My thoughts.... They may be incorrect, If a duplexer was constructed so that the Inner conductor to outer conductor ratio was constant the length to the tunning rod, you wouldnt have the swr dips. Sure the main part is the proper ratio, but none of the designs take into account the evil impedence mismatch on the "trombone" at the bottom of the rod. So while the top half of the cavity is 50 ohm, the botom is somewhere between 60 and 85 ohms. Most likely 75 ohm considering materials avalible. The solution of using a solid threaded rod is one way to circumvent this problem, but the materials avalible (half inch) limit the cavity to a relitivly low Q factor.
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