WSJT-X operating tips

The WSJT-X program enables popular digital modes like FT8 and FT4 among several other modes. WSJT-X is generally easier to use than FLdigi, another popular digital program that handles a wide range of other ham radio data modes. WSJT-X decodes multiple signals at once, and discriminates between overlapping signals. WSJT-X includes multiple high performance digital modes, each tuned for challenging propagation conditions from LF to microwave.

Control transmitter RF output power with the sound card output level, NOT with radio RF power control. Optimize transmitter RF cleanliness by:

  1. set the RF power control to well above the intended transmit power level (say 6 dB to 10 dB higher)
  2. set microphone gain to about midrange
  3. set computer audio volume to precisely set output RF transmit power

If the radio transmitter is driven into ALC by using RF power control to limit transmitter level, the radio will splatter, interfering with adjacent frequencies. While some operators use QRP ≤ 5 watt transmit power for WSJT-X supported modes, many others use 25 .. 100 watts. This accounts for the discrepancy one might experience in a QSO between the received signal strength by each operator. Some use a very minimal amount of RF power for a “kilometers per watt” challenge.

When using CAT control for PTT, RFI (for example, common mode RF interference) can cause the radio to fall out of transmit, even with very low RF transmit power like 100 milliwatts. With the compromise HF antennas enabled by these efficient digital modes, one must be just as mindful of proper grounding and RFI as with a 100 watt SSB or CW station. Diagnosing this problem can be done by setting the transmitter power to as low as measurable, and seeing if the radio still drops out of transmit. If so, then the problem is likely RFI or common mode RF on the USB control cables.

Older radios may have too-narrow receive filters to capture the whole audio passband used by modern data-friendly transceivers. If the radio has “IF shift” or “pass band tuning”, it might receive from 400-2800 Hz. The “split” transmit feature of WSJT-X shifts the radio RF transmit frequency and audio frequency to optimize the transmit audio passband to minimize transmitted audio harmonics.

A radio that receives on multiple HF bands simultaneously greatly increases the data gathering capability for ionospheric studies. SDRs oriented toward “traditional” ham radio use typically receive one or two RF bands at once. Simultaneous multi-band reception can be accomplished with a bank of SDR receivers or a broadband direct-sampling SDR.