Cavity filter not working?

History and origin wise, is it not Latin vs Greek for the du or di? Seriously though the technical use of these terms relates to either directional properties, so things like couplers separate by signal path direction while the other does it with filters to separate them by frequency?

Origins aside, the long standing common usage of the two terms is now being changed. For no reason.

We had an understanding on which was meant, but now with the terms being used interchangeably, people just getting into the hobby will be more confused than ever. It don’t matter if it is a battle between greek or latin origin, it’s about what has made sense in our language since radio was invented. In both cases, the separation is with respect to frequency, not direction. One is broadbanded and the other is very selective, its not really a direction thing. Directional couplers and isolators/circulators are a thing on their own, none of them are filters (even though their tuning can make them behave as such).

Not sure I agree - a cavity filter for a repeater is directional surely? The in and out ports are used directionally, despite reverse working being perfectly possible. The little boxes splitting UHF and VHF I have always termed a diplexer, and a typical repeater style cavity a duplexer - that’s how I use the term with this topic in mind.

We agree then on the terms, diplexers for separating out of band signals and duplexers for in-band separation. But both are still just filters, regardless of whether they work in reverse as well as they do the way they were tuned. I wasn’t trying to destroy a thread by starting a big debate, just took pause to rant about something I figured anyone serious about radio would agree with (that being why MFJ is calling a diplexer a duplexer). For a company that knows the difference, I can only assume they are trying to scam newcomers in this age of “everyone wants their own repeater without doing thier homework”. No offense intended to the OP. My frustration there is toward MFJ and the like. I wasn’t trying to take away from the OP’s issue by ranting about terms, which is where this is headed, so I don’t really care what anyone calls it…

I really hope he invests in a nanoVNA or similar analyzer. If he had one, we could walk him through this step by step… It is nearly impossible to help someone set up a repeater system when they cannot even measure their antenna impedance.

My recommendation is to take a pause from the repeater project and study up on what you are working with and maybe get the tools necessary to do this stuff.

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OK, thats good to know.
Would a UHF antenna still work OK for receiving VHF even if not tuned for that band?
I believe its not so crucial for receiving the signal

Ive got a nanoVNA, but have been advised they are not accurate enough, my mate tuned in on what i think was called a rigmaster? I had tuned with the VNA, but it was way out he told me.
He said the big issue with these is you have to keep calibrating the unit to get an accurate measurement.

What model and hardware version is your nano? A nanoVNA is certainly underrated for tuning a duplexer. If you have a good nano, you might get it close enough for it to function. For antennas, it would be just fine, assuming you have it calibrated properly. Not sure what a rigblaster is (i believe computer radio audio interfaces), but I have heard of rigexperts (which are probably no better than a good nano).

Long before I knew anything about radio, I managed to set up a VHF repeater well enough to work a 50 mile radius with 25w mobiles from a 30’ tower holding a 5/8 wave maxrad GP antenna. The duplexer tuning was done with nothing but a radioshack wattmeter, dummy load and a TK-705d transciever that was good at reducing power during mismatches (those kenwood tk radios can take a beating!!!)… Was it perfect, heck no, but it did work quite well, so I imagine you can “make it work” too with a cheap nano, even if it isn’t ideal.

If he tuned the cans and it didn’t work at all, DO NOT mess with the tuning until you rule everything else out. Like the shielding of the cheap unshielded radios…

How do you calibrate your nano, and do you have any known loads other than the included 50Ω to compare it to to verify calibration? If you don’t, I could solder a couple SMD resistors to a female SMD connector and check it on my siglent. I could then mail you that load and impedance plot so you could compare it with your nano’s calibration load. I could make a coupe different ones. Sometimes you get a cheap calibration load with the knock-offs and simply using a better calibration load can make a world of difference.

What sort of coax and connectors you are using to get the nano connected to the antenna? Are you calibrating at the nano (and hopefully port extending to the end of the coax) or are you calibrating to the end of that coax directly?

I cant seem to post links sorry, but the VNA is the 3GHz model. I calibrated it using the dummy loads supplied, I believe its firmware is the latest revision.
The model is a AURSINC NanoVNA SAA-2N V2.2

I found it was easier to plug it into the PC with a USB cable as the sampling rate was very slow on the screen, but was faster on the PC, I felt it would have given me reasonable results, but I know they are not as accurate as a more expensive industrial model.

The unit he used was called a rigexpert sorry, its an antenna analyser.
Model was a AA-650 zoom, is a much more expensive piece of gear compared to the nano VNA.

He said he tunes them with a very precise dummy load and basically tuned each side of the filter until the SWR came right down to 1 on each side, says thats all you need to do.

I was told this fella is the local go-to in the HAM community for everything about cavity filters, so I assume he knows what he is doing, he had a pile of filters sitting all over the place.

That might be worth trying out some more accurate loads for VNA calibration as you say if you can provide me something better, but as a starter I guess i should really replace all the coax on these radios to start with?
I have a cheap 50W dummy load off amazon, seems to measure very close to 50 ohms on my meter. Its a cylindrical looking object with a PL259 connector at the end.
I do wonder if these baofeng radios could be a big part of the problem too.
Would placing each radio inside a die case alloy box be enough?

I clearly missed stuff last time I scrolled down this thread, been catching up…

There is no way a rigexpert could pull off tuning a reject cavity. In fact, lets talk about that. Is a reject filter, right? So exactly how is he connecting his rigexpert and looking for a flat SWR to indicate reject frequency? Is he using a tee with a dummy load and tuning each cavity individually? Probably not when those cavities are hardlined together. So he is teeing into the end with a dummy load on the tee with the rigexpert set to the reject frequency tuning so the cans essentually disappear leaving only the dummy load (as an indication of rejection)? Did he also have a dummy load at the other end? Doesn’t matter, because with maybe a mW output from that rigexpert, it wouldn’t stand a chance at measuring a reflection 50dB down, and 50dB down just isn’t gonna cut it. You would be better off with a high power UHF transmitter you didn’t mind damaging, a couple dummy loads, and a very sensitive wattmeter. Not to hurt the feelings of your local “expert”, but you should really find a new “go-to” guy. You simply cannot do that with a rigexpert. You won’t even get close with a rigexpert. You need someone else to tune them.

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You mentioned using an uhf antenna for vhf. Are we still talking for the cavity filter? If so, the. No, because it needs to present a 50 ohm load or the tuning will be off again.

OK, interesting, it definitely was a rigexpert he used.
He said that this will work fine is used with a “precise” dummy load.
He basically connected it to both the tx and rx ports at a time and had a dummy load on all other ports.
I saw him adjust the cavities until the SWR came right down to 1.
Yes its a reject filter, so it notches out the opposite frequency you want to operate on. i.e you filter out the tx frequency on the rx side and the rx frequency on the tx side.
Ive been told by other HAMs that he knows his stuff, so I have no idea, but may see if someone else can assist. It might be connecting the VNR back up to it too.

You don’t tune a filter that way? Or at least not if you want good performance out of close channels by tuning for minimal VSWR - You tune a cavity by watching the display on an analyser, because every tiny tweak of the filters in the RX chain, have an impact of the TX chain - so you tweak, and tweak until the display shows maximum RX rejection of the TX frequency and vice versa.

He is no expert if he tunes cavities trial and error. This video is pretty good at showing the process with the same analyser I have. If he did not do it this way, then your problems are explained, I think.

The three cavities combine to produce the best and deepest filter, but each one could easily produce a good VSWR, even if they are not aligned. VSWR is a guide as to what is happening, but how would you possibly align all 6 cavities with VSWR? I don’t think you can.

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Yeah I thought something was out when I saw him do that, I actually asked him if it was that easy and he said something along the lines, that yes its that simple as long as you have an accurate dummy load!
Just shows that its hard to find people in the HAM community that actually know everything, i guess its called amateur for a reason! Either way, I guess what he has been doing works for him?

It must not be working, you said he had a pile of filters laying around. If they worked, they would be in service! You could do a far better job with your VNA than he did with a RigExpert for sure. W2AEW has some videos (#270, #318, and #334) where he tunes a duplexer very similar to yours, and in one of his videos (#334), he uses a nanoVNA. Its worth checking out how its actually done before proceeding further.

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Yeah, ive figured I cant trust anyone, other than a few i know who have worked in the field for many decades.
It looks a simple enough procedure with the right equipment.
I will check out the channel you are talking about, I remember watching a video on youtube where someone tunes one with a nanoVNA and then compares it to the more expensive tektronix VNA he had. This person didnt think it was good enough to tune a filter with however, but said it was a good guide to give you a general idea.

In fairness, we did used to tune cavities without analysers and other modern gizmos, but they tended to be individual ones that could be tuned separately. So you could tune each one with a Bird, dummy load and a power meter and measure each filter’s performance. Then you connect them together. It was done that way for years. It’s just a rotten way of doing a multi cavity device where they are always coupled. It’s not impossible, but lots of trial and error. With 6 adjustments, each impacting on the others, it’s a silly process to consider.

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