[222mhz] Notch stub on a WACOM duplexer

[Dr. Gerald N. Johnson]

You won't be able to tune it close enough by cutting heliax. It is 
mostly a variable capacitor that varies the coupling coefficient between 
the link and the main resonator. With the mutual inductance set right 
you get the split between pass and notch. The last Wacom I worked with, 
the stub was made of smooth tubing with a bit of dielectric bored to 
slide in and out. The outer stub was split I think with a hose clamp to 
clamp on the dielectric once set. That dielectric could be polystyrene, 
teflon, or polyethylene (as in solid coax insulation). Since these 
materials are available in 3/8" and 1/2" diameter rods, I'd make the 
stub of hobby shop tubing 1/2" ID and with a center conductor say 1/4" 
OD and hope to find a plastic already drilled. I see McMaster-Carr has 
teflon 1/2" OD and 1/4" ID for less than $6 a foot. I'd get it before I 
got the hobby shop tubing because it has a +.032 tolerance on the OD and 
-.032 on the ID so it might need 17/32" ID brass for the outside and 
7/32" brass for the center conductor. Since its primarily a variable 
capacitor, the characteristic impedance isn't all that critical.

Alternatively you could mount a 15 or 20 pf piston trimmer in a UHF 
connector and tune that as the series capacitor for the link. Other 
makers of duplexors did that to adjust the coupling. DB Products rotated 
the link to get the same effect.

When tuning be sure your equipment is well shielded and use double 
shielded coax for all the cables, else the notch won't be there when you 
connect the radios and will move when you move the cables. I'd never use 
BNC in a duplexor because the shield connection is too unreliable at the 
connector. TNC perhaps.

73, Jerry, K0CQ

On 3/9/2011 2:06 PM, Chris Boone wrote:
> I have a 4 can BpBr WACOM that is missing one notch stub..I can make another
> if I knew what the materials are.the brass and elbow UHF connector is not a
> problem.I take it this is acting as a variable capacitor.I could use a piece
> of 1/2in Heliax as a stub and cut it to the right length while watching it
> on the spectrum analyzer.but would like to make an exact replacement...TXRX
> told me a few years back they could make it and send me the specs in a
> PDF.NOW they say, no they cannot make a replacement (the cost they quoted me
> was about $100 or so.which I thought was ok).
>
>
>
> Asked Telewave and all they said was "We can sell you a WHOLE new duplexer."
> I thought they could give some technical help..(I sure miss Kit.he would
> call ME on the phone if I had a problem..or made the order wrong..like I did
> on a WP652 I wanted Ns on.he called me at home and said "nope.the cans wont
> meet spec with Ns.you get deeper notches with the UHF connectors." I was
> sooo glad he did call and explain it to me.(Hint: TXRX used BNCs because
> they give even a better notch.IF the connector was RIGHT on top of the can
> with no vertical rise, the notches would possibly be doubled their normal
> specs!)
>
>
>
> All I have on the 4th can that is naked is the female SO239 port..just
> wanting to put this one back in service.
>
>
>
> Thanks!
> Chris
>
>