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 A 440 Cheap-"Squeaky Clean Signal"
A 70CM Duplexer
Designed by Leonard WB3AYW 10/31/06
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 assembled.

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 duplexer.

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 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.

TUNE UP:

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 mixed up!

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.
 

Parts List

QTY	Description	Where
4	36oz. or larger coffee cans, I prefer the 48oz size
4	brass tubing 3inches long, to be soldered to the can	(Ace Hardware)
4	lbrass tubing the length of the coffee can that is a slip fit inside of the above brass tubing	(Ace Hardware)
4	#16 wire 2 inches long
4	#16 wire 4 inches long
8	¼-20 brass nuts
4	¼-20 brass screws 3 inches long
4	connectors of your choice, NEVER USE PL259/SO239’s on a duplexer
4	Plastic lid from can
4	Aluminum foil larger than can bottom

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Dedicated to "Playing Radio"
and all things that are "FM"
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