How do I tune this notch duplexer?

A Band Reject duplexer is commonly referred to as a notch duplexer.  The transmitter side of the duplexer is tuned to the receiver frequency, which it notches out.  The receiver port is tuned to the transmitter frequency, which it notches out.
Notch only duplexers are readily available in the used / surplus market and eBay is also a good source.  These were commonly used in mobile repeater applications and were made by Celwave, Phelps Dodge, and others.

When the duplexer has the same number of cavities on either side of the antenna connector, the transmitter and receiver ports are likely identical and can be used in reverse if the configuration of the duplexer from the factory was inverted, or your repeater is upside-down from the common commercial high-in low-out configuration.  If the tag on the duplexer is still intact, use the original configuration as a guide for determining which way is best for your situation.  These duplexers are somewhat asymmetrical.  If the duplexer has a label that has the previous frequencies on it (like TX 453.500 RX 458.500), you should maintain the same low-port/high-port relationship.  That might mean that you end up transmitting in to the port previously labeled "REC" and receiving on the port previously labeled "TRANS" if your offset is the opposite of what the original factory- labeled setup was.  What I am trying to make sure you don't do is re-tune the factory tuned higher notch to the low side and vice versa.  This will cause the duplexer to have excessive insertion loss and not provide stated isolation.  Keep the notch's on the same side as original.  If the duplexer is built with a different number of cavities on either side of the antenna connection, you likely cannot invert the receive and transmit connections from their factory locations.  Why?  It is usually more important to provide more transmitter sideband noise suppression than to provide more carrier suppression.

The duplexer provides isolation by notching transmitter power from going to the receiver and by notching transmitter noise from going to the receiver.  This may sound confusing, but this is what it does.  With this in mind, let me give an example:

Repeater TX = 444.000 and RX = 449.000

The transmit side of the duplexer is tuned to notch the receiver, so.... in other words, the transmit side of the duplexer is tuned to the receiver frequency.

In this example the transmit side of the duplexer is tuned to 449.000 and the receiver side is tuned to 444.000.  Remember you are tuning the duplexer to eliminate power or signal, not pass them.

I commonly hook my signal generator to the antenna port and send one of the frequencies into the duplexer. It is best to terminate the open port with a quality 50 ohm load.  I choose which side is best for the receiver and connect a hand-held to that port tuned to the repeater transmit frequency.  I tune the duplexer sections back and forth to eliminate (notch) out the reading on the hand-helds S-Meter.  You'll need to keep increasing the output of the generator as you provide more notch depth.  Keep going till there is no further ability to notch the signal out.

Now, tune the generator and handheld to the receiver frequency.  Switch the hand-held and dummy loads connections to the opposite ports and tune the other side of the duplexer to notch out this frequency.

After both have been done, do it again as there is some interaction between sides.

The last thing is to verify proper insertion loss of the intended pass frequency.  Connect the hand-held to the antenna port, and the generator to the receiver port.  Tune the hand-held and generator for the receiver frequency.  There should be little loss in the duplexer.  You can connect the hand-held to the generator directly and tune the output of the generator for say a half scale S-Meter reading.  Now replace the duplexer in line as above and there should be little difference in the S-Meter reading.
Do the same for the other side and verify results.

Properly tuned 4 section duplexers will only exhibit about 1 dB of loss on the intended frequency and will provide about 60 dB of notch depth.  6 cavity duplexers will have about 1.2 to 1.4 dB of insertion loss and provide 75 dB or more rejection.

More information on the Celwave or Phelps Dodge mobile duplexer can be found here:

Copyright August 16 2000, Kevin K. Custer  W3KKC
Updated March 2007 W3KKC
All Rights Reserved.