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Notch duplexer question help please!

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kb3avz

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I have a question concerning a notch duplexer in a motorola gr1225 repeater. The duplexer was tuned by a radio repair shop for us for use on 462.675/467.675 gmrs. Since another repeater has decided to come on the air and is causing us interference, and we have to move frequencies. My question is can I just program the repeater to the new frequencyof 462.00/467.700 without an issue? How sensitive/tight are these notch duplexers tuned? Thanks

Frank
wqtb834
kb3avz
 

mkewman

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I have a question concerning a notch duplexer in a motorola gr1225 repeater. The duplexer was tuned by a radio repair shop for us for use on 462.675/467.675 gmrs. Since another repeater has decided to come on the air and is causing us interference, and we have to move frequencies. My question is can I just program the repeater to the new frequencyof 462.00/467.700 without an issue? How sensitive/tight are these notch duplexers tuned? Thanks

Frank
wqtb834
kb3avz
I personally wouldn't. You'd probably be ok for a couple weeks or months but depending on how much usage the repeater gets, you'll probably damage the repeater eventually, and since the PA for 1225s is no longer sold by Motorola, I don't recommend doing it. *Some* people might say it's close enough, but If you want to get the most power out of that thing, and get 20 more years out of it, I'd do it right.

That being said, tuning a duplexer is relatively simple (the GR1225 is a little tight and sensitive to metal screwdrivers so it's slightly harder) and should only take about a half hour to an hour. Even less time (probably 15 minutes or less) since you're so close in frequency Drop in on a local radio shop and see if their bench tech will cut you a deal to do it on their lunch break for a 6 pack of beers or something. Honestly making a good connection with a local mom and pop Motorola tech that way will benefit you more in the long run as well. You never know when your 1225 will have some issues and that tech may stockpile some of those discontinued parts. Making a friend could give you access to those parts.

The GR1225 is a very good repeater that can last you a long time, I highly recommend doing it right and getting it tuned. If you lived near Northern California I'd probably be able to do it for you.
 

prcguy

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So Cal - Richardson, TX - Tewksbury, MA
If its a flat pack mobile duplexer and it was tuned right and well centered on your original frequency its usually no problem to go +/- 50KHz which is 2 GMRS channels away. Going up or down one channel should not have any affect at all since the mobile notch type duplexers are not that sharp.
prcguy
 

ElroyJetson

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Florida, where you wish you were!
I tune duplexers almost every day and have on some occasions have had need to tune duplexers to be used on a small range of frequencies like your situation. As long as all frequencies are within about a 100 KHz frequency range and the duplexer is tuned to match, it'll be fine.

There's NO problem regarding damaging the transmitter output, either, as the duplexer's TX side is tuned to reject a narrow notch of frequencies matching the receiver frequencies. TX power should not be significantly affected. The duplexer is a notch duplexer, not a pass band duplexer, if it's your regular compact mobile style unit, so the notch in the TX side is 5 MHz away from the transmitter frequencies. (Or 3 MHz away in the UHF T band)

If you have the right tools (a spectrum analyzer and tracking generator, or a network analyzer) then
tuning a duplexer takes 5 minutes once it's connected to the tuning setup.
 
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