Our Repeater Duplexer As recalled by Gerald Johnson KØCQ (January 2005)
I built the duplexer from cavities I bought at a Cedar Rapids Hamfest long ago. I recall it was one at the 5 Seasons convention center but maybe Hawkeye Downs. Anyway, it came as a collection of 7 DB Products cavities, no cables. The seller said, "insult me with an offer" so I offered $25 a cavity (actually $150 for the collection) and he took it nearly instantly. When I got the cavities home, I found that three of them had been apart and the center conductor had sleeves of cruddy (mixture of green corrosion and paint) 1-1/2 inch copper pipe supported on long self tapping screws. I think they hoped to get the cavities down to 145 MHz. I cleaned them up and made up connecting cables of RG-223 and they give enough isolation for a 25 watt repeater and have proven to have good stability. In tuning them I did discover the importance of well shielded cables for generator and receiver and the importance of well shielded generator and receiver. Using a Yaesu multimode and HP 608F didn't quite meet those. Moving cables changed the notch depth. I think the attention to shielding the receiver and transmitter parts of the repeater have added to its performance compared to those created by others to the south that seem to have lifetimes under a couple years. Using the Polyphasors has to have helped too. The controller was way out front of the state of the art that has probably caught up and passed.
