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History of circular waveguide
This story came from Professor Jin (albeit second hand) and University of Illinois.
In 1897 Lord Rayleigh performed the first theoretical analysis of a wave in a circular waveguide, but by the time World War II came around, the men in charge of the war effort did not think to look at Lord Rayleigh's manuscripts for guidance. The legend of its rediscovery is this: The US military (US Army Air Force (thanks for the correction, Doug!) was transmitting radio waves using glass dielectric waveguides, using it much like we now use optical fiber, but they needed something a little more bendable and fluid than glass. So, thinking about the most fluid thing around, they decided to try water as a dielectric medium. They designed an experiment, built their water waveguide, and behold, the water was a great dielectric waveguide. Then, being good scientists, they wanted to re-test their results the next morning before writing up the official report. They tested their waveguide again and found that it was even better than the night before! Then one of the technicians found a puddle of water underneath their waveguide. This puzzled the men, so they opened up the pipe that had contained their water waveguide and found that it was empty. All of the water had leaked out in the night. After some thinking, they figured out that the water had nothing to do with the propagation of the wave. They realized that the reason the wave was confined in their dielectric (air/water) was because of the iron walls of the pipe. Thus the first practical metallic waveguide was a round air-filled pipe. It was not until some time later that it was found that Lord Rayleigh had predicted the behavior (but not necessarily the application) of metallic waveguides.
Standard sizes of circular waveguide
Visit our page on rectangular waveguide dimensions, and download "Uli's List". Circular waveguide internal cross sections (apertures) are given in chapters 6 and 7. At this time Microwaves101 does not have a list of standard circular flanges to offer you.
More to come!