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The Chain Home radar system

The following article from "The GEC Journal of Research", Vol. 3 No.2 1985 pages 73-83 describes the RAF Chain Home (CH) air defence radar system and has been reproduced with the kind permission of the Editor. The copyright of the material remains with the owner.

As the author remarks, CH was the first radar to be organised into a complete air defence system and the first such system to be used in wartime operations. The essential point to remember whilst reading this article is that CH did not rely on any new radio techniques. Watson-Watt and his team adapted the available technology into a highly effective air defence system and herein lies their genius. If Britain had waited until new technologies were forthcoming then the air defence system that Watson-Watt's team developed would not have been available or operational at the outbreak of World War II and the Control and Reporting System would not have been in place; and the final outcome might have been very different - for Britain at least.

Good historians and researchers should be aware of the pitfalls when looking back in time, for there is always the danger of judging the past in the context of the present. In this context it is interesting to view the American perspective on early British radar development, where D. F. Winkler1 perhaps misses this point when he describes CH as "primitive" whilst Major Gregory Clark2 appears scathing of the HF technology used, dismissing it as "a dead end technology with serious shortcomings". Primitive it was, by todays standards, however it was the combination of available technologies and the pioneering work on effective Control and Reporting Systems that made CH so successful. It is interesting to speculate on what the outcome at Pearl Harbor might have been if the US had protected Hawaii with a CH radar system along with its associated Control and Reporting System! For all its faults (HF technology, poor low level cover below 2o, slow data transmission and so on) the CH flood-light radar system represented the best general early warning system that could then have been devised given the technological capability and the level of scientific knowledge available in Great Britain at that time. CH held the fort until new technologies such as the cavity magnetron developed by Messers Randall and Boot could be brought into service use.

There is an interesting adjunct in this article; towards the end the author mentions "Big Ben", this was the code name given to the use of the CH system to detect German V2 (A4) missile launches. This use of the system as a method of detecting ballistic missile launches predates the US BMEWS system by more than ten years - another first for CH!

1. Winkler, David Frank, Searching The Skies - The legacy of the United States Cold War Defense Radar Program, Prepared for United States Air Force Headquarters Air Combat Command, UG612.3W56, 1997, Chapter 1 page 9.

2. Clark, Major Gregory C., "Deflating British Radar Myths Of World War II", A research paper presented to The Research Department, Air Command and Staff College, AU/ACSC/0609F/97-3, March 1997. This paper can be down loaded in .pdf format from the Federation Of American Scientists web site at http://www.fas.org/man/dod-101/ops/docs/97-0609F.pdf, abstract on page vi.

Note: A number of the sites listed in The Radar Pages offer documents in the .pdf format. This format is an excellent way to transmit documents but you need a special viewer to view them on your computer. You can down load Adobe's free Acrobat Reader by clicking on the Acrobat logo.


CH - THE FIRST OPERATIONAL RADAR

B. T. NEALE

The CH (Chain Home) radar was the first to be organized into a complete defence system, and the first to be used in wartime operations. This paper describes its construction and performance. It had a very long range against high-flying targets, could measure height, and was well-equipped to deal with likely enemy counter-measures.

INTRODUCTION

Chain Home transmitter tower (photo - CHIDE)Fifty years ago on February 26, 1935, a simple experiment in a field near Daventry conclusively demonstrated that aircraft could be detected by radio. The experiment arose directly from the need to prove, to a first order, calculations that suggested that if an aircraft was 'illuminated' by radio waves, enough energy should be 'reflected' to permit detection on the ground by a sensitive receiver. The story of this significant experiment and its immediate consequences has been told many times and is well documented.

It is sufficient to record here that it led to a massive programme, backed by the highest priority and virtually unlimited finance, to design, build and install a chain of early warning systems around the coast of Britain. The importance of this courageous decision to a country whose only early warning of air attack was, at that time, a huge concrete acoustic mirror on the Romney marshes, cannot be over stated. Key stations in the south east of England were operational and integrated into a vast reporting network just in time for the air battles to come.

The purpose of this article is to describe in some detail the principal component of this remarkable system: The 'CH' station (AMES Type 1), the direct descendant of the 'Daventry Experiment'.

GENERAL PRINCIPLE

The basic operating principle of CH is very simple: the volume of sky to be kept under surveillance is literally 'floodlit' with r.f. pulsed energy; the back-scattered pulses or echoes' from all aircraft within this volume are received back at the ground station by a set of crossed-dipoles connected to a low-noise, high-gain receiver and displayed as a Y-deflection along the time base of a CRT. The aircraft range is simply a precise measurement of the elapsed time between the transmitted pulse and the 'echo', and the bearing a measurement of the ratio of 'echo' strengths of the X- and Y-components of the crossed-dipoles. These principles are illustrated in figure1.

Principles of CH (Chain Home) R.D.F. system

Choice of frequency, polarization and p.r.f.

The choice of radio frequency was principally influenced by three factors: the 'state of the art' technology in 1935, feasible antenna dimensions to provide the vertical polar diagram and antenna gain required for efficient floodlighting and heightfinding; and a wavelength that was considered at that time likely to give the best 'echo' from a typical bomber of the period. Very little, if anything, was known at that time about the effective back-scattering cross-section of an aircraft or how it varied with frequency.

Chain Home transmitter tower at RAF Bawdsey (photo - Posford Duvivier)It was earlier thought that an approaching aircraft could be regarded as a half-wave, horizontally polarized dipole; in fact, a typical enemy bomber of that period, a Heinkel 111 with a wingspan of 22.5 metres, closely matched the original 'Daventry Experiment' frequency of 6 MHz. This theory was later abandoned.

The original plan was for each station to have the choice of operating on any of four allocated spot frequencies in the band 20 to 55 MHz as a counter-measure to possible jamming and as alternative frequencies should interference or propagation effects cause operational problems.

 

(This article is taken from "The GEC Journal of Research", Vol. 3 No.2 1985 pages 73-83 and has been reproduced with the kind permission of the Editor. The copyright of the material remains with the owner.)

 

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