I have a few questions regarding a Celwave PD-497 6-cavity VHF duplexer. I have tuned quite a few duplexers in my time, but this one has me baffled. I have a decent Tektronix spec analyzer with tracking generator, and also an HP8920A with tracking generator. I have tuned all kinds of UHF duplexers and a few other brand VHF units with no trouble, but this Celwave unit is an animal.
First off, trying to tune the rx and tx sides with all three cavities in series is a real PITA to find the peak of each cavity. I have also followed the Celwave instruction sheet about tuning each cavity separately, then reconnecting the interconnection cables and testing it. I even used 6db pads as called for in the manual. I have the unit tuned, and my tracking generators show about 1.25db insersion loss, and 98-100db or rejection at the reject frequency. However, once connected to a 100w repeater with 0.25uv receiver sensitivity, the repeater experiences desense to a degree where once the tx is above 50w or so, the receiver is brought to its knees and needs about 0.75uv or better to obtain 12db sinad. The duplexer is as supplied from the factory. The duplexer was originally delivered from the factory on 147MHz. The original RG214 jumpers between cans are still intact. The interconnecting cables are all RG214 double silver shield (including the patch cables from the pa and rx to the duplexer. The same repeaters (I've tried two with this duplexer) and cables have been connected to two other duplexers, a Telewave 1554 and a Wacom 674(?) VHF 4-cavity duplexer. Both sets of duplexers work great with the repeaters. With the tx at 100w, the receiver is almost as good as with the tx off. I must be missing something with this PD497 duplexer, or its specifications are grossly inflated. I am starting to wonder, because another group I am affiliated with has a PD497 as well, and they tell me they have to keep their tx at 50w or less to avoid receiver issues. Their unit was tuned by Celwave some years ago. Is it possible there has been corrosion inside the cans? Or is there some special procedure unique to the PD series that I am missing? Right now I am running the repeater with the Telewave cans (which are rated at 77db isolation), and it is working great. But I'd like to have the 100db of the Celwave cans, if I could bet them to deliver in the "real world"! Eric KE2D www.w2njr.org
