How to Receive FT8 and WSPR without Transmitting

[Pavel]

Short answer: Receive-only FT8 and WSPR need an HF-capable SDR, accurate computer time, USB audio or direct I/Q routing and the correct dial frequency for the selected band. Transmission is not required to decode public amateur signals.

Before you start​

• Identify the band, mode and expected occupied bandwidth before tuning. Shortwave broadcast AM, amateur SSB and narrow digital modes require different filters and demodulators.
• For HF, reduce household noise first: compare battery-powered reception, disconnect network cables and test away from displays and switch-mode power supplies.

Step-by-step method​

1. Step 1: Choose a band with current activity and set computer time accurately using network time synchronization. Errors approaching a second can prevent time-slotted decoding.
2. Step 2: Tune the published dial frequency in USB mode, use roughly 3 kHz audio passband and disable squelch, noise reduction and audio enhancements.
3. Step 3: Route clean audio to WSJT-X or WSPR software through a virtual audio cable or supported direct interface. Keep levels below clipping and ensure the decoder sees the expected sample rate.
4. Step 4: Run several 15-second FT8 periods or two-minute WSPR periods, then compare decode count while adjusting gain and audio level separately.

Concrete example​

For FT8, the receiver remains on the band's USB dial frequency while individual signals appear at audio offsets within the waterfall. Do not retune the RF center to every trace.

How to judge the result​

The chain is correct when time remains synchronized, decoder audio stays unclipped and calls are decoded over repeated periods without any transmit device configured.

What to record​

• Record UTC time, frequency, mode, filter width, antenna and signal-to-noise ratio because propagation changes with time and path.
• For FT8 or WSPR reception, decode count and clock accuracy are more useful than loud speaker audio.
• A lower visual noise floor is only an improvement when wanted signals remain and their signal-to-noise ratio increases.

Common mistakes​

• Changing several hardware, software or RF variables at once, which removes the controlled comparison needed to identify the cause.
• Treating one autoscaled screenshot or one unusually good result as proof without recording the settings and repeating the test.
• Using AGC or audio processing that pumps levels within each time slot can reduce weak-signal decoder performance even when speaker audio sounds louder.

Final check​

Return to the same band at a comparable UTC time and repeat the receive test with the recorded mode, filter, antenna and gain. Keep propagation changes separate from hardware changes.