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/
