to construct manoeuvrable narrow-beam arrays to scan the skies.   Thus CH transmitting aerials, static with simple reflectors, radiated broadly in the direction from which attacks were likely to come, while direction-finding of echoes was achieved by separate receiving aerials and goniometers as in traditional DF systems.  For this system, in which every target within a broad area is illuminated by every transmitted pulse, a pulse repetition rate of 25 per second was entirely adequate.  This contrasted with narrow beam sky-scanning radars where much higher repetition rates were essential avoid the missing of targets.

It was the British use of  the long wavelength and low pulse repetition rate that misled the German Zeppelin spying mission of  August 1939 - just a month before war was declared.  The airship carried a technical crew equipped with receivers with the aim of assessing whether we had an operational radar warning system. We most certainty had, even though the war had not begun.  CH tracked the Zeppelin, flying close to our east coast from Essex to Scotland, the German technicians failed to recognise our transmissions as radar and reported back to Germany that Britain had no such system.  They had been expecting to detect shorter wave transmissions, with a mid-range musical sound in their headphones, rather than the slow tick-tick 25 times per second. That frequency was actually derived at each CH station by halving the common mains supply, at 50 Hz, and so phasing it that stations did not interfere with each other.  In itself that added to the mystery for the Zeppelin crew because the ticking hardly varied during their coastal trip, giving the impression of interference from sparking of the UK high voltage grid distribution system.   The outcome of the Battle of Britain was determined in advance..

So much for a brief comparison of British and German radar development but that is not the whole story.   There was another significant difference between the radar organisations of the two countries to which attention has rarely been drawn although an exception lies within the extensive work of the late American author, Louis Brown. He was a physicist of the Carnegie Institute, Washington who, after years of rigorous research produced his truly massive book, A Radar History of World War II (Institute of Physics, London, 1999).

According to Brown’s research the extent of technical ability at German radar stations was minimal.  Staff were trained to operate the equipment with only the barest amount of background information about how it worked.  If the well-designed and well-made equipment should fail they needed only to remove and replace the suspected unit and have it returned through the appropriate channels to the manufacturer for repair.     This was in contrast to the RAF whose radar mechanics were trained almost - as Brown puts it - to the technical standard of professional engineers.   Thus repairs and modifications could be done on site, a system which became of enormous advantage when, as the war developed, hastily made equipments had to be pressed into service after only minimal operational testing. Close co-operation often took place during radar trials between senior scientists and ordinary RAF radar mechanics - a situation apparently unlike the more rigid procedures in Germany.        

How were the RAF radar mechanics trained so completely and effectively?  And how were the radar operators, both men and women, given such deep insight into the workings of the radar chain and its equipment that they took to the work with such outstanding dedication and enthusiasm? 

The answer lies in the activities of the RAF’s secret ‘radio’ schools.

No. 8 Radio School, Cranwell, and No.9 Radio School, Yatesbury, were concerned primarily with the early and continually developing long range radar defence equipments in service through throughout the war, while the slightly later developments of airborne radar were taught at No. 2 Radio School, London, in a site near the Albert Hall.

The aim here is to describe the radar work at Yatesbury during the war.

Latham

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