Tuning Quantar VHF 6-cavity base duplexer

[kcbooboo]

I'm working on one. Each side has pass tuning (knob attached to slide tuning shaft) and reject tuning (one or two trimmer caps). Each section gets tuned separately. Brand new unit from Motorola.

I tuned the low-side pass freq first, then went back to tune the reject freqs. When I finished the notch, the pass side was way off, as in over 1 MHz. So I tuned it again, then retuned the notch, and the pass was off again. Can you see where this is going? After a few attempts I ran out of pass tuning range; the shaft is all the way in.

High-side is working the same way.

Simple TEE connector with cables going to the service monitor, set for spectrum analyzer / tracking generator mode with a 1 MHz horizontal span. The two freqs are about 4 MHz apart and both are in the 150-155 MHz range. The duplexer IS the right model for this range.

Also, on the low-side cavities, there are TWO notch tuning caps, where there's just one on each of the high-side cavities. The tuning instructions just say to tune them equally, but it doesn't say whether they should be set to approximately the same depth. Either one will tune the notch adequately but I made them fairly equal.

Am I missing something or doing something wrong?

Bob M.

[The Pager Geek]

These get some "getting used to"

Are you tuning each individual cavity first? Not a "section" (high vs. low)?

I was losing my mind until I read the RFS / Celwave instructions.. not the one in the Quantar manual

tpg

[kcbooboo]

Yes, I disconnected the entire harness (marking it first) and attacked each section separately.

It all seemed to tune nicely the very first time I did it, then I had to retune it because one freq I was given was incorrect. That's when it all went downhill.

All I could find was the info in the Quantar manual, because this unit only had Motorola part numbers on it. If you can summarize the RFS/Celwave info, that would be great. If you have a link to the RFS/Celwave info, or an actual file, please send me a PM.

Thanks.

Bob M.

[Bill_G]

I leave the harness attached. It's part of the tuning. The cavities interact.

Terminate the low side. Sweep and tune the high side. Then move the load to the high side. Sweep and tune the low side. It should dial right in unless your freq pair are too close. You said they were about 4 meg apart.

I also tighten the sweep as I get close. I start wide (1M to 5M) so I can rough in the pass and notch. Then I go down to 100K so I can refine the top and bottom.

[kcbooboo]

Thanks. After I tuned it the very first time, I did check the TX-to-ANT and RX-to-ANT pass and reject and they seemed to be fine. But repeating the tuning just seemed to screw things up. I'll try it all cabled together.

Bob M.

[kcbooboo]

I tried looking at the LO-to-ANT port (HI port terminated). Something's all wrong. About 20dB of loss and the leftmost cavity just doesn't tune properly. I'm not sure if it's a function of the two trimmer caps or the cavity itself is somehow defective.

I didn't bother looking at the HI-to-ANT port. How or why this unit is not functioning is a mystery to a couple of people now.

Bob M.

[Wowbagger]

I'd suggest backing everything out to midrange, then trying again. I've had cases where I was working with the wrong "notch" (there can be more than one notch due to the reactance of the cavity and the notch cap), and managed to get things totally out of whack.

[kcbooboo]

I tried that on one section. Pulled the tuning knob/shaft out until (surprisingly) it fell into my hand (oops). Pushed it back in and watched the reject notch pass the center freq, followed by the start of the passband peak, and the tuning rod was almost all the way into the cavity, further than it was before I started. there's only another half inch to go, and I still hadn't reached the peak. This was with all the sections disconnected and it was the first LO section. I wouldn't think there's anything inside that could have gotten broken or destroyed during tuning; how much signal can a service monitor put out.

Bob M.

[AEC]

I do no t play favorites with which side ai start on, but I terminate one port, and run the ANT prt to the monitor, tuning the input port sie forst, then reverse and start agan, then do the notch and switch again. Only a few swaps usually gets me a good return pattern on both low and high ports.

I never remove any cabling between cavities as they are integral to the entire duplexer(as Bill stated).
I also start with a 10dB. level and open the span up a bit to show a 3-6Mhz. shot, then as I narrow the range and tweak the cavities, I narrow the span to under 3 Mhz. and continue until I get a good dip and stable notch on all cavities. Everyone gets faked with seeing the wrong signal on screen, and the depth of the pass/reject should indicate you are not within the proper adjustment. I do not know a single person that tunes any cavity that has not erred on the wrong signal and assumed all was fine.
This is a trial and error learning curve, it takes experience to know exactly what to look for, it does not tell you.
I once spent several hours on a 6 cavity set simply because I was not happy with the insertion loss. I finally squeezed that last 1/2dB from it and saw a 102dB transmit to receive separation.
I.L was under 2.0dB, which is super, as the factory 'suggested' a loss of 3.0dB. That felt like everything was going in slow motion, tune, retune, adjust and retune again. Frustrating at times, until you place it into service and they hear like never before!

[kcbooboo]

I've tuned lots of UHF duplexers in the past. Normally, I would use a return-loss bridge connected to the cavity and tune for the lowest level at the pass freq, but all I had available was a 2600-series service monitor with tracking generator, so I had to use that. I even looked at the LO-to-ANT ports with the HI terminated and things were still way off. I ran the tuning shaft for the first section all the way in and all the way out until it was a foot away; the peak just never came into the 151 MHz freq it needed to be on. The notch tuning always worked fine. I'm also concerned about the fact that there are TWO notch tuning caps on the LO sections but only ONE notch tuning cap on the HI sections. I don't know if they're supposed to be at about the same depth, or if one might have an effect on the passband tuning. The instructions in the Quantar manual just say to turn them in equal amounts but it doesn't say to preset them in any specific way.

The very first time it tuned great, but I didn't go back and look at the passband tuning. It was only when I had to retune it because I was given the wrong freqs that things went downhill. As soon as I connected the first cavity, I could see the PB was nowhere near where it had been, and from that point on it just got worse.

I've given up on it. Thanks to all the people that commented.

Bob M.