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A 440 Cheap-"Squeaky Clean Signal"
A 70CM Duplexer
Designed by Leonard
Published by ARRG.US January 29th, 2007
To build a duplexer for 440 to 450
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. Measure out from that hole 1 ½ inches and punch
another hole, this is where your connector goes; this hole gets
enlarged to fit the connector to be used. Cut one piece of the
larger tubing 3 inches long and slide it into the coffee can bottom
so about ½ inch is sticking out of the can, and solder in place. 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.
Bend the 4 inch wire at ¼ inch at a ninety degree angle, then at a
ninety degree angle at ¾ inch the other way, then 1 ¼ inch to a 45
degree angle back towards the ¼ inch” measurement.
Bend the 2 inch long wire in the middle into a vee, and solder to
the pick-up loop, just made, spaced ¾ inch from the can bottom when
Solder this assembly to the connector on the can as shown in the
pictures with the vee spaced ¾ inch from the can bottom.
Bend the vee wire in the assembly to ¾ inch away from the bottom of
the can; this sets the pass frequency at the 5 Meg split on the
duplexer 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
Measure up from the open end of the
can 2 inches and drill a hole so a ¼-20 screw will slide through
easily, and solder a brass nut to the outside of the can. 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
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 the notch to your highest
frequency to be used and then pull up the tubing 1/16 inch more,
then tac-solder the two pieces of tubing together.
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.
This design is by WB3AYW, Leonard Shick.
Make your Phasing harness with ¼ wave lines for the frequency to be
used. See the two meter design, and use ½ wavelengths or multiples
to the receiver and transmitter.
All cables MUST be 100 percent shielded!
FINAL TUNE UP:
Mark all cavities so they do not get mixed up!
Mark all cavities so they do not get
Hook up the duplexer and re-tune the ¼-20 screw for the deepest
notch on the opposite frequency. After tuning I use a lock nut on
the adjustment for stability.
Do both sides of the duplexer. This completes the duplexer tune up.
||36oz. or larger coffee cans, I prefer the 48oz size
3inches long, to be soldered to the can
||lbrass tubing the length of the coffee can that is a slip fit
inside of the above brass tubing
||#16 wire 2
||#16 wire 4
screws 3 inches long
your choice, NEVER USE PL259/SO239’s on a duplexer
larger than can bottom
Dedicated to "Playing Radio"
and all things that are "FM"
6M - 2M - 1 1/4M -
70CM - 33CM