Milestone-Proposal:Cavity Magnetron


To see comments, or add a comment to this discussion, click here.

Docket #:2021-09

This is a draft proposal, that has not yet been submitted. To submit this proposal, click on "Actions" in the toolbar above, then "Edit with form". At the bottom of the form, check the box that says "Submit this proposal to the IEEE History Committee for review. Only check this when the proposal is finished" and save the page.


To the proposer’s knowledge, is this achievement subject to litigation? No

Is the achievement you are proposing more than 25 years old? Yes

Is the achievement you are proposing within IEEE’s designated fields as defined by IEEE Bylaw I-104.11, namely: Engineering, Computer Sciences and Information Technology, Physical Sciences, Biological and Medical Sciences, Mathematics, Technical Communications, Education, Management, and Law and Policy. Yes

Did the achievement provide a meaningful benefit for humanity? Yes

Was it of at least regional importance? Yes

Has an IEEE Organizational Unit agreed to pay for the milestone plaque(s)? Yes

Has an IEEE Organizational Unit agreed to arrange the dedication ceremony? Yes

Has the IEEE Section in which the milestone is located agreed to take responsibility for the plaque after it is dedicated? Yes

Has the owner of the site agreed to have it designated as an IEEE Milestone? Yes


Year or range of years in which the achievement occurred:

1939-41

Title of the proposed milestone:

Development of the Cavity Magnetron, 1939-1941

Plaque citation summarizing the achievement and its significance:

In this building John Randall, Harry Boot, and James Sayers advanced earlier cavity magnetron designs to generate, for the first time, microwave energy at high power. As Britain lacked large scale production facilities Winston Churchill dispatched a mission for the manufacture of these cavity magnetron designs in America and this enabled the first deployments of airborne radar systems. These magnetrons are found today in every microwave oven.

In what IEEE section(s) does it reside?

Region 8 UK and Ireland Section

IEEE Organizational Unit(s) which have agreed to sponsor the Milestone:

IEEE Organizational Unit(s) paying for milestone plaque(s):

Unit: Region 8 UK and Ireland Section
Senior Officer Name: Dr Mona Ghassemian

IEEE Organizational Unit(s) arranging the dedication ceremony:

Unit: Region 8 UK and Ireland Section
Senior Officer Name: Dr Mona Ghassemian

IEEE section(s) monitoring the plaque(s):

IEEE Section: Region 8 UK and Ireland Section
IEEE Section Chair name: Dr Mona Ghassemian

Milestone proposer(s):

Proposer name: Peter Grant
Proposer email: Proposer's email masked to public

Please note: your email address and contact information will be masked on the website for privacy reasons. Only IEEE History Center Staff will be able to view the email address.

Street address(es) and GPS coordinates of the intended milestone plaque site(s):

Poynting Building, University of Birmingham, Birmingham UK

Describe briefly the intended site(s) of the milestone plaque(s). The intended site(s) must have a direct connection with the achievement (e.g. where developed, invented, tested, demonstrated, installed, or operated, etc.). A museum where a device or example of the technology is displayed, or the university where the inventor studied, are not, in themselves, sufficient connection for a milestone plaque.

Please give the address(es) of the plaque site(s) (GPS coordinates if you have them). Also please give the details of the mounting, i.e. on the outside of the building, in the ground floor entrance hall, on a plinth on the grounds, etc. If visitors to the plaque site will need to go through security, or make an appointment, please give the contact information visitors will need. University building housing the School of Physics

Are the original buildings extant?

yes

Details of the plaque mounting:

standard external wall mounting at appropriate height

How is the site protected/secured, and in what ways is it accessible to the public?

These University buildings are open to the public during working hours.

Who is the present owner of the site(s)?

University of Birmingham

What is the historical significance of the work (its technological, scientific, or social importance)?

Summary

Following the independent development of RADAR in many countries from 1934-9 there was a need to efficiently achieve high power microwave energy in a lightweight generator to enable the advance from land based into airborne RADAR. This was enabled by the development of the cavity magnetron. John Randall and Harry Boot at the University of Birmingham produced, in 1940, their first working example of a major advance in the cavity magnetron design which enabled the generation of hundreds of Watts of power at 10 cm wavelength. Shortly afterwards engineers at the General Electric Company, London, had improved this to well over a kilowatt, and eventually on to over 100 kW by 1941. These high power microwave pulses could be transmitted from an antenna only centimeters long, reducing the size of practical RADAR systems by orders of magnitude, enabling the realisation of long range night-fighter and anti-submarine aircraft systems. As Britain had limited funding and did not have the capability to mass-produce these magnetrons in the required volumes, Winston Churchill agreed that Sir Henry Tizard should offer this magnetron design to the Americans in exchange for financial and industrial help. Several examples of an early 10 kW version, built in England at the General Electric company, were taken to America on the Tizard Mission in September 1940. This design was immediately adopted to implement airborne radar systems for wartime use.

Preamble

In December 1935, the British Treasury appropriated £60,000 for a five-station early RADAR system called Chain Home [1], covering the approaches to the Thames Estuary to identify the presence of incoming hostile aircraft and direct their interception by fighter aircraft. This system, which used high energy pulses generated at 20 or 30 MHz, was a rather basic RADAR design employing tall wooden and steel towers to support the physically large antennas. Randall and Boot spent the summer of 1939 visiting one of these Chain Home installations [4c] and this encouraged them to attempt to develop high power generators at microwave frequencies. To expand the Chain Home into an airborne RADAR required the use of much higher microwave frequencies to achieve the smaller sized transmitter and receiver antennae designs required for inclusion in an airframe. Early efforts at generating these microwave frequencies were often based on klystron and magnetron tubes but their power output was found to be far too small to achieve the required range to build an operational airborne RADAR system.

The Magnetron

The magnetron name originally arose out of work by Albert Hull in 1921 at GE Schenectady in New York on the use of magnetic fields to control the current in a vacuum tube.[2] The development of magnetrons with multiple cathodes was initially undertaken at Bell Telephone Laboratories in America from 1934, by Philips, the UK General Electric Company (GEC), Telefunken and others, but these devices were limited to typically modest 10 W output [2]. In the 1930s, the British expert on magnetron design was Eric Megaw at GEC who published several reviews of the mechanisms for their generation, including split-anode magnetron designs. The resonant cavity magnetron design used the interaction of a stream of electrons with a magnetic field while moving past a series of open metal cavity resonators. These early cavity magnetron designs, which incorporated internal resonant elements to control the operating frequency, had been investigated and patented by several independent groups in the late 1930’s [2,3a,3b], such as Hansen at Stanford, Samuel at Bell Labs, Hollmann and Engbert at Telefunken, Posthumus at Philips, Ponte and Gutton in France, Alekseev and Malairov in Leningrad, Okabe in Japan etc. However, in spite of the many published papers and patents, not all the designs were actually built as prototypes and, generally, the resonator concept had been rejected as somewhat inflexible.[3a] Nowhere in the world in 1939 was there a working, pulsed, cavity magnetron capable of generating 10 kW or more peak power at wavelengths of 10 cm or less, which had a compact portable size, used a small permanent magnet and which was readily capable of being manufactured at scale.[3a]

Design Breakthrough

The radical improvement in power and manufacturability was achieved by the cavity design of John Randall and Harry Boot group, working under a British Admiralty contract, at the University of Birmingham, England in 1940 [4b,4c] and they secretly patented their device that August, along with their Admiralty sponsor.[4a] As mentioned above, cavity magnetron designs were really “a simultaneous invention” in many different nations [3b] but dissemination of the various design details was somewhat patchy! Randall arrived at the inspirational idea of using concentric cavities when he researched the design of the original Hertz oscillator, which was an open single ring. Randall had earlier visited, while on holiday, the University College bookshop in Aberystwyth where he found and acquired a copy of Jones's translation of Hertz's "Electric waves".[3a,4c] What is somewhat difficult to establish is precisely how much Randall and Boot were inspired by the cavity klystron of Hansen and other cavity-anode ideas that had existed since 1934? It is generally claimed by the group at the University of Birmingham that they were not fully aware of these other developments, [4b,4c] and they are now largely credited with implementing the first high-power version of this microwave device which was easily reproducible and readily adapted for mass production.

Randall and Boot developed their cavity magnetron design in November 1939 and they showed their first copper block to Lawrence Bragg and Edward Appleton when they paid a visit to the Birmingham laboratory during that month.[3a] In late 1939 Randall and Boot had converged on a 6 cylindrical resonator geometry, slotted so that the slots were parallel to the cathode axis, which they opened into the anode-cathode space as a cylindrical extension of the original Hertzian dipole. The resonating chambers were thus built with a physical size which was matched to the wavelength of the operating microwave frequency, to boost the signal. Randall’s structure is claimed in [2] to differ from the prior cavity designs (hollow spheres, cubes and “doughnut” shaped cavities) as he was thought to be the first person to introduce the cylindrical symmetry into the magnetron design, but [3b] somewhat contradicts this claim! However, a significant difference between the Randall & Boot cavity magnetron design and those previously patented was that nearly all the others had their anode system inside a glass envelope containing a vacuum whereas the Birmingham valve had its vacuum system inside the anode structure. This was the design feature which achieved much more efficient cooling of the anode system to permit the higher power dissipation and generate the larger output power.

When initially switched on Randall’s device produced, on February 21 1940, about 400 W continuous wave at ~10cm wavelength. Randall and Boot provide their reminiscences on this development in [4c] with [5] covering subsequent industrial developments and [2,3a,3b] assesses their achievements in the context of the other innovators. Randall and Boot’s technical innovation is often seen as an unprecedented achievement compared to all the pre-existing magnetron designs.[6] In retrospect Randall and Boot’s design is now viewed as a major technological revolution as they made the novel, innovative steps which paved the way for generations of magnetron devices at exactly the right time for the war effort. Randall and Boot’s device was a laboratory prototype, not suited for field operation. In April 1940, the British Admiralty signed a contract [3b] with the General Electric Company (GEC) at Wembley to produce an operational device. Their device was to operate with neither vacuum pumps nor an external generator of the magnetic field. The GEC engineers, led by Eric Megaw, performed the required industrial development on the academic prototype device by deploying their industrial techniques to improve the design of the vacuum seals and thus remove the pump requirement. GEC further combined the multi-resonator system of Randall and Boot with a large oxide coated cathode to turn it into a manufacturable device with a small permanent magnet to achieve its more rapid introduction for service use. This enabled the power output to immediately be improved to well over a kW, within months to 25 kW, and on to over 100 kW by 1941!

Another key individual who further contributed to the Birmingham development was Jim Sayers, while working at the University of Birmingham on a Contract, noted the excessive frequency noise and suggested in August 1941 [4d] strapping the cavities to constrain them to generate oscillations in one or more particular modes, to the exclusion of others over a wide range of operating conditions, which provided the major improvement in frequency stability. Strapping continues in use and in patent awards today. Sayers technical innovation was acknowledged when he joined Randall and Boot in securing their 1949 award. The “Royal Commission on Awards to Inventors” was responsible for judging how much Civil Servants should receive if the “British Crown" profits from their inventions. The Commission was responsible for initiating the award of the very significant £30,000 prize in 1949 to Randall, Boot and Sayers. Such a financial award would be worth more than £0.5 M today!

Airborne Radar

The high power magnetron pulses were generated from a device the size of a small book and broadcast from an antenna only cm long, reducing the size of practical RADAR systems by orders of magnitude. The cavity magnetron thus enabled new compact RADARs to be designed for deployment on aircraft such as submarine hunters and night-fighters and also on the smallest of escort ships. A 10 cm wavelength RADAR achieves superior angular resolution and different objects have very different RADAR signatures; water, open land and built-up areas of cities and towns all produced quite distinct returns and this enables the RADAR to map the ground below the aircraft to assist navigation as well as targeting munitions delivery, even through cloud. The most widely-used British RADAR which used the cavity magnetron was called H2S. This operated at S-band (around 3 GHz), and was last used in anger during the Falklands War in 1982. Some H2S units remained in service for more than 50 years, until 1993. The US equivalent operated at X-band (around 10 GHz), was denoted H2X, and was in service from October 1943. From 1940 onwards the Allies of World War 2 held a technical lead in RADAR systems that their counterparts in Germany and Japan were never able to close, even though they had previously patented earlier versions of these magnetron devices. The development of the cavity magnetron was so sensitive that aircraft were not permitted to fly over Germany until 1943 and they were fitted with explosives to destroy the magnetron if the plane was shot down, to ensure that the enemy were unable to learn about the device. In February 1943 a Stirling bomber carrying a cavity magnetron powered 10 cm H2S RADAR crashed near Rotterdam when the Germans finally acquired a complete H2S system design and they quickly copied it but it was too late to have a significant effect on the outcome of the war. By the end of World War 2, practically every Allied RADAR was based on a cavity magnetron. After the war in 1947 British cavity magnetron production was taken up at EEV in Chelmsford and they lobbied for additional financial reward for the inventors. Today the cavity magnetron is located in the microwave oven in 93% of UK households, with US sales exceeding 10 million new microwave ovens p.a.

Technology Transfer

Because France had just fallen in World War 2 and Britain was strapped for the funding and manufacturing capability for the cavity magnetron on the required massive scale, Winston Churchill agreed that Sir Henry Tizard should offer the magnetron to the Americans in exchange for their financial and industrial help. The 12th device, a 10 kW version, built in England by the General Electric Company London, was taken on the Tizard Mission in September 1940.[7,8] This cavity magnetron was a small, air cooled, compact device and, from no. 12 onwards, they were designed with 8 cavity resonators. As the discussion turned to RADAR, the US Navy representatives detailed the problems with their short-wavelength systems, complaining that their klystrons could only produce 10 W. One of the British mission members pulled out from his briefcase a cavity magnetron and explained that it could already produce 1000 times that power. Subsequent testing at Bell Labs showed that the new design produced 10 times the output power at 5 times the frequency of the best performing American triodes![2] Bell Labs quickly began making copies, and before the end of 1940, the Radiation Laboratory was set up on the campus of the Massachusetts Institute of Technology to develop various types of RADAR system using this cavity magnetron design. (This laboratory later expanded into the Lincoln Laboratory at Lexington MA to further improve the U.S. air defense capability.) The 1940 cavity magnetron thus became the heart of more than 150 new RADARs of all categories designed between 1941 and 1944.[3b]

Recognition

Randall Boot and Sayer's innovative development of the magnetron into a high power manufacturable device was "massive technological breakthrough" and "deemed by many, even now, to be the most important invention that came out of the Second World War"! Professor of military history at the University of Victoria in British Columbia, David Zimmerman, states: “The magnetron remains the essential radio tube for shortwave radio signals of all types. It not only changed the course of the war by allowing us to develop airborne radar systems, it remains the key piece of technology that lies at the heart of your microwave oven today. The cavity magnetron's invention changed the world.[7]”

Further, the official historian of the Office of Scientific Research and Development, James Phinney Baxter III, wrote: "When the members of the Tizard Mission brought the cavity magnetron to America in 1940, they carried the most valuable cargo ever brought to our shores."[8]

Further discussion of the significance of these technical developments are contained in the reminiscences by Churchill’s wartime director of scientific intelligence, R V Jones, latterly Professor of Natural Philosophy at the University of Aberdeen.[9]

What obstacles (technical, political, geographic) needed to be overcome?

Britain had the design but not the manufacturing capability for the very large number of required units and this necessitated the assistance of the American engineers to produce these components on the required scale.

What features set this work apart from similar achievements?

Nowhere in the world in 1939 was there a working, pulsed, cavity magnetron capable of generating 10 kW or more peak power at wavelengths of 10 cm or less, which had a compact portable size, used a small permanent magnet and which was readily capable of being manufactured at scale. This was the design breakthrough at Birmingham University.

Supporting texts and citations to establish the dates, location, and importance of the achievement: Minimum of five (5), but as many as needed to support the milestone, such as patents, contemporary newspaper articles, journal articles, or chapters in scholarly books. 'Scholarly' is defined as peer-reviewed, with references, and published. You must supply the texts or excerpts themselves, not just the references. At least one of the references must be from a scholarly book or journal article. All supporting materials must be in English, or accompanied by an English translation.

[1] Hugh Griffiths, “Some Reflections on the History of Radar from its Invention up to the Second World War”, Maxwell Foundation Newsletter No 8, Spring 2017. ISSN2058-7511 https://www.clerkmaxwellfoundation.org/Newsletter_2017_Spring.pdf

[2] Proceedings International Conference on the ‘Origins and Evolution of the Cavity Magnetron’, IEEE Cat. No. CFP10771-CDR, ISBN: 978-1-4244-5610-9, Library of Congress: 2009938875, 19-20 April 2010.

[3a] Mike Diprose, ‘The cavity magnetron: known knowns, known unknowns, but are there any unknown unknowns?’ 11th South Yorkshire Network IET Annual Radio/Radar lecture, South Yorkshire Aircraft Museum, Doncaster, UK, October 2015, PDF supplied.

[3b] Yves Blanchard, Gaspare Galati and Piet van Genderen, “The Cavity Magnetron: Not Just a British Invention”, IEEE Antennas and Propagation Magazine, Vol. 55, No. 5, pp. 244-254, October 2013. doi‎: ‎10.1109/MAP.2013.6735528

[4a] J.T. Randall, H.A.H. Boot, and C.S. Wright [Director of Scientific Research, Admiralty, London], “Magnetron: Improvements in high frequency electrical oscillators”, UK Patent GB 588185A, filed Aug. 22, 1940. https://patents.google.com/patent/GB588185A/en?oq=GB+588185

[4b] H.A.H Boot and J. T. Randall, “The cavity magnetron”, Journal Institution of Electric Engineers, vol. 93, Issue 5, pp. 928-938, 1946. doi: 10.1049/ji-3a-1.1946.0183

[4c] H.A.H. Boot and J.T. Randall, “Historical notes on the cavity magnetron,” IEEE Transactions on Electron Devices, vol. 23, no. 7, pp 724 – 729, Jul 1976. doi: 10.1109/T-ED.1976.18476

[4d] James Sayers, “High Frequency Electrical Oscillator”, US Patent 2546870 awarded 27/03/1951 on priority 03/10/1941. https://patents.google.com/patent/US2546870A/en?oq=US2546870

[5] E.C.S. Megaw, “The High-Power Pulsed Magnetron, a Review of Early Developments”, Journal of the Institution of Electrical Engineers, pp. 977 – 984, vol. 93, Issue 5, 1946. doi: 10.1049/ji-3a-1.1946.0187

[6] Allison Marsh, “From World War II Radar to Microwave Popcorn, the Cavity Magnetron Was There”, IEEE Spectrum, November 2018. https://spectrum.ieee.org/tech-history/dawn-of-electronics/from-world-war-ii-radar-to-microwave-popcorn-the-cavity-magnetron-was-there

[7] Angela Hind, "Briefcase 'that changed the world'". BBC News February 5, 2007

[8] B. Schroter, "How important was Tizard's Box of Tricks?", Imperial Engineer, 8: 10, Spring 2008.

[9] R.V. Jones, “Most Secret War”, Hamish Hamilton, London, 1978

Supporting materials (supported formats: GIF, JPEG, PNG, PDF, DOC): All supporting materials must be in English, or if not in English, accompanied by an English translation. You must supply the texts or excerpts themselves, not just the references. For documents that are copyright-encumbered, or which you do not have rights to post, email the documents themselves to ieee-history@ieee.org. Please see the Milestone Program Guidelines for more information.


Please email a jpeg or PDF a letter in English, or with English translation, from the site owner(s) giving permission to place IEEE milestone plaque on the property, and a letter (or forwarded email) from the appropriate Section Chair supporting the Milestone application to ieee-history@ieee.org with the subject line "Attention: Milestone Administrator." Note that there are multiple texts of the letter depending on whether an IEEE organizational unit other than the section will be paying for the plaque(s).