k2aau wrote: > I have heard of repeater owners using pre-amps on the receive side of the > duplexer and adding 1 pass-reject cavity after the preamp and placing a > pre-amp on the pass reject cavity to enhance more receive. > > Does this work or is it a myth?
It does work on UHF. I have no experience on VHF because the quality of the preamp needed has always been out of my budget, but I believe it could work here as well. Basically, you separate the duplexer receive path between the first and subsequent stages, place a quality preamp between these sections. Then take the output of the receive path of the duplexer arrangement and feed a bandpass cavity - that output feeds a second preamp; which feeds the repeater receiver. I have used this theory on Motorola MICOR repeaters with a common 4 cavity WACOM UHF duplexer. It should only be considered if you have an extremely clean site and quiet nearby RF environment. You need to insure the interstage preamp is not driven into non linear operation, especially when the transmitter is on (sigh). Chip Angle manufactures preamps with a very high overload point - well suited for this type of experiment. Basically you are taking advantage of minimizing the losses ahead of the first preamp while insuring you aren't blowing it away. Then using a subsequent preamp (really as a post amp) to finish adding gain to make up for any additional losses. Obviously we can't put a preamp in the antenna line of a duplex repeater using one feedline, but, getting the preamp as close to the antenna as possible (without destroying it) allows us to take advantage of better S/N ration to the first active device - making the system noise figure better. Kevin Custer
