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Well, since it is usually better to learn how to fish than to have to buy fish... Try starting with the Wikipedia. Another excellent source for repeater information is Repeater-Builder.com. Here is a page from them on duplexers. But, to get you started... very basically, each cavity performs one of two functions. It either: Filters out everything except a specific frequency (a notch filter)Filters out just a specific frequency (a band-pass filter)As you add more cavities (I.e. the more times you filter the signal) the effect of the filter becomes more pronounced. The low cost, so-called mobile, duplexers consist of only notch filters - 3 for receive, 3 for transmit. The transmit cavities (attempt to) filter out any signal other than the transmit frequency. The receive cavities do the opposite, filtering out everything except the receive frequency. Adding a band-pass filter will boost the effectiveness of the duplexer. For example, a receive band-pass cavity would be turned to attenuate the transmit frequency. Note that most high-end and commercial duplexers consist of both types of cavities.

Sounds like a kid I once heard say he put 7.5" pistons in his 6 cylinder Mustang engine to get higher compression for his supercharger... and now it runs 9 seconds in the quarter. I never laughed so hard in my life.

I build repeaters, tune duplexes, transmit combiners and I have no idea what you are trying to say here.... What?