ft8.gr

Transmit Level Adjustment in WSJT-X for FT8

A clean transmit signal is the other half of a good FT8 station. Setting the audio drive correctly keeps your signal readable far away and keeps the band free of splatter for everyone else.

Theory

The FT8 transmission is a constant-carrier FSK mode. This means that we transmit continuously for the whole duration of the cycle (15 seconds). If we over-drive (over-deviate) the transmitter, we generate ghost signals that affect other stations and us. Unfortunately this is difficult for us to check ourselves. What we need to do is to adjust our transmitter properly. On the waterfall display an over-driven transmitter shows extra copies of its signal on both sides of the real one.

We gain nothing from it, but we do become "popular" and everyone curses us.

Adjustment

  1. 1

    Set the transceiver to almost its maximum power (100 W) from the controls of the radio.

  2. 2

    In WSJT-X, lower the PWR slider on the right side to the minimum.

  3. 3

    Select [TX 6] CQ.

  4. 4

    Press [Enable Tx] and, as soon as the transmission starts, slowly raise PWR in WSJT-X while watching the output power of the transceiver.

  5. 5

    When the power starts to rise, continue more slowly until you see 100 W. At this point the ALC should be almost nonexistent.

  6. 6

    That's all, folks!

Even Better

While the setting above is the ideal one, it may be somewhat unstable, so raise PWR in WSJT-X a little more until you see a little ALC on the meter.

The example uses the IC-7300, but the same applies to every transceiver, whether it has a built-in sound card or an external one.

Why the ALC Matters

The ALC of a transceiver limits the output when the audio drive is too high. On a single-tone digital mode it should do as little work as possible: every time it acts it changes the signal and can add the unwanted products that show up as ghost signals on the waterfall. Keeping the ALC at or near zero is the simplest check that the drive is right.

Questions and Answers

Should I always transmit at full power?

No. The procedure uses full power only to find the point where the ALC starts to act. Many FT8 contacts are made with much lower power, and the same rule applies at any power level: no more drive than the transmitter needs.

Do I need to repeat the adjustment?

It is worth checking again after changing the sound card, the cable, the band or the settings of Windows, since any of these can change the audio level that reaches the transmitter.