This evening I came across a terrible signal on 15m FT8. The audio from a Cuban station sounded distorted. I won’t name and shame the station. The audio fundamental was about 600Hz, and the waterfall showed strong signals at 1200Hz and 1800Hz matching the 2nd and 3rd harmonic of the audio signal. This caused significant QRM across a significant part of the FT8 segment. Almost certainly the station was overdriving the audio, either causing distortion from their soundcard, and/or distortion in the rig’s modulation chain.
There are several things you can and should do to avoid this, especially when using significant power and/or gain antennas:
- Don’t use full output from your soundcard. Set at no more than 50% volume level. Play a pure tone at that level and listen to it on the soundcard output as you adjust the level.
- Some rigs, such as the Elecraft K3, have a dedicated data audio input and data mode that removes audio processing like compression or other ALC artifacts. Use this if possible.
- For rigs without dedicated data modes, disable all audio compression and set the RF power to a desired amount, say 25W, and start with a low audio signal so the RF power is much lower than that. Then increase the audio level using the soundcard volume and rig audio level until just below the desired RF output while keeping the sound card level in the range 10% to 50%.
- Use an audio fundamental of at least 1400Hz. If necessary, tune the rig to adjust the dial frequency. For example, say you are using FT8 on 20m where the band segment starts at 14.074Mhz. You see a free frequency on the waterfall of 400Hz, and you want to start calling there. If you transmitted a 400Hz tone, then the 2nd, 3rd and 4th harmomics at 800Hz, 1200Hz, and 1600Hz will all be in the audio passband of the rig. Hence the rig will transmit the desired signal at 14.074400, as well as undesired signals at 14.074800, 14.075200, and 14.075600. To avoid this, retune the rig 1kHz lower, to 14.073Mhz and use an audio frequency of 1400Hz. This will generate the same RF for the audio fundamental, but the 2nd and higher audio harmonics will be mostly outside of the rigs audio passband and will be suppressed from the RF output.