The phenomenon you are seeing is because the 1/4 wave size at VHF is 3/4 wave at UHF.
I believe the hybrid duplexer design uses 2 or 4 notch only cavities. If you haven't drilled holes in the side 2 of the band pass cavitiys I believe they can be converted to notch only by simply removing the 2nd coupling loop and puting a metal slug in to cover the hole. Then I would use the 3rd cavity in a Pass/Reject configuration. Piston trimmers should work fine on the coupling loop. Should be more tempreture stable than Vynal. I believe WACOM uses Delrin. You may be able to get this or Teflon rod from small parts inc if you wish to take the same approch and build stubs. On 4/12/05, derek_mcintyre <[EMAIL PROTECTED]> wrote: > > > Hello group, > I have been working on building a duplexer for 222 MHz from old 150 > MHz cavities. I have three Decibel products cans, which look the > size of regular 150 MHz cans, but have been told they are actually an > old 450 MHz all pass-band duplexer. I don't know the model number > but they are beige in color and look really old. They will tune very > good throughout the 440-470 band, but also work good on 144-174, > too. The plunge rod is nearly identical to the old Wacom 2M ham > duplexers. Since it is just a pass band cavity, there are two > coupling loops, one input, and one output. > > Using the same loops, I cut one of the rods down nearly 4 inches, > then spliced with a 1 inch copper coupling and soldered together. > Now the can tunes a good pass band at 222-225 MHz also. > > I really wanted to create a hybrid ring 3 cavity duplexer for a new > 220 repeater, two cavities on RX and one for TX, band-pass, band- > reject, like the Wacom deals with the rod on the side which has a > plexiglass looking dielectric that slides in and out in order to > determine the position of the notch. > > I took my existing 220 duplexer (Wacom) loops out and the reject stub > also, and tried to duplicate it as closely as possible. So far the > results look promising. I am using 3/8" hard drawn copper for the > sleeve, and a piece of #10 wire for the inner conductor, and some > vinyl tubing to slide in and out to vary the capacitance. I couldn't > find any better means of making a capacitor than this, and my first > question is, will this be stable in an uncontrolled environment? 90 > deg in summer and 40 in winter. > > Second, I measured the capacitance of the Wacom stubs and get > somewhere around 7 pF. I have a bunch of piston type caps that will > give this value, plus or minus, but don't know exactly how to > incorporate this into the hybrid ring design. I am tired of randomly > trying pieces of copper straps for different lengths of coupling > loops and wonder if anyone has a proper way to determine coupling > loop lengths? > > Third, the repeater will not be operating in a high RF environment. > Would it be more practical to use each can as a notch only (reject) > cavity? If so, how in the world do you make coupling loops to work > for band reject only? I have tried taking one loop, running it down > the cavity for several inches, then directly back up and to ground > again. Then taking a tee connector and going across it, but with the > tracking generator, my notch is only like 6 dB deep which obviously > won't work. > > Any suggestions or links you can think of to help me build a 220 > duplexer out of the parts I have mentioned above? I have a decent > shop and can do light precision machine work. > > Thanks. > 73, KC4FWC > > http://www.w4dex.com/kc4fwc/224480.htm > > > Yahoo! Groups Links > > > > > Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
