I was wondering if I could get a quick sanity check/feedback on how I go about tuning Bandpass duplexers. Some recent follies have had me questioning whether I'm doing it right.

1. Unharness all cavities
2. Take a direction coupler/return loss bridge and quality 50 ohm load and go cavity by cavity, tuning each cavity for best return loss
3. Normalize all cables and hook up to each individual cavity for insertion loss, adjust loops for desired insertion loss if applicable, set notches
4. Harness up all cavities
5. Now, after I harness up all the cavities, should I readjust each pass for best return loss? That's what I normally I do but while working on a duplexer, it was said not to touch the pass tuning after harnessing back up. But if I don't, I'm wasting insertion loss and return loss match and some performance.
6. Set notches on each pass

What's the proper method here or what are people's thoughts on this?

Have you watched the tuning video from Sinclair that I sent to you before?



Mick - W7CAT

1. Unharness all cavities

That's the only one you got right ?

After unharnessing, the first thing you do is set the coupling to the desired or spec’ed insertion loss. For a bandpass cavity, each loop has to be set such that the return loss is identical in both directions (S11 and S22).

As you adjust loop coupling, the resonant frequency is going to move around, so you have to do this first, not late in the game. Also, the return loss is going to vary with coupling. The tighter the coupling, the less insertion loss there will be, and the better the return loss will be. As you loosen the coupling, the insertion loss goes up (and so does the Q), and return loss degrades. A properly-operating bandpass cavity will typically yield 20 dB of return loss when the coupling is set for 1 dB of insertion loss – you can use that as a sanity check.

At the risk of repeating myself, it is imperative that S11 and S22 be identical. If you don’t have a VNA to do this, you’re going to spend/waste a lot of time swapping cables around and going back and forth between the two loops iteratively until you get it right. So, unless you know that somebody has messed with the coupling, or if you see S11 <> S22, you may not need to even touch the coupling.

Once the coupling is set, then you tune them for the frequency of interest based on the best return loss. The harness everything back together and check that it makes spec. If everything was done correctly, and the cable lengths are correct, you should not need to do any "touching up".

6. Set notches on each pass

How would you have notches in a bandpass duplexer??!?!?

--- Jeff WN3A


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It’s my Sinclair Q2330e

On 11/8/2022 7:11 PM, Jared Smudde wrote:

It’s my Sinclair Q2330e

Oh - well - okay - that's not a bandpass duplexer.