CHAPTER 8

TUNING

Experience shows a duplexer to be relatively easy to tune, even in the field. Obviously the best tool to use is a spectrum analyzer with a tracking signal generator, or a communications monitor. You can, however, successfully use a transmitter, such as an HT, that can transmit on the input and the output frequency, a dummy load and a low power wattmeter.

The Basic Concept

Both sides of my 440 MHz duplexer have two notch cavities and one bandpass cavity. The bandpass cavities are the first ones after the antenna tee connector. For other bands and cavity configurations, you can apply these concepts and methods with equal success.

On the TRANSMIT SIDE, bandpass tune to pass the TRANSMIT frequency and notch to reject the RECEIVE frequency.

On the RECEIVE SIDE, do just the opposite. Bandpass tune to pass the RECEIVE frequency and notch to reject the TRANSMIT frequency.

In my case, the receive side has one additional bandpass cavity, inserted after a GASFET preamp. Tune it to pass the receive frequency.

If you don't use a preamp, it is not necessary to use the extra bandpass cavity. If you do, you will need a tuned cable between it and the last notch cavity. It should be the same as the cable that connects the existing input bandpass cavity to the first notch cavity on the receive side. Tuned lines are not required before the first or after the last cavity of a duplexer. Only lines between cavities need to be tuned.

Tuning

Begin by tuning each cavity separately, disconnected from the duplexer.. I recommend this technique even for a minor tuneup. A good individual-cavity tune will get you quite close without further adjustment.

Begin by applying low power from a transmitter to either connector on the individual cavity. The cavities are symmetrical. Connect a low power wattmeter, terminated by a good-quality 50 ohm dummy load, to the other connector. A poor dummy load or one specified for lower frequencies will not work.

Tune bandpass cavities to maximum, notch cavities for a dip. Make certain the cavity is still in tune after you have tightened the locking nut. This takes a little practice.

Be especially careful with the notch cavities. Their tuning is the most critical. You may wish to cautiously use a receiver with an S meter in place of the wattmeter for fine adjustment, once you have the tuning close. I recommend a 50ohm pad in front of the receiver to prevent too much power accidentally getting into the receiver's front end. It is also a good idea in that many receivers do not show a 50 ohm input impedance. A 50 Ohm pad corrects this. A 50 ohm load on the other side of the cavity is mandatory.

After tuning each cavity separately, disconnected from the duplex, reconnect the entire unit.r. Then connect the transmitter to the antenna tee, the wattmeter to one port and a second dummy load to the other port. Set the transmitter to the pass frequency of that side of the duplexer and tune the bandpass cavity only for maximum output power. Do not tune the notch cavities with the duplex completely assembled. Then reverse the connections and the frequency and tune the other side of the duplexer for maximum power.

If you have a spectrum analyzer or a communication monitor, the process is essentially identical. You will merely be able to obtain a finer adjustment. You will also be able to fine tune the notches. The duplexer,however, will perform very well without being tuned with the sophisticated test equipment. I have hill-top tuned many duplexers by the simple method.