Duplexer Tuning Problem

[ajanke99]

Hello everyone,
I recently acquired a Decibel Products duplexer, model DB-4072. It is a 6 cavity notch only duplexer. I am tuning this for a gmrs repeater I put together with the frequencies 462.550/ 467.550. On the low side of the duplexer I get a great notch at 467.550, at somewhere around -80 db, and great insertion loss numbers. On the high side however I have run into an issue. Separately, each of the cans seems to be working properly and I get 2 of the 3 to line up with one another and I get a decent notch (around -50 db), but when I try to bring the third notch onto the other two, the notch becomes very wide and does not want to go below -45 db. The insertion loss at the RX frequency is also very lousy. I've tried swapping cables and connectors around from the low side as it seemed to work great, but it made no difference. I'm just stumped at this point and can't really find much online about my particular issue. Any help would be greatly appreciated.
- Thanks

 

[prcguy]

What was the original frequency the duplexer was delivered on? Have any of the cables been replaced? Wrong cable lengths are usually the culprit when all the single cans tune the same but they don't play nice together.

Hello everyone,
I recently acquired a Decibel Products duplexer, model DB-4072. It is a 6 cavity notch only duplexer. I am tuning this for a gmrs repeater I put together with the frequencies 462.550/ 467.550. On the low side of the duplexer I get a great notch at 467.550, at somewhere around -80 db, and great insertion loss numbers. On the high side however I have run into an issue. Separately, each of the cans seems to be working properly and I get 2 of the 3 to line up with one another and I get a decent notch (around -50 db), but when I try to bring the third notch onto the other two, the notch becomes very wide and does not want to go below -45 db. The insertion loss at the RX frequency is also very lousy. I've tried swapping cables and connectors around from the low side as it seemed to work great, but it made no difference. I'm just stumped at this point and can't really find much online about my particular issue. Any help would be greatly appreciated.
- Thanks

 

[ajanke99]

What was the original frequency the duplexer was delivered on? Have any of the cables been replaced? Wrong cable lengths are usually the culprit when all the single cans tune the same but they don't play nice together.

The original frequency pair was 450/455. No cables have been replaced to my knowledge. Everything inside looks factory to me. After I tried tuning to my frequency pair I then tried to retune it to the original pair but still encountered the same issue. I neglected to see what frequency it was tuned to when I received it. I'm leaning towards improper cable length based on what I've read online.

 

[prcguy]

Sometimes with problem duplexers its worth the extra effort to take all the cables off, tune each cavity separately, then start putting cables back on one by one and check the tuning. A cavity can sometimes get inadvertently tuned way out of band and disappear messing up the rest of the chain.

After tuning all cans separate I would start adding cables on the radio end working towards the T as you touch up tuning after each cable is added. If it still doesn't tune up try on the original shipped frequency or even in the amateur band, as someone could have diddled the cables to make it work there. If that doesn't work its time for another duplexer. I have several here that just don't want to tune up anywhere and its probably due to wrong cables. Finding the perfect cable length is very difficult outside of the factory.

The original frequency pair was 450/455. No cables have been replaced to my knowledge. Everything inside looks factory to me. After I tried tuning to my frequency pair I then tried to retune it to the original pair but still encountered the same issue. I neglected to see what frequency it was tuned to when I received it. I'm leaning towards improper cable length based on what I've read online.

 

[Project25_MASTR]

I'm with @prcguy in suggesting tuning the cavities individually and then putting it all back together.

Looks like those are single port cavities so you'd just be removing the cables from the T and hooking your TG up there. Once you get it put back together it should look a lot better (you'll need to fine tune with it all back together). If it doesn't, it would suggest a bad jumper between the cavities (which are length specific).

 

[ajanke99]

Thank you for the suggestions. Will give them a try and hope for the best.