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Author Topic: Can you tune a 2M duplexer with only a VNA  (Read 253 times)

KG4RUL

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Can you tune a 2M duplexer with only a VNA
« on: November 16, 2019, 06:00:06 AM »

Is it possible to tune a 2M duplexer using only a VNA?  If not, what else, beside the VNA, would be needed?
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N8AUC

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Re: Can you tune a 2M duplexer with only a VNA
« Reply #1 on: November 16, 2019, 06:17:54 PM »

I suppose it depends on the VNA.

If the VNA can be swept over a wide enough frequency range to enable you to see the filter skirts of the duplexer, I don't see why you couldn't do it with just a VNA. After all the VNA is a device that provides stimulus and measures response. Which is basically what you'd do with a sweep generator and a spectrum analyzer.

The benefit of using a VNA is that it also allows you to see phase response as well as amplitude response as a function of frequency.

73 de N8AUC
Eric


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K5LXP

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Re: Can you tune a 2M duplexer with only a VNA
« Reply #2 on: November 18, 2019, 08:29:49 PM »

Duplexer implies three ports.  You can tune this with a 2-port VNA provided you terminate the unconnected port with a load.  You'll have to go back and forth a few times and touch up the tuning if there's any interaction.

Part of my day job is tuning microwave duplexers.  I use a 4-port VNA so I can see all the ports at once, so that any interaction between them (port isolation, mostly) can be directly observed.  One limitation I can see to an inexpensive VNA is due to the limited dynamic range you may not see any deep nulls/notches.  But should be more than adequate for setting and measuring passbands.

Mark K5LXP
Albuquerque, NM

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N4DBM

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Re: Can you tune a 2M duplexer with only a VNA
« Reply #3 on: November 19, 2019, 05:22:58 PM »

When most techs think of using a "VNA," they are often thinking of one-port return loss.  When using a two port analyzer, most are thinking tracking generator. 

You can use a one-port VNA under return loss operations to tune a pass filter with a correct 50 ohm load attached to the other end.  You can tune the filter for best return loss and pretty much bet that the filter is optimized for best pass-through characteristics at the frequency you are tuning.  This is helpful at times when tuning a pass cavity between a known 50 ohm device and a slightly mismatched antenna.  You can look into the input of the cavity and tune the cavity for best return loss while looking into the slightly mismatched antenna and compensate slightly for the mismatch. 

I have found that the best way to tune a pass/reject, or a reject only duplexer is with a two-port VNA and run the plot in "insertion loss" mode.  The top of the graph should be set to zero dB and the bottom at -100 for instance.  You need to use phase-stable cables between the analyzer and the duplexer.  Be sure to terminate the other side of the duplexer with a 50 ohm load while you're tuning the one side.  You will be able to see the notches and pass curves in a visual manner and get them right on target with your marker table, depending on your analyzer.

If you are asking can you tune a duplexer with a one-port VNA using the return-loss method, I would say no.  You can expect the return loss to be low at frequency X when the cavity is properly tuned to frequency X, but it would be near impossible to tune the reject notches with a one-port VNA.  Although in theory you would think that really bad return loss at a certain frequency would mean a reject notch, it isn't always dead-on.  You really need a two-port analyzer, calibrated, and using phase-stable cables to do your very best work.

Best regards and best of luck,
N4DBM.
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KB8VUL

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Re: Can you tune a 2M duplexer with only a VNA
« Reply #4 on: December 16, 2019, 04:44:52 PM »

Well you could possibly tune a duplexer with a single port VNA using two 50 ohm terminators.
The easiest way would be to set a marker for the center frequency of one of the frequencies and then begin by tuning each can for pass and reject by disconnecting the can to can jumpers and putting the terminator on one port and the VNA on the other.  I would start with a 2.5 or 5Mhz sweep for this.  You may need to increase the sweep at first if the cans are tuned far from the desired frequency.
Once you have all the cans tuned for their specific pass and reject frequencies, you will need to put all the jumpers back in place and terminate both the RX and TX ports on the duplexer and sweep at 1 Mhz and trim up the tuning on the cans for the final time. 
You will want two markers for this.  One for the TX and one for the RX.  That should work.
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KB8VUL

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Re: Can you tune a 2M duplexer with only a VNA
« Reply #5 on: December 16, 2019, 04:48:19 PM »

All that being said.  Most any VNA is two port.
For tuning with a two port VNA, you still tune each can individually then you inject into the antenna port, and connect to the RX or TX port and terminate the other port with a 50 ohm load.  You are looking for the same thing, pass and reject.  On the TX side you are looking for best pass of the TX freq and best reject on the RX frequency.  Opposite is true on the RX side. 
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