Duplexer Tuning

I have received several Emails asking for help (or how do I) tuning a duplexer, so I will make my best attempt telling you the methods I and my partners in crime use to tune a duplexer.

Before we start with the procedure...
This article assumes that you do not have access to a spectrum analyzer or a signal generator. If you do have access to the mentioned test equipment, you don't need to be reading this. You can create a signal generator from an amateur radio and many feet of RG58 (which will attenuate the signal) or attenuator pads. You can also use a signal sampler, more on that later. I'll leave it up to you to put together the right amount of feed line and attenuation to generate the required signal. If you need help, see the Amateur Radio Handbook for detailed instructions.

In the context of this paper, a duplexer allows a radio system to transmit and receive concurrently on a single antenna/feedline system.

About the signal sampler...
If you donít have one then one go get one. RF Parts offers a very nice signal sampler, which will supply between 20-80 dB of attenuated signal. If you own or maintain a repeater system you can't live with out one. Read on and Iíll show you why. There are articles elsewhere on this site to show how one is built.

Now to the procedure...

Tune the transmitter band pass.
If the duplexer has not been tuned before for your frequency, you may want to tune each cavity first by disconnecting them, getting it close, and then tune them together. Connect your Watt meter between the signal output of the band pass cavity and the load (antenna or dummy load) and adjust for maximum RF power. Repeat for each band pass.

Tune the receiver band pass.
Again, if this is the first time the duplexer is being tuned, you may want to tune each receiver band pass first, and then fine tune them together. Cable the duplexer for operation, that is, cable all of the cavities, two (or three) on the receiver and two (or three) on the transmitter. Connect the duplexer to the radio. Connect your signal sampler between the output of the duplexer and the load. Set the signal samplerís attenuation to minimum, open the receiverís squelch, and tune up the volume. Connect your signal source to the signal samplerís port and inject an on frequency signal ample enough so it is noisy but can be heard. A 1 kHz tone applied to the injected signal will help you hear the quality of the injected signal.

Now tune each receiver pass band for the clearest signal. You may have to increase the amount of attenuation on the signal sampler to keep the signal a bit noisy as you improve the pass band tuning, doing this will allow you to heard the gains made. Continue with this procedure until no more improvement can be heard.

Tune the transmit notches...
This eliminates transmitter side band noise at the receive frequency. Key up your transmitter and once again adjust the attenuation so you can hear a noisy signal. Adjust each of the receiver notches until no more improvement can be heard. As you gain improvement you will need to increase the attenuation of the injected signal. Repeat until no more improvement of the received signal is notice.

Tune the receive notches...
This eliminates the transmit power in the receive leg. Key up your transmitter and once again adjust the attenuation so you can hear a noisy signal. Adjust each of the receiver notches until no more improvement can be heard. As you gain improvement you will need to increase the attenuation of the injected signal. Repeat until no more improvement of the received signal is notice.

Always tune the notches last since the notch adjustments track with the band pass adjustments.

OK, I did just what you said, and to the letter and I still have a problem. Well, bummer. What are you hearing?

If you hear cracks and/or pops, or de-sense fading in and out you have a problem past the duplexer, either your feed line or your antenna, or both are causing the grief, this is assuming your receiver is working properly. Don't use a Ringo or an ARX-270 or any other cheap antenna, they are fine for simplex operation but not for duplex operation. Use a good, one piece DC grounded antenna. Don't use LMR-400 (or any LMR cable) for feed line.

If you still have desense in your system and the desense is constant, then find a spectrum analyzer, return loss bridge (or VNA) and tune the duplexer by the factory procedure. Another possibility is that your duplexer does not provide the required attenuation needed. For a 100 watt transmitter and a receiver with a sensitivity spec of .5 uV you need about 88 dB of attenuation.

The following table lists the typical attenuation required between a transmitter and a receiver which will result in no more than a 1 db degradation of the 12 db SINAD sensitivity. This information is based on a receiver with .5uv sensitivity, and a typical solid state thansmitter.

Transmit Power
2 Meter
600 kHz. Frequency Separation
70 CM
5 MHz Frequency Separation
25 Watts 82db 55db
50 Watts 85db 58db
100 Watts 88db 61db
250 Watts 92db 65db
350 Watts 98db 70db

If you are using a solid state RF amp that is over 100 watts,  I think you will find this article dealing with solid state power on a duplexed system very interesting.