Fullduplex operation in FT4/FT8 via QO-100

By Matthias, DD1US, January 30th 2023

FT4 and FT8 are getting very popular digital operation modes on QO-100. Most radio amateurs seem to use the software WSJT-X in combination with SDR-Console.

Even though those digital modes need very little uplink power (ERP) we can often see, that many users are transmitting with excess uplink power.

Unfortunately, WSJT-X does not support full duplex operation which helps to monitor the own signal, adjust the uplink power and in addition check the signal quality. Also, in full duplex mode possible collisions of the transmission with other users can be detected and the uplink frequency can be accordingly adjusted rather than increasing the uplink power.

Searching for a solution I contacted Uwe DG2YCB, who is part of the development team of WSJT-X. Uwe was very responsive and helpful and recommended to use 2 instances of WSJT-X. It turned out that this is a very nice solution not only helping to monitor the own signal but also enabling to monitor the same frequency simultaneously for FT4 and FT8 emissions.

Below I will explain how to setup the software. Many thanks to Uwe DG2YCB for his helpful advice. By the way Uwe is the source for the versions “WSJT-X_improved” and “JTDX_improved”.

You can always run multiple instances of WSJT-X (or “WSJT-X_improved”) or JTDX (or “JTDX_improved”) at the same time. They will then work independently of each other.

To track your signal on the QO-100, you only need to run a second instance using your downlink signal from the satellite as audio source. In fact, the two instances of WSJT-X can share the same port of a virtual audio cable. I am using VAC from Eugene V. Muzychenko.

I assume that you start WSJT-X via a hyperlink on the screen and work with Windows, but with Linux or macOS it works in principle analogously.

The instances work as follows:

Of course, now both of your new instances will start with fresh settings, so you would have to re-enter all your data. But you can also easily copy the existing data. This is how it works:

From now on, work only with your new two instances, because starting the "zeroth" instance at the same time would "shoot down" the other two. Of course, the "zeroth" instance will continue to work as usual, as long as the two new ones are not open. You can run only two instances independently of each other, if they are started with the extension --rig-name=XYZ).

If you are using SDR-Console and have enabled in “Transmit Options” the option “Mute receiver during transmit”, then the spectrum and waterfall display will be muted or dimmed according to the set volume reduction. It is better to disable the option “Mute receiver during transmit” when operating FT4 or FT8 in full duplex mode.

Also make sure that the receiver you are using does not have the squelch function enabled.

Here are some screenshots how my setup looks like. I am using 6 receiver windows to monitor the two CW beacons (10489.5 and 10490.0MHz), the PSK beacon (10489.750MHz), the Multimedia beacon (10489.9935MHz), listen to a SSB-QSO (10489.690MHz) and to operate in FT4/FT8 (10489.540MHz):

When operating in SSB mode I am using the Transmit setting “Normal”, when operating FT4/FT8 I am using the setting “Other”. Thus, I can adjust my output power (by drive and gain settings) for each mode and switch simply between them.

Another option is to change the transmit power in the WSJT GUI (slider on the right).

You have now two waterfall windows, the upper called “WSJT-X – TX” and the lower “WSJT-X – RX”. In the lower window you can see your own transmit signal (marked with the red bar) and also other signals transmitted while you are transmitting. You can see there also signals and thus avoid transmitting on the same frequency as they are using.

If you are not in a QSO you can also use the “WSJT-X – RX” instance to monitor a second digital mode. For instance, you can use the instance which is used for transmit (“WSJT-X – TX”) to monitor FT-8 signals while the other instance (“WSJT-X – RX”) is used to receiving signals in FT-4. If you then find an interesting station, you want to have a QSO with, you simply switch both windows in the same mode and use the “WSJT-X – TX” instance to make the QSO.

I hope this description will help others to optimize their operations in FT4/FT8 via QO-100.

Many thanks to Uwe DG2YCB for his kind support.

Questions are always welcome. Please send them to my Email address given below.

Kind regards