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US January 29th, To build a duplexer for to band get 4 coffee cans at least 36 ounces or larger. Punch a hole in the center of the bottom with a small screw driver, and enlarge it later to the size of your larger brass tubing that will be soldered to the can bottom. Put connector on can, solder or your choice of assembly to the can, must have good contact to the can. Make pick up loop similar to the 2 meter design. The more capacitance the wider the split is.
Cut the smaller tubing the length of the can, and insert into the larger tubing soldered to the can. This is your coarse adjustment for the notch frequency on the duplexer. Do this on the opposite side of the can from the connector. Put the brass screw through the nut from the inside of the can, this is your fine tune adjustment for the notch frequency; turn all the way in.
Get a piece of aluminum foil that is larger than the can bottom. Place the foil over the plastic lid and put the lid on the open end of the can. Set your cavity to the opposite frequency you want it to pass and adjust the screw for the deepest notch, this pre-tunes this cavity.
All cables MUST be percent shielded! Mark all cavities so they do not get mixed up! After tuning I use a lock nut on the adjustment for stability. Do both sides of the duplexer. This completes the duplexer tune up.
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