I appreciate all of the suggestions. Two cavities wouldn't work here, I can pass a couple of frequencies 500 Kc apart, but the insertion loss is too high between them, as shown in the plot. This typically takes several cavities, for example, here's one for high band that Telewave sells with 6 cavities, I think it's about 1 MHz wide: https://www.telewave.com/product/tpcp-1556-6-cavity-receiver-preselector-148-174-mhz/ WB5WPA: Yes, the plot is two cavities in series on the same freq (the upper freq) with a critical length cable, and the type of filter I'd like to be able to build is similar, but with more cavities, as shown in the link above. What I'm trying to figure out is how they manage to do it as wide as they do with only a couple dB insertion loss. I can't seem to make it work with simple stagger-tuning, so I assume there is something else going on, like the conjugate matching I did with the critical cable, maybe in combination with some other tuning method with the cavities themselves.