Duplexer Tuning

I have received several Emails asking for help (or how do I) tuning a duplexer (note, duplexer is not duplexers) 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…
Most of us do not have access to a spectrum analyzer or a signal generator, I assume that if you do have access to the mentioned test equipment, you don't need to be reading this. I do have access to both a spectrum analyzer and a signal generator but seldom use the spectrum analyzer. I do use the signal generator to develop test signals at the correct frequency and level. You can create a signal generator from an amateur radio and many feet of RG58 (which will help attenuate the signal). I suggest you spend fifty bucks on 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.

Do you really have a duplexer…
I would like to point out that all repeater duplexer are band pass, band reject. Most of the newer designs use two band pass cavities for transmit and two band pass cavities for the receiver and each can has a reject circuit, or notch. Each cavity should have two RF ports, if your cavities only have one RF port then you have a notch filter which is not going to get you the attenuation you need. If your cavities have two RF ports but do not have a notch adjustment then you have only a band bass cavity, you are out of luck. In the context of this paper, a duplexer allows a radio system to transmit and receive concurrently on a single antenna/feedline system.

One more thing you should know…
When inserting any in-line devices in to your repeater's feed line, be sure any added feed line(s) used to connect the device (like a watt meter) are at least one half wave length of your transmit frequency. You can calculate the electrical length required by dividing your frequency in to 5904 and multiplying the result by the velocity factor of your feed line. This will yield the 1/2 electrical wave length in inches, tip to tip including the RF connector. An example, 5904/442.275 = 13.35, now multiply 13.35 by the velocity factor of the feed line, I'll use 66%, 13.35 * .66 = 8.81 inches. You can use any multiple of the result 8.81, 17.6, 26.43, 35.24. Also try to use a good watt meter like the Bird, the less expensive watt meters are not a true power measuring device but a glorified voltage meter which can be easily fooled by hi Q as found in a duplexer.

About the signal sampler…
If you don’t 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 I’ll show you why.

Now to the procedure…
If this is a new installation or the duplexer is way out of tune, I suggest you rough in the duplexer tuning in to a dummy load first before fine-tuning in to the antenna system. To get optimal performance from the duplexer, tuning the duplexer to the antenna system is a requirement.

Tune the transmitter pass bands…
If the duplexer has not been tuned before for your frequency, you may want to tune each transmit band pass cavity first, and then tune them together. Connect your watt meter between the signal output of the band pass cavity (be sure the receiver band pass cavities are not connected) and the load (antenna or dummy load) and adjust for maximum RF power. Repeat for each band pass.

Tune the receiver pass bands…
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 sampler’s attenuation to minimum, open the receiver’s squelch, and tune up the volume. Connect your signal source to the signal sampler’s port and inject an on frequency signal ample enough so it is noisy but can be heard. A 1kHZ 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 your receive frequency from the transmitted signal. 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 frequency from 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.

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

OK Bob, 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. Since you are an amateur, I won't go in to the feed line issues.

If you still have desense in your system and the desense is constant, then find a spectrum analyzer and a signal generator and tune the duplexer by the factory procedure. Another possibility is that your duplexer does not offer the required attenuation needed. For a 100 watt transmitter and a receiver with a sensitivity spec of .5 uV you need 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.

If you are using a solid state RF amp that is pushing around 100 watts or better I think you will find Kevin's paper dealing with solid state power on a duplexed system very interesting. You can find Kevin's research at his Repeater Builders site.

Copyright © 1996 by RFW Research Company, all rights reserved.