How MIT’s Rad Lab rescued D-Day

But the dreadful European weather—overcast and stormy, particularly in the late fall, winter, and early spring months—rendered all that firepower virtually useless. By the end of 1943, with D-Day only months away, the Allies had averaged only seven completed bombing missions a month; 70% to 80% of the year’s planned missions had been scrubbed or recalled because of the weather. Even if the Norden bombsight was truly capable of dropping a bomb into a pickle barrel—George Valley, for one, didn’t think so—it was useless if the bombardier could not see the target through the cloud cover.

The scientists

In 1904, the German physicist Christian Hülsmeyer had demonstrated that radio waves could be used to detect ships. Through the 1930s, scientists in England, America, Germany, and other technologically advanced nations had been working on using reflected radio waves to detect and measure the distance to and direction of objects that could not be seen because of darkness, clouds, precipitation, fog, or distance. Eventually, this new technology became known as radar, an acronym for “radio direction and ranging.”

In 1940, radar was still in its infancy. Radio wave reflections bouncing back from large objects provided little detail about the nature of the object, and small objects could not be detected at all. What’s more, the antennas required to send the low-frequency transmissions seeking the presence of these objects were too large for mobile deployment. Yet despite these limitations, radar was effective for defensive purposes. Anticipating war, England installed a chain of radar stations on its coastline to detect incoming German bombers and provide vector coordinates to fighter planes so they could intercept them.

H2X radar scope image off the Normandy coast from a bomber over the English Channel shortly before the landings. Assuming the range rings are set at 10-mile intervals, Cherbourg would be about 40 miles west of the plane’s position. By changing the range ring setting, the radar operator could zoom in closer or out for a wider view.

Radar scientists understood that transmitting higher-frequency radio waves would enable them to “see” more detail in the signal returns, detect smaller targets, and reduce the size of the antennas for mobile uses. But no device existed that could transmit high-frequency radio waves with sufficient power to detect objects at long distances.

As Britain watched Nazi Germany rebuild its military might through the 1930s, British scientists were urged to advance radio-wave technology for communications and radar detection. Two months after Germany invaded Poland in September 1939, physicists John Randall and Harry Boot, working at the University of Birmingham, sketched out a new concept for a radio-wave transmitter. (It later came to light that scientists in Russia, France, and Japan had come up with similar ideas but hadn’t developed them.) It took Randall and Boot four months to beg, borrow, and steal the components and equipment needed for a test. When they finally turned on what looked like a rat’s nest of wires, electronic components, transformers, electromagnets, vacuum pumps, and metering devices, they found that their resonant cavity magnetron generated a thousand times more high-frequency power than any known radio transmitter.

At the time, radio-transmitting tubes like the popular Klystron could generate at most 20 watts of power at microwave frequencies—only enough to detect objects that were relatively close. An MIT microwave radar system set up in a rooftop laboratory before the war could detect nearby planes and clock moving automobile traffic on the other side of the Charles River, but its reach was too limited for military applications.

The cavity magnetron would make radar portable: Radar systems could be designed using antennas small enough to be installed in boats, trucks, and planes.

Randall and Boot’s resonant cavity magnetron, however, could generate high-frequency radio waves at power levels three orders of magnitude greater than the Klystron—and high-frequency radar systems with more powerful signals would detect smaller objects in greater detail at much greater distances.

What’s more, by increasing the frequency of radar transmission from 30 megahertz to 3,000, Randall and Boot had shortened the signal wavelength from 10 meters to 10 centimeters. They realized that their new device could make it feasible to develop radar systems that could count approaching bomber planes or detect a surfaced submarine even at a great distance—and do so with much smaller transmitting antennas.