HOW THE ATLANTIC WAS WON

The crucial battle of the Atlantic was finally won in 1944 by a little electronic tube called a magnetron that came to airborne radar in 1943-44. I was then an 18-year old radar mechanic on 311 Czechoslovak Squadron of the Royal Air Force in Britain. It was allotted to Coastal Command which hunted U-Boats in the Atlan-tic and the North Sea, so I know a little about this tube which today powers your microwave oven and is about to be used for scrubbing he pollutants out of the smoke in electric power plants.

The principle is still the same, but I will describe the magnetron I used to know, though that was 40 years ago and I would not swear to the details, such as the number of resonant cavities used then. But the principle has not changed.

How did the magnetron win the Battle of the Atlantic? When the batteries of a submarine are charged, they consume consider-able quantities of oxygen and produce dangerous gases, so the sub-marine has to go to the surface to ventilate its interior during the charging. (Another good reason to have nuclear subs.) When it did so, it was detected by the airborne radar of the anti-submarine patrols, which attacked it by rockets and, when it dived, by depth charges.

The Germans defended themselves against this by a device called Schnorkel, a small air duct bent at the top by which the U-boat took in fresh air while staying submerged. A radio wave is well reflected only by objects much larger than its wavelength, and thc original airborne radars worked on a wavelength a little shorter than 2 m (about 6 feet), so that the Schnorkels were invisible to them. What was needed was a powerful transmitter at the then fantastically short wavelength of about 3 cm (1.18"). The transmit-ter tube that achieved this, packing some 10 kW of peak power into a pulse of several microseconds (repeated some 150 times a second, so that the average power was quite low), was the mag-netron.

The heart of the magnetron are its resonant cavities. A simple case of a resonant cavity is an organ pipe. The wedge in the way of the air stream produces sounds of many pitches, which are all reflected back and are partially or totally canceled by the incoming sound wave of the same pitch. Only some frequencies, whose wavelengths are integral multiples of the length of the pipe, get reinforced by the incoming sound wave of that frequency, and the wave (standing, not propagating) builds up until it is very strong, very much stronger than the unwanted sound waves: it resonates, with the pipe forming a resonant cavity. Of course, when the in-coming stream is cut off, the wave will die out, but even then it reverberates longer than any of the other frequencies.

In the same way one can make a resonant cavity for electromag-netic waves, e.g., a rectangular box which will resonate when its three dimensions are integral multiples of the (half) wavelength exciting the box whose walls reflect it back and forth. Since radio waves generally have a much longer wavelength than sound waves, there is usually only enough "room" for only a single frequency in-side the box. When the cavity is a sphere the mathematics get a little hairier, but the principle remains the same.

The resonant cavities of a magnetron, to be described below, were spherical and resonated at a frequency in the 3 cm band.