World War II Day by Day

Washington, D.C. • September 8, 1940

On this date in 1940 the seven-mem­ber British Tech­ni­cal and Scien­tific Mis­sion to the United States, or Tizard Mis­sion as it was infor­mally known, assem­bled in the nation’s capital. Sir Henry Tizard, bril­liant vision­ary scien­tist and head dele­gate, had been in Wash­ing­ton, D.C., since August 22, meeting with notables, including Presi­dent Frank­lin D. Roose­velt. Tizard’s imme­di­ate task: for­mu­late ground rules for British and Amer­i­can scien­tists and their mili­tary advi­sors to exchange each nation’s trea­sured war-related research and devel­op­ment (R&D) secrets with the other. The British dele­gates brought with them what one Amer­i­can his­to­rian called “the most valu­able cargo ever brought to our shores,” a single cavity magne­tron that emitted 10‑centi­meter wave­length signals, or radar waves. (Radar is the acro­nym for Radio Acquired Detection and Ranging.)

During the 1920s and ’30s British radar R&D had led Bri­tain to deploy a chain of trans­mitter and recei­ver towers along its south­ern and east­ern coast­lines designed to detect incoming enemy air­craft as far out as 80 miles (150 km) or more using radio wave­lengths of between 10 and 13 meters, the famous Chain Home Net­work. Radio waves from the trans­mitter tower reflected off the incoming radar target (i.e., enemy air­craft) and returned to the receiver tower, pro­viding crucial infor­ma­tion about the target’s rough location and speed.

With the outbreak of World War II in Europe in Septem­ber 1939, Tizard, chair of the national research com­mit­tee that had orches­trated the devel­op­ment of British radar in the 1930s, came to the con­clu­sion that his coun­try, pressed by the imme­di­ate demands of war-related pro­duc­tion, could not success­fully engage its Euro­pean adver­sa­ries with­out the indus­trial, scien­tific, and funding resources of the United States, then a neu­tral nation. His auda­cious sug­ges­tion that Britain hand over its most sacred tech­ni­cal and sci­en­tific secrets to the U.S. in exchange for Amer­i­can pro­duc­tive and research capa­city was widely mocked until France and the Bene­lux coun­tries, Britain’s con­ti­nen­tal allies, fell victim to Germany’s mili­tary jug­ger­naut in May and June 1940. Prime Minis­ter Win­ston Chur­chill con­ferred with his White House coun­ter­part, Pre­si­dent Roosevelt, and the two agreed that a joint dele­ga­tion should explore the pos­si­bil­ity of exchanging closely guarded secrets in Washington, D.C.

American and British scientists and their mili­tary advi­sors began dis­cus­sions on Septem­ber 12. As the days passed both sides became ever more com­for­table sharing infor­ma­tion about ways to advance radar tech­nol­ogy. (Other topics apart from radar were dis­cussed but are off topic here.) Amer­i­can radar experts lamented that they were at loose ends trying to find a high-power vacuum tube that was able to gen­er­ate enough power (watt­age) to make for a fea­si­ble centi­meter-band radar sys­tem. On cue two Tizard dele­gates opened a wooden box they carried with them and pulled out an object small enough to fit in the palm of a hand—a super-secret resonant cavity magnetron, or simply cavity magnetron.

The British-made cavity magnetron was a high-power vacuum tube that con­trolled elec­tric cur­rent flow. It served as an elec­tronic oscil­la­tor, spitting out high-power pulses of micro­wave radio energy on a short wave­length of about 10 cen­ti­meters. The shorter the wave­length, the better the radar was at finding smaller objects. But the shorter wave­length needed more power to emit a coher­ent beam capa­ble of traveling miles to a target and back. The cavity mag­ne­tron was not only the key to making micro­meter radar work, it carried the poten­tial to bolster British—and by exten­sion Allied—mili­tary capa­bil­ities across the board and give their armed forces the upper hand in what truly was a technological war.

The high-power pulses of radio energy gene­rated from a device the size of a small book and trans­mitted from an antenna only cen­ti­meters in length reduced the size of prac­ti­cal radar sets by orders of mag­ni­tude. New micro­meter radars appeared on air­craft and even the smallest escort ships, and from that point on the Allies of World War II held a lead in radar that their foes never closed.

Cavity Magnetron: One of the Keys to Allied Victory in World War II

Left: Chain Home (CH) Network’s long-wave radar installation at RAF Poling, Bawdsey Manor, Sussex, 1945. (In May 1936 Bawd­sey Manor was the site of the first oper­a­tional radar sta­tion in Great Britain.) On the left are three (originally four) in­line 360 ft (100 m) steel trans­mitter towers that sent out pulses of radio energy. On the right are four 240 ft (73 m) wooden receiver towers that picked up the faint echo of enemy air­craft more than a 100 miles (160 km) away. The CH Net­work was the world’s first com­pre­hen­sive early warning radar system, which the British called radio direc­tion finding (RDF), oper­a­ting round-the-clock, rain or shine. Never had radar been used to blan­ket such a large area for air defense. Twenty-one CH stations covering most of the east­ern and south­ern coasts of Great Britain, along with a com­plete ground net­work of thou­sands of miles of pri­vate tele­phone lines, were ready by the time the Euro­pean con­flict erupted in 1939. Chain Home proved deci­sive during the Battle of Brit­ain in 1940. CH systems could detect German fighters and bombers while they were forming over the Euro­pean con­ti­nent, feed action­able flight data (number, type, height, flying direction, etc.) to a cen­tral plotting facil­ity, and give Royal Air Force com­man­ders ample time to mar­shal their entire force directly in the path of the approaching enemy. This had the effect of mul­ti­plying the effec­tive­ness of the RAF to the point that it was as if the British had three times as many inter­ceptors, allowing them to defeat frequently larger German forces.

Right: This anode block was developed for the original 6‑cavity mag­ne­tron. Two physi­cists at the Uni­ver­sity of Bir­ming­ham in England looked into the prob­lem of why current vacuum tubes pro­vided insuf­fi­cient power to be use­ful as a radar trans­mitter. By late Febru­ary 1940 they had in­vented the much more power­ful (resonant) cavity mag­ne­tron, which was fitted in an experi­mental radar by May 1940. Their first cavity mag­ne­tron proto­type pro­duced about 400 watts at 10‑cen­ti­meter wave­length, an extraor­di­nary achieve­ment. Within weeks engi­neers at Britain’s General Elec­tric Com­pany’s research labo­ra­tory had improved this to well over a kilo­watt, and within months 25 kW, over 100 kW by 1941, and pushing towards a mega­watt by 1943. By the end of the war in 1945, prac­ti­cally every Allied micrometer-band radar was based on a magnetron.

Left: (l-r) Sir Henry Thomas Tizard, Alfred L. Loomis, and Lee Alvin-DuBridge exam­ine a cavity mag­ne­tron on a table. Loomis was chair­man of the 2-month-old U.S. National Defense Research Com­mit­tee. On Septem­ber 29, 1940, he was pre­sent when Edward George Bowen, the young Welsh radar expert on Tizard’s mis­sion, care­fully revealed to his Amer­i­can counter­parts the dimin­u­tive cavity mag­ne­tron, one of the first 12 pro­duc­tion copies of Britain’s mys­te­ri­ous radar trans­mitter. Standing to Loomis’s right is Alvin-DuBridge, who became the director of Massa­chu­setts Insti­tute of Tech­no­logy’s Radi­a­tion Labo­ra­tory (“Rad Lab”), the cover name to dis­guise its true iden­tity as a radar R&D facility.

Right: The reasons the diminutive cavity mag­ne­tron was so vital to Britain’s sur­vi­val during the early years of the Euro­pean con­flict were its com­pact­ness and power. Placed inside the nose of an RAF fighter-inter­ceptor, for in­stance, a 10‑centi­meter pulse in minia­turized air­borne-inter­cep­tion (AI) radar systems would be sharp, losing little con­cen­tra­tion as it made its way to a fast-approaching enemy air­craft and bounced back. Longer radio waves in the multi­meter range like that of the Chain Home Net­work could only detect large fields of enemy air­craft, but a micro­meter beam pos­sessed the reso­lu­tion to pick out small objects like a single air­craft or several air­craft—even a U‑boat periscope or sea mines—for successful interception.

Battle Stations: Radar in World War II