Understanding Circulary Waveguide - Experimentally

1Notes appear on page 48. Waveguide is an excellent microwave transmission line, with low loss and predictable performance, usable at any frequency by choosing the proper dimensions. The most common type of commercial waveguide is precision rectangular tubing, which is only affordable on the surplus market. Elliptical waveguide is also used commercially for microwave transmission line. Many microwave structures, particularly antennas, have a round cross section and are better suited to circular (cylindrical) waveguide. Unfortunately, commercial circular waveguide is rare and unlikely to be found surplus. As luck would have it, ordinary copper water pipe works just fine and is universally available at low cost. In particular, 3/4-inch copper pipe is perfect for 10 GHz. Most microwave design today is done with the aid of computers. However, only a few programs handle electromagnetics with the capabilities required for circular waveguide, and their prices are somewhere between a fancy car and a new house. Solving the problems without good software involves some difficult math, and the solution is probably only approximate. The remaining alternative is old-fashioned empirical microwave engineering. The ham’s favorite design technique, reverse engineering (copying something that works!) is made difficult by the lack of commercial circularwaveguide examples. Reference books have extensive information on rectangular waveguide but very little information on circular waveguide. The pioneering work on waveguides was done by George Southworth before World War II, but I only recently located a copy of his book.1 I was afraid that I had duplicated some of his work, but that’s a good way to learn. On first reading, it appears that he did a lot of work on waveguide fundamentals, but very little on waveguide-to-coax transitions, probably because good coaxial cable and connectors were not available prior to the wartime development.