My fingers thank you for the description Jeff... I wasn't looking forward to typing all that information out.
> "Jeff DePolo" <[EMAIL PROTECTED]> wrote: > It started out as a stock six-cavity notch duplexer (I have one of > these in storage - I think it's a DB-4021?). Let me explain that > part (the stock duplexer) first. > > Before I get into it, realize that some notch cavities are > asymmetrical in terms of the notch response - they have somewhat > of a high-pass *or* low-pass characteristic to them. That is, > the insertion loss above the notch frequency may be a few dB more > than it is below the notch. > > To start with, the three cavities on the right are one side of > the duplexer, and the three in the middle are the other side. > Easy enough. > > The three cavities on the right have traditional tees right on > the loops. The three in the middle have the cavities stood off > what would normally be a tee using a length of coax. In other > words, instead of the tee along the length of the harness being > attached directly to the cavity loop, there is a short piece of > coax between the tee and the cavity loop. By standing the cavity > off by a length of cable (typically 1/4 wave), the high-pass or > low-pass characteristic of the cavity is flipped. So, if we > assume the right three cavities were high-pass, the middle three > would be low pass using this technique. > > Now, to get back to the stubs. They are added right at the cavity > tee to provide additional notch depth and enhance the > high-pass/low-pass response. Depending on the length of the cable, > they can look like either L or C in parallel with the loop. > > So, that explains the duplexer. That leaves the three cavities on > the left. Those are just more notch cavities, again with the stubs > to enhance the response. > > HOWEVER, like Bob, what bothers me is that these three extra > cavities are configured without the standoff coax between the tee > and the cavity, thereby making them the same pass-response > configuration as the RIGHT three cavities, whereas they should be > configured the same as the MIDDLE three cavities. Not sure what > to make of that...my guess would be that it doesn't work right... > > --- Jeff > > > On Behalf Of Bob M. > > It would seem like the middle and right columns were > > originally a simple duplexer, then they added the left > > column to that. If you can trust the labeling, the > > bottom left is the TX input and the top right is the > > RX input. > > > > The middle and left columns have shorted loop stubs > > attached to each cavity. These are stuffed down > > between the cavities and you can see the shorted ends > > on a couple. The right column doesn't have these > > stubs. > > > > The signal path seems to go from the bottom middle > > column, up to the top middle, then over to the top > > left and down to the bottom left. The blue sticker > > shown seems to be for the 152-153 MHz range, although > > that could have been for just the left column's > > cavities. It only shows one pass freq and one reject > > freq; a duplexer would have a high-pass and a > > low-pass; this looks more like a specially designed > > filter than a real duplexer, although there are labels > > on the top middle and top right cavities indicating RX > > and TX respectively. > > > > It's strange that the left and middle columns are > > configured differently, with short pieces of coax > > between the TEE fittings and the cavities in the > > middle column that are not present on the left column. > > > > Bob M. > > ====== > > --- skipp025 <[EMAIL PROTECTED]> wrote: > > > > > Re: When 4, 6 or 8 Cavities just won't due... > > > > > > Another Ebay gem: > > > > > > DB PRODUCTS 9-CAVITY RADIO REPEATER > > > DUPLEXER-100DB-HAM > > > Ebay Item number: 250120910164 > > > > > > I don't know to be impressed or just laugh at all > > > the hardware (number of cavities used). > > > cheers, > > > skipp
