Data Modes
Mike Richards G4WNC continues with his primer on operating FT8.
Mike Richards G4WNC continues with his primer on operating FT8.
This month I’m continuing my introduction to FT8 with a selection of operating tips to help you get the most from the mode.
FT8 Operating Tips
USB or LSB?: Those new to data modes are often confused as to whether or not they should follow the SSB convention and use LSB for transmissions on 7MHz and below. This convention is often built into the firmware of your rig so can happen inadvertently. The correct setting for all bands is USB. Data modes operators use USB because we are sending audio tones, AFSK (Audio Frequency Shift Keying), to emulate true FSK (Frequency Shift Keying). In this system, a 1kHz audio tone will raise the carrier frequency by 1kHz and a 2kHz tone will raise it by 2kHz, Fig. 1A. If we were to use LSB, the 1kHz tone would lower the carrier by 1kHz and the 2kHz tone would lower it by a further 1kHz to 2kHz, Fig. 1B. While this still produces an FSK-like signal, the shift would be inverted, thus making it unreadable. Remember, you should always use USB for data modes, regardless of the operating band.
RTTY and USB/LSB: While you should generally use USB for all data modes, there are dual standards in operation for RTTY. North American operators normally use LSB while USB is often used in Europe. This situation is exacerbated because some data modes software assumes you will be using LSB. For this reason every RTTY program has a Reverse button that’s used to correct inverted tones. If you’re using FLDIGI, then you should use USB. For other software packages, check the user guide and select the appropriate sideband.
Audio Drive: While we are all aware of the splatter problems associated with overdriving an SSB transmitter, the use of software generated audio signals introduces an, often neglected, link in the chain. While digital audio systems produce excellent fidelity, they are unforgiving and produce hard clipping and severe distortion products when overdriven. This distortion can pass to the SSB modulator and have a disastrous effect on your signal. You will not be popular! Once aware of the issue, it is easy to control. For those of you with recent transceivers using a USB link for the soundcard and CAT, life is much easier because the audio remains in the digital domain. In this case you should start by configuring your rig for the desired output power.
As FT8 is a weak signal mode, you should always use the minimum power required to support the link and I generally start at 5 watts. Once you have configured your rig, connect a dummy load and click the Tune button in WSJT-X. This will put the rig into transmit and send an audio tone to the transmitter. You can adjust the audio tone level using the Power slider at the bottom right of the WSJT-X main screen, Fig. 2. I aim to run the Power slider at around 75%. If all is well, you should see your transmit power change as you move the WSJT-X Power slider. If the output power doesn’t change, you are probably overdriving the audio and need to investigate. Those with older rigs that are feeding the audio from WSJT-X to the Mic or accessory input of the rig need to watch the rig’s mic or data gain setting. With the tune button activated, set the WSJT-X Power slider to about 75% and adjust the rig’s Mic or Data gain controls for the desired output power. When complete, make sure you can control the output power with the WSJT-X Power slider. If the transmit power stops rising as you increase the WSJT-X Power slider, that’s an indicator that some part of the chain is limiting and you should back-off slightly.
Once you’re confident that you have the drive levels under control, you can move on to set the optimum drive level for each band. This is often required because many rigs need different drive levels to achieve a given output power across the bands. WSJT-X can help with this by remembering the drive level for each band. Leave your dummy load connected and do the following:
In the File menu choose: Settings - Audio Ensure that ‘Remember power settings by band - Transmit’ is ticked.
Click OK
For each of the bands you intend to use, do the following:
Retune to the desired band
Click the CQ message and Enable TX During the transmit cycles, adjust the Power slider for the desired output
Choosing the transmit power: As FT8 is a specialist weak signal mode you can generally operate with low powers in the region of 5 watts. However, you can use the received signal reports to trim the transmit power. Most expert users suggest adjusting the RF power so that your received signal strength is between 0 and -10dB. As WSJT-X measures your signal-to-noise ratio at the far end, these signal reports are far more useful than the manually reported 599 results given in some other modes.
Signal monitoring: The availability of so many cheap SDRs means that it is easy to build your own off-air signal monitor to check the quality of your transmitted signal. In most cases, you will only need a very small whip antenna of just a few inches to gather sufficient signal. Another technique, for those fortunate enough to have a digital scope, is to take a tap off the transmit line and feed it to a scope in spectrum analyser mode. For my setup I use a home-made 40dB RF tap that feeds a sample of my signal into a PicoScope 3203D 50MHz scope in spectrum analyser mode, Fig. 3. That way I can see a 50MHz wide spectrum and directly measure the harmonic distortion of my signal.
Split Frequency Operation: Most newcomers to FT8 operate using the default settings and these are a great way to get started. In its default state, you select a station to call by double-clicking on the desired callsign. This sets both your transmit and receive frequencies to those of the called station, just as you would for a normal SSB QSO. However, when you do that, your transmit signal is directly competing with any other stations that are calling. This is fine if the bands are quiet or you have a booming
signal, but not so good if you’re running a QRP station. As WSJT-X can simultaneously receive all signals in the receive passband, we can use split frequency operation and transmit on a clear channel. Not only does that improve our chances of being received, but it helps the called station because WSJT-X will display a list of all the calling stations, Fig. 4. That station can then stay on air until everyone has been contacted. Finding and setting separate transmit and receive frequencies is done using the Wide graph display and there are a few things we need to configure to get the best from this helpful display.
There are two sections to the Wide Graph. At the top is a waterfall display that is useful for showing band occupancy, while the boittom section is a spectrum display that gives a better view of the signal levels. The first task is to set the display to match the passband of your transceiver. We can use the spectrum display to measure that passband. Begin by doing the following:
Set your transceiver to the mode you will be using with WSJT-X
Set Bins/Pixel to 4
Start to 0Hz
N Avg to 3
Palette to Default
Adjust the two lower sliders for a display similar to Fig. 5
Using the spectrum display, you should be able to clearly see the passband of your receiver, Fig. 5. The next step is to match the display to your passband as follows:
Adjust the Start frequency to the lower edge of your passband, typically around 300Hz.
Resize the Wide-Graph window to the width that works best for your screen size.
Adjust the Bins/Pixel so that the top-end of your receiver passband fits within the resized Wide Graph window.
That’s it!
Those of you with SDR receivers will generally be able to set a wider passband. Now that you have the Wide-Graph passband configured, you can move up to the waterfall as that’s what we’ll be using to run split frequencies. The first task here is to trim
the two waterfall sliders, Fig. 6, to give the clearest display. With the Wide Graph optimised you can start using it for split-frequency operation.
The first task is to return to the WSJT-X main window and ensure Hold Tx Freq is ticked. When this is ticked the transmit frequency will remain wherever you place it. On the Wide Graph you need to observe a few 15 second FT8 cycles to spot an empty or lightly used channel. When you find that channel, press and hold the Shift key while clicking with the mouse on the left-hand edge of the slot you have chosen. You will see the red square bracket symbol appear on top of the slot. That shows your transmit frequency. There’s no need to set the receive frequency because that will be determined by the station you want to work. You can now put out a CQ call by enabling Tx and clicking message 6 to start calling CQ in the next available timeslot. If you don’t have any success, try moving your Tx frequency as there may be QRM on that frequency at the distant end.
Managing your CQ calls: For those new to FT8, trying to scan the list of received stations and to choose which station to work can be just too confusing. To solve that problem, the Call 1st option was introduced. When ticked, Call 1st will automatically select and start an automated QSO to the first operator to respond to your CQ call.
Working multiple stations after a CQ call: This is very easy to do. All that’s required is to reselect Enable Tx at the end of each QSO and WSJT-X will automatically pick up anyone who is still calling you.
Errata: In Fig. 3 of my August column, I incorrectly highlighted the WSJT-X Call 1st tickbox when I should have highlighted the Tx even/1st box. It is the Tx even/1st box that forces FT8 to transmit on the 1st and even timing cycles. This is a convention used when calling for DX contacts. Under normal operation, the software automatically looks after cycle selection when you double-click on the station you want to work.
That’s it for this month, but I’ll have more on FT8 and some other modes next time.