We describe the design procedure for and realization of microwave bandpass filters that use two direct-coupled cylindrical cavities. Use of rectangular-to-circular waveguide transitions scrounged from commercial waveguide variable attenuators solved two problems: how to mate the filters to our microwave system that used rectangular waveguide and how to realize filters cheaply and quickly that could be used at different frequencies by changing only a small number of easily fabricated parts.
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In case the reader does not know, there is a trick for drilling holes through flat material so thin (in our case, brass shimstock 4 mils thick) that it would wrap around the drill bit if drilled directly and be destroyed. One must sandwich the thin material between two flat pieces of metal stock. (Aluminum or brass work fine, but the harder the metal, the smaller is the burr mentioned below.) If the pieces of metal stock are not free of flaws that would prevent them from being held tightly together with clamps, one should wet sand them on a clean granite or metal surface plate and then clean them afterward. Clamping the sandwich together allows one to drill through it and produce a clean, round hole in the thin material. The tiny burr left on the hole caused no problems for us.
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