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How to Get A Cheap-"Squeaky Clean Signal"
A Two Meter Duplexer
Designed by Leonard WB3AYW 10/31/06
Published by ARRG.US December 5th, 2006

Go to a Quick Oil Change place. Get four, (4) 14 inch diameter, sixteen (16) gallon drums, approximately 26 inches tall with lids with a plug in the center. The flat lids are less stable but useable. Transmission fluid comes in these drums, do not clean the oil out of the drums.

Remove gasket from lids. This can be time consuming, but must be done for good contact between the lid and the top of the drum. Drill the insert out of the plug that is used for tightening it. They are usually Spot welded in, at 6 places, drill and remove insert.

Mount a BNC connector (or a connector of your choice) 3 ľ inches from the center of the plug in the lid where the center tuning rod will be soldered. Do not use SO239ís and PL259ís; they do not make real good RF contact all the time. Drill the plug to accept the larger Brass tubing and solder to the plug.
The larger the diameter of the center rod, the more critical the tuning adjustment will be.

Make a pick up loop for each cavity, measure and cut four (4) pieces of #16 solid copper wire 23 inches long. Start and bend the wire 1/2 inch from the end at a right angle; this solders to the connector after bending is complete. From the first bend at the connector, bend the wire towards the tuning rod a length of 1 3/4 inches or so, the next bend is 3/4 inch from the tuning rod, then bend along the rod for 4 inches, then bend out away from the center rod 5 1/2 inches; continue the bend toward the lid at a 45 degree angle and cut wire 1 inch above the lid. The pickup loops should be 3/4 to one inch from the center rod when installed on the connector; if not bend the loop at the connector so it is. Closer is more coupling; I like the 3/4 inch spacing.

Cut 4 pieces of wire 7 inches long and bend in a vee at the center (3 Ĺ inches). Solder one piece to each pickup loop just made and installed pointing out away from the connector, parallel to the lid and spaced 1 inch to the top of the wire; this is the capacitor. (Remember readjusting the capacitor later WILL CHANGE the notch frequency.) The notch frequency is now set to 600 KCís.

Mark the cavities so they do not get mixed up!

Tune up. Adjust the High Pass side of the duplexer on the lower frequency side for the deepest notch on these two cavities.

Tune up. Adjust the low Pass side of the duplexer on the higher frequency for the deepest notch on these two cavities.

When pre tuning only bend 3 tabs on the lid. After assembly and before final tuning all of the tabs on all lids are bent with pliers to make a good tight contact with the drum.
This completes the duplexer pre-tune-up.

The phasing harness!
Cables of a quarter wave length in coax are needed for the hook up from the center of the tee's (All cables MUST BE 100 percent shielded coax, not really as critical in length as I thought but try to be fairly accurate). Remember the velocity factor for your coax, and subtract the length of the tee from the coax length. The coax to the receiver and transmitter must also be 100 percent shielded in multiples of one half wave lengths of coax.

A tee is needed at each cavity to connect the phasing harnesses, and one more tee at the antenna junction for a total of 5 teeís. The one tee is different from the other 4 teeís; this one connects the 2 harnesses, to the antenna feed.

Attach the harness and then retune the notches with the center rod as before; this completes the duplexer tune-up.

There is very little loss going thru the pass side of the duplexer on the high pass low notch side. The loss is determined by the spacing of the coupling loop to the center tuning rod which is pre set in the design. Wide spacing has less loss, readjusting this spacing also affects the frequency setting of the center rod.

At approximately 40 KCís from the notch center is the 3 DB up point. This makes the notch width about 80 KCís wide.
The isolation in mine is 90 DBís plus for the two cavities on each side. This is a total of 180 DBís of isolation plus or minus for the final duplexer.

Tuned properly this is enough isolation even with some temperature drift in frequency for a repeater running about 100 watts. More than 50 watts is seldom used for a two meter repeater system. This power is out of the transmitter, not ERP.
The isolation could be better, but I am using steel on the inside of the drums instead of copper or silver plating and copper or brass for the center rod, instead of silver plating it also.

The pick up loops and the capacitor wires are #16 solid copper.
Space needed for this Duplexer is three feet by three feet by 28 inches high.


Click on any photo for a larger view

Total Cost of this project should be less than $100

Parts List



4 13 gallon oil Drumís with lids Quick Lube outlets
4 small Brass tubes  3 feet long for slip fit (Ace Hardware)
4 larger Brass tubes 3 inches long to be soldered to the lid (Ace Hardware)
4   #16 wire7 inches long  
4 #16 wire 23 inches long  
5 teeís  
2 phasing harnesses  


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