With our advancements in the world of D-Star Hotspots (thanks LARGELY to Steve, KB4DNQ), some of you will undoubtedly be looking at creating your own hotspots – whether simplex (easiest), or full-duplex (most capable). If you ‘re thinking of full-duplex, then you’ll need a duplexer. UHF units are the cheapest, due to their small size.

Some of you have asked   “… how difficult is it to tune repeater duplexers..?” As in all things in life, the answer is “it depends”.

If you have a spectrum analyzer with a tracking generator, as well as a return loss bridge, the procedure is straightforward and takes maybe 60 minutes.

If you have just basic tools, it’s a bit more difficult and a lot less certain (you can’t SEE the results, only HEAR them). While the basic method can get you in the ballpark, it can sometimes be deceiving, in that you could be tuning into a cavity’s harmonic “sweet spot” and therefore not getting optimal results. There is no substitute for using the right tools for the job, BUT… when you don’t have (or didn’t BRING the right tools), it’s always good to be able to fall back to basics.

So here goes, using a signal generator, an attenuator, a power meter:

You can create a signal generator from an amateur radio and many feet of RG58 (which will help attenuate the signal).  Signal samplers can be had for $50 if you look on eBay, with even more elaborate units for around $100 which are laboratory qulity, more on that later.

A couple hundred feet of the worst RG-58x is about the right amount of feed line and attenuation to generate the required signal (more is better – and the lossier the better). If you need help, see the Amateur Radio Handbook for detailed instructions.

Almost 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 cavity 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, since there will not be enough rejection of the other side of the repeater (transmit or receive).

The signal sampler is a critical component for this procedure. RF Parts offers a very nice signal sampler, which will provide between 20-80 dB of attenuated signal. Another source is eBay – use the search term “Step Attenuator”. I got a couple of beautiful units manufactured by JFW (Model 50DR-055) which were “Factory Seconds” with only minor (almost un-noticable) cosmetic belmishes for less that $100 each. The units are rated from DC-2,200Mhz, 50-ohms, N-connectors, attenuation range from 0-30DB in 1-DB steps.  If you own or maintain a repeater system you can’t live with out one.

Tune the TRANSMITTER PASSBANDS - 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 signal source to one side of the transmit cavity – you will be injecting TRANSMIT frequency
  • 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)
  • Inject a signal at the TRANSMIT frequency
  • Adjust the cavity for maximum RF power.
  • Repeat the process for each bandpass cavity
  • When all individual cavities are tuned,
  • Connect all transmit bandpass cavities together, then
  • Repeat the procedure on the cavities in SERIES and adjust for maximum RF power

Tune the RECEIVER PASSBANDS - 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.

  • Connect your signal generator to one side of the receiver cavity – you will be injecting RECEIVE frequency.
  • Connect your signal sampler between the output of the receive cavity and the load.
  • Set the signal sampler’s attenuation to maximum (high DB),
  • Open the receiver’s squelch, and turn-up the volume.
  • 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 receive bandpass cavity  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 bandpass tuning, doing this will allow you to hear the improvement made by your tuning adjustments.
  • When you’ve achieved the maximum signal clarity (listen for the 1khz tone quality), then
  • Proceed to tune all subsequent receive cavities
  • When all receive cavities are independently tuned,
  • Cable the duplexer for operation – cable all of the cavities, two (or three) on the receiver side and two (or three) on the transmit side.
  • Reconnect the signal generator and signal sampler – but this time for the SERIES of the receive cavities.
  • Repeat the tuning process with all of the receive cavities in series. Note that the cavities will interact with each other, so changes on one cavity will affect the signal quality coming from the other (this can be an extremely frustrating process – and is where MOST of the time is spent).

Tune the TRANSMIT NOTCH FREQUENCIES  - this eliminates your receiver signal from the transmitter input.

  • Connect your transmitter to a SEPARATE ANTENNA – not to the duplexer..!! You will be transmitting on the RECEIVE frequency.
  • Connect your receiver to the transmitter cavity where the signal generator used to be
  • Listen on the receive frequency thru the transmitter cavities (in series) and adjust the attenuation so you can hear a noisy signal. Adjust each of the transmit notches in order to REDUCE the received signal (rejecting the received frequency)

Tune the RECEIVE NOTCHES - This eliminates the transmit signal from the receive input.

  • Connect your transmitter to a SEPARATE ANTENNA – not to the duplexer..!! You will be transmitting on the TRANSMIT frequency.
  • Connect your receiver to the receiver cavity where the signal generator used to be
  • Listen on the transmit frequency thru the receiver cavities (in series) and adjust the attenuation so you can hear a noisy signal. Adjust each of the receive notches in order to REDUCE the received signal (rejecting the transmit frequency)

Always tune the notches LAST, since the notch adjustments track with the bandpass adjustments.

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

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. There are a number of folks from Daytona thru Jacksonville who may be able to assist (cheap – for a bottle of beer..!!)

If you are using a re-purposed Commercial duplexer, you may need to change the interconnecting cables between the cavities, since each of those cables are cut-to-frequency. Although usable, it will not be optimal without exchanging. If you are using Tx/Rx Systems duplexers, we can provide you with the exact length of both VHF and UHF interconnect cables for amateur service.

Another possibility is that your duplexer does not offer the required attenuation or frequency separation 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 (which is pretty crappy).

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