Well as mentioned earlier equipment finally started to arrive. I got the RD982i, and the two Sinclair Q3220E factory adjusted duplexer's, the 1/2 hardline and connectors, rack enclosure, plus some other stuff, among them all the 2-foot RF interconnecting jumpers made of RG142. Still waiting on the Telewave antenna as factory suddenly out of the blue pushed back delivery to late this month after the order had already been confirmed and processed.
Besides the Sinclair duplexer I still wanted to use at least a single stage circulator on each repeater so I stared looking around and came across a listing on ebay of someone that was offering a complete dual stage UHF IM Panel made by Decibel from around the 90's. It has two DB4325-B 450-512MHz circulators, one DB4332-B 450-470MHz LP Filter, one 100W and one 300W Decibel loads. All neatly mounted on an open aluminum rack, and of course only SS or aluminum hardware to avoid affecting the circulators. Obviously will have to take the pieces and remount individually on each repeater. Seller was asking $175, but after a week of no takers went down to $150. I then had a couple of back and forth chit chats and then politely offered $100 to which he agreed. My shipping was $75. I think the two loads alone are worth more than what I paid so I think that was not a bad deal even if I end up not being able to use it for the current project.
I put the new repeater on the bench with the duplexer and one circulator, plus the LP filter at the output of the circulator and everything seems to be working fine. The circulator has about -0.4dB forward insertion loss, and about 23.8dB isolation in reverse. These circulators don't seem to be adjustable, but worked just fine even though I doubt that they where operating anywhere near where I am which would have been a big coincidence. They seem to be rather broadband, hence the no adjustment? I thought all circulators had adjustments, but on these nothing under the labels, no adjustment holes at all. Also checked the LP filter between 400-960MHz and it is almost flat up to about 475MHz after which it starts to drop so looks good to suppress any 2F or above products.
So all in all on the TX path I got -0.8dB for the duplexer HP, -0.4dB for the circulator, and -0.3dB for the LPF. Total insertion loss -1.5dB. Given I am planning to set the repeater to 20W (-43dBm), excluding feed line loss that should give me about 14.2W (+41.5dBm) at the antenna. I figure this would be enough for the portables to operate within the designated 1 mile of the repeater. Will have to test and adjust if necessary, but off the bat want to keep TX power as low as possible.
Now I'm still short of one LPF, but could not find any of the same anywhere, not used or new. Contacted dbSpectra which is still carrying the same DB4332-B filter, but they said lead time would be 6-8 weeks. It seems everything nowadays is the same, and the "in stock" just died. There is also a similar filter made by EMR Corp but I have not yet contacted them, not sure how good they are, but the name strangely seems to ring a bell in my mind from back in the 70s or 80s and more affiliated to 27MHz CB products. But I'm probably wrong on that.
So took the small DB filter apart, its very easy to open with 4 screws the back cover comes off. Its housed in one of those Pomona metal utility boxes with one NM bulkhead connector on one end, and a NF connector on the other. Inside the box it actually says Pomona. So there are 4 Johanson brand trimmers (these have absolutely no markings but its easy to recognize the piston type), and 3 short coils. I disconnected one of the coils so I was able to check one of the trimmers, which turned out has an adjustment range from close to zero to 10pf. And the coils are made out of #18 enameled wire, one has 3.5 turns, and the other two have 2 turns. All 3 have a 7/64" diameter.
I figured that it should be quite easy to duplicate this filter. But Pomona does no longer use anything but BNC connectors on their boxes, and they are smaller in size. So went to look on ebay and found the same box as the original only with PL male/female connectors. Then also separately ordered one NM and one NF chassis mount to swap out the PL connectors. Next found several sellers offering almost identical looking Johanson trimmers of the value I needed, two sellers where even offering them in 4-packs. So went with the one that looked more similar in size and got the ones supposedly rated at 150V. There where others same value rated at 250V but they where clearly physically longer than the originals. Lastly got a small spool of #18 enameled wire. So now waiting for the stuff to arrive and see if I can duplicate the filter, which seems quite doable.
Any comments and suggestions welcomed. Thanks.