Exploring 40MHz Propagation with WSPR
Exploring 40MHz Propagation with WSPR
Over the past year, I have been running a 40MHz WSPR beacon under a UK Ofcom Trial and Innovation Licence. Operating at 1W into a dipole (08:00–20:00 UTC daily), this beacon has provided a unique opportunity to study propagation at a seldom-used part of the spectrum — sitting between the well-known 10m (28MHz) and 6m (50MHz) amateur bands.
WSPR (Weak Signal Propagation Reporter) proved invaluable here. By capturing reports far below the noise floor, and thanks to the WSPRnet permanent archive the dataset reveals detailed trends in signal-to-noise ratio, distance, and time of day across thousands of reception reports.
Key Findings
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Sporadic-E dominates: Most spots fell in the 800–2,200 km range, especially during summer.
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F2-layer events — rare but remarkable:
On 29 November 2024, the beacon was received over 8,600 km away, a clear signature of F2-layer propagation. The F2 region, forming 250–400 km above Earth, can occasionally support very high maximum usable frequencies (MUFs) when solar activity is strong.
That day, the solar flux index (F10.7) was elevated at 175 sfu, while Kp remained low — a combination that produced excellent ionospheric stability. Ionosonde data from Juliusruh even showed foF2 values above 11 MHz, meaning MUF(3000) comfortably extended beyond 40 MHz.
For a 1W beacon to cross oceans via F2 reflection is exceptional, and these long-haul receptions highlight how WSPR can capture propagation at the very limits of the ionosphere’s capability.
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Seasonal trends: Es in summer, F2 in winter following the already well known pattern.
Sporadic-E dominates: Most spots fell in the 800–2,200 km range, especially during summer.
F2-layer events — rare but remarkable:
On 29 November 2024, the beacon was received over 8,600 km away, a clear signature of F2-layer propagation. The F2 region, forming 250–400 km above Earth, can occasionally support very high maximum usable frequencies (MUFs) when solar activity is strong.
That day, the solar flux index (F10.7) was elevated at 175 sfu, while Kp remained low — a combination that produced excellent ionospheric stability. Ionosonde data from Juliusruh even showed foF2 values above 11 MHz, meaning MUF(3000) comfortably extended beyond 40 MHz.
For a 1W beacon to cross oceans via F2 reflection is exceptional, and these long-haul receptions highlight how WSPR can capture propagation at the very limits of the ionosphere’s capability.
Seasonal trends: Es in summer, F2 in winter following the already well known pattern.
Why 40MHz Matters
The 40MHz band sits at the upper edge of regular ionospheric reflection making it a sensitive area of changing conditions. Sporadic E, solar driven F2 and even rare modes like TEP all come into play here. Because of that, low-power WSPR beacons at 40MHz can act as effective “early warning” tools for openings at higher VHF frequencies.
Looking Ahead
The beacon results confirm that archived (the keyword here is "archived") WSPR data is an invaluable resource, not just for radio amateurs but for researchers studying ionospheric behaviour. It is important to appreciate that there is no database of spots or flags from FT8 transmissions so you can't go back say 2 years and download everybody's spots to see what was happening then - wspr is the only mode that has that facility.
I downloaded my 8m WSPR beacon spots for 2024/25 and asked ChatGPT to analyse the data and provide a comprehensive technical report - its available here if interested
https://drive.google.com/file/d/12Ik7uCBD2g4VXJNMtAeOrb4he7M1kM_c/view?usp=sharing
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