Notch Duplexer Tuning

[WSGB808]

Morning.

Does anyone know of a radio shop located in the Austin/Buda Texas area?

I’m on a mission to set up a public repeater in the Buda area and I’d like to locate someone local that may be able to tune or assist me in tuning my inexpensive notch duplexer. 

I have a VNA and I think I know how to do it based on my research, but it would be nice to have a local contact with more experience.

Any advice would be appreciated.

Thanks all!

[WRUE951]

I tried to tune one myself using a Nano VNA and watching on line videos..  It didn't go very well  .  Ended up sending the Duplexer to BuyTwoWayRadios.com.  I live in Calif i think i sent to a Florida address. Overall it took about 8 days to and from at a $60 buck fee..   There are people that advertise on Ebay offering Duplexer service as well, fees are about the same..  I'v always had good luck with BuyTwoWayRadios, personally i'll stick with them..  Good luck  

[Flameout]

I bought one of these cheap, Chinese spectrum analyzers and I was pretty successful in tuning the flat pack duplexers. I tried tuning one, then took ot to a local guy that had REAL equipment and he was surprised at how close I had it tuned. They are a pain to work with though!

https://www.ebay.com/itm/353219998270?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=_4IuJVqfS0m&sssrc=4429486&ssuid=0zihXCkJQWG&var=&widget_ver=artemis&media=COPY

 

This will give you an idea of what it looks like while tuning

 

[WRYZ926]

I have to agree with everyone else, I would not try to tune a duplexer using a NanoVNA. We have member in our local club that retired from Sound Solutions and he still has access to all of the proper equipment to tune duplexers for us. A person really does need the proper equipment if they want to do it right.

@WSGB808 I suggest reaching out to any local amateur radio clubs near you and ask them.

[WRKC935]

While I disagree with it needing to be done by 'someone with proper knowledge and equipment' the nano VNA is fine for a NOTCH duplexer, in my opinion, that's the opinion of a guy that's been donig this for 15 years professionally and over 20 years in general. 

The little flatpack notch setups aren't going to be tight enough that if they are only close they will not be close enough.   And there are tests that can be done to ensure they are at least that close.  I have tuned them with nothing more than a radio, attenuator, RF signal meter and a bit of know how.  And when I put them on a regular VNA they were dead on. 

Biggest concern is does the VNA output the frequency it says on the display.  Easy test is put a frequency in it that you have a receiver for and verify it can be heard in the receiver.  If you can hear it as a carrier, then it's on frequency enough to tune a flatpack.

If you decide to take it on yourself, you need to know a few things.  First is the open port on the fatpack needs to be connected to a dummy load.  If it's not terminated, ti's not going to tune up right.

Second is how you actually move the notch.  Guys without any experience will just start cranking on whatever cavity they feel the need to and NOT move a little at a time.   Once you move one cavity off far enough the notch in that cavity will disappear.  Once that happens, it's a PITA to figure out.  So make your changes up or down about a half turn at a time and do ALL three cavities with each step move.  You will want to set your VNA with a wide frequency range to start.  Once you are close, then you drop the bandwidth down to 1 Mhz and finally 500Khz to walk it in all the way.   Remember that you are notching the TX out of the high side (receive side) and the RX out of the low side (transmit side).  They should be marked high and low.  So follow that and you will be fine.  DO NOT however attempt to tune one the way I talked about before with just using a radio, attenuator and RF meter.  I have tuned hurdreds of duplexers, built up combiner networks from parts, learned all the math and crap that goes with doing that.  It's not stuff for a beginner to attempt.

 

[marcspaz]

No idea about local shops, but as mentioned above, local amateur radio clubs should have someone with the right tools and experience.

 

Just my opinion based on experience, using the cheap Nano VNA's that you get off of Amazon are not good enough to tune a duplexer well.  You don't have to spend a fortune.  I only paid about $130 for the one I use for 2m/VHF Pass/Reject duplexers with 0.600 MHz splits.  I am getting -100dB notches that are only 25KHz wide.

 

I would also recommend asking whoever tunes it for you, if they would be willing to teach you how to do it.  A good portion of amateurs I know actually like helping people learn.

[WRUE951]

1 hour ago, WRKC935 said:

 Once you move one cavity off far enough the notch in that cavity will disappear.  Once that happens, it's a PITA to figure out.  

 

Exactly what happened  to me with a UHF Fiplex..    In fact i was worried when i sent it in to BuyTwoWayRadios of getting  a message saying 'No Go'...   I just picked up a Panorama UHF on Marketplace for $25 bucks.. Basically got it to mess with and try to learn the tuning art using a Nano...  Going to also order a Tiny SA for a sig generator source...   

[WRYZ926]

While one can tune a duplexer with a NanaVNA, how accurate is that going to be. And you really want the duplexers spot on when there are other repeaters on site and/or other GMRS repeaters in the area.

As we all know RF can do weird things. I'll say it again, ask around with your local amateur radio clubs and see where they get their duplexers tuned. Most of those people will be more than happy to help.

[WRUE951]

1 hour ago, WRYZ926 said:

While one can tune a duplexer with a NanaVNA, how accurate is that going to be. And you really want the duplexers spot on when there are other repeaters on site and/or other GMRS repeaters in the area.

As we all know RF can do weird things. I'll say it again, ask around with your local amateur radio clubs and see where they get their duplexers tuned. Most of those people will be more than happy to help.

In my case, i'm just wanting to learn the tunning process.. Unless i can compare to a professional tune i wouldn't depend on the Nana tune.. Another reason i got a 'el-cheapo' duplexer...   

[WRYZ926]

46 minutes ago, WRUE951 said:

In my case, i'm just wanting to learn the tunning process.. Unless i can compare to a professional tune i wouldn't depend on the Nana tune.. Another reason i got a 'el-cheapo' duplexer...   

And nothing wrong with that either. I find that I learn best by hands on.

[WSGB808]

Thanks all for your input and feedback.

I took a stab at tuning it with a NVNA. I’m sure it’s not as good as it could be,  It’s inexpensive equipment so good enough is something I’m okay with for now. 
 

I did ensure my VNA was frequency accurate, thank you for that. 
 

Getting about -70dB with a 25mhz spread from both hi/lo. I don’t have the resolution to get much more and it got to the point where I was just chasing it so I’ll live with -70 for now.
who knows, it’s a cheap duplexer so maybe it can’t even get better than that.
 

I did happen to connect everything backwards at first which caused an interesting and frantic full volume loop at midnight. Wife was not happy.

Thanks again everyone.