How to Get A Cheap-"Squeaky Clean Signal"
A Two Meter Duplexer
Designed by Leonard
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
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
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
||13 gallon oil
Drumís with lids
||Quick Lube outlets
tubes 3 feet long for slip fit
tubes 3 inches long to be soldered to the lid
||#16 wire 23
Dedicated to "Playing Radio"
and all things that are "FM"
6M - 2M - 1 1/4M -
70CM - 33CM