CARSON'S RULE BANDWIDTH = 2 X (PEAK DEVIATION + HIGHEST MODULATING FREQUENCY) Most 2M off the shelf radios I have seen lately in wide band FM hit at least +/- 5kc peak deviation, and 6 to 6 � is not at all uncommon. Measurements using HP8921 or IFR COM120B. I have not run a swept transmit audio response test using an audio sweep generator into the mic input @ 60% full system deviation, but I suspect it would not come to a dead stop at 3 kc of audio response. Hence the 16 khz or more of occupied band width. Now add a little bit for slightly off frequency operation, and a little over modulation and you really crowd the channel bandwidth. Add to that a little excessive high pass in the post limiting audio filter and the bandwidth again increases. The selectivity of a VHF FM radio is largely determined by the IF section, and crystal filters. The front end helical resonators or tuned lines from the old days, are going to be more than 200 kc wide at 3 db points. Even a couple of band pass cavities in front of the receiver will be very large in bandwidth compared to the overlap of the two repeater input frequencies. Using a MastrIII or MSF5000 would have better front end performance, but adjacent channel rejection (when the adjacent channel shares part of the desired channel) is not going to be very good. Going to a Micor or M2 or adding a helical preselector will help, but mostly by adding insertion loss and not in bandwidth shrinkage. At best, changing the IF filters might narrow the response, or offsetting the mixer frequency on the receiver, or mistuning the IF away from the neighboring input might help, at the expense of degrading performance on the desired operating frequency. Introducing loss in the front end may also help. I wonder how a signal generator at the input of the repeater generating on the offending frequency would act? If a -80 dbm. Signal at the antenna port modulated at 1 Kc tone with 4 or 5 kc of deviation bothers the repeater then it will be difficult to ask your neighbor to turn things down. Anyhow, best luck with the project and let me know how it turns out, Steve
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