KA3SGM
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Premium
join:2006-01-17
West Chester, PA
clubs:
 ·Verizon FIOS
| reply to RFJUNKIE
Re: modifyng duplexers
said by RFJUNKIE  :Cleaning up or 145.310 repeater... we are in a shared building and had the big 10" Motorola cavaties took to much shelf space.. I acquired a set of six Decible 5" 148-170mhz pass cavities(db4001-2) they will not tune tight enough on 145.310 they start losing Q at abt 148mhz can i modify the loops to move them down in freq??? or do i have to lengthen the tunable shunt also .... I cant find much info on mods on cavities...I can look at them on a spectrum analyzer but don't want to hack a good set of cavities then find it wont work.. I am figuring that you already have run the external tuning stubs out as far as possible???
The physical cavities being smaller are probably not capable of going below 148MHz, but the tuning stubs are more of a limitation.
I am thinking the cavities themselves might be able to go lower in frequency, but longer tunning stubs would be required, and as mentioned that as the Q drops off at 148MHz, the signal loss would also be going through the roof.
If you have a spectrum analyzer available, it would be interesting to see if you can set the notch frequencies to spec, and find out how bad the loss is.
If you don't get a better answer here, try posting the question at:
»www.repeater-builder.com/rbtip/
That is the best repeater owner/builder site that I know of, with a lot of long time experts to ask questions.
(KeysCapt please take note of that site as well.  )
Maybe you could trade off with someone else that has a repeater at +147MHz with a + .600 offset that might be more compatible?? 
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