Distance-dependent frequency offsets in 40 m WSPR data — observations & question

[Stephen MacDonald-Brown]

Hello everyone,

I’ve been doing some informal analysis of WSPR data recently — very much in the spirit of “that looks interesting… can I test it?” — and a pattern turned up that I hadn’t set out to look for.

Looking at ~30–40+ hours of 40 m WSPR data and separating reports by distance, a fairly consistent structure appears:

• short paths (<200 km) tend to sit in a persistent negative offset regime
• intermediate paths behave more variably
• longer paths (>400–800 km) show different, often smoother behaviour

The key point is that these groups don’t move together — they behave like separate populations rather than a single global shift.

I tried the obvious next step of comparing this against solar/geomagnetic context (GOES X-ray flux, solar wind, Bt/Bz), but couldn’t find a consistent or repeatable relationship. I also reduced the dataset down to a single receiver to remove spatial averaging effects — the structure remains, but still no clear solar-linked behaviour emerges.

So at this stage, I’m treating this as a repeatable observational pattern rather than a defined mechanism. This is very much exploratory rather than a controlled experiment.

I’ve written up the analysis (plots + method notes) here:
https://macalchemist.net/Interesting_stuff/#entry-5

I’d be very interested to know:

• Has anyone seen similar distance-dependent behaviour in WSPR or PSKReporter data?
• Does this align with known propagation geometry effects (e.g. takeoff angle / reflection height differences)?
• Would ionosonde data (hmF2 / foF2) be the logical next comparison?

Thanks — and I’m very open to being told this is already well known under a different name!

Steve (M7SQI)

[David Eckhardt]

You are detecting single vs double vs. triple hops off the ionosphere.  For the short distances of 200 km or less, these are certainly single hop returns.  For the 400 to 800 km bin, this is likely double hop propagation.  

Dave - WØLEV  

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Dave - WØLEV

[Stephen MacDonald-Brown]

Thanks Dave — that makes sense in terms of general propagation geometry.

I’ve reworked the analysis slightly since my original post, mainly to remove per-reporter frequency bias (which was influencing the absolute offset scale). As expected, that collapses the large ~−25 Hz shelf — but interestingly, the distance-separated behaviour itself remains.

What still stands out is that once the reports are split by distance, the groups don’t just sit at different path lengths — they show different and largely independent behaviour over time. The shortest paths form a fairly coherent regime, the 200–400 km range behaves more like a transition zone, and the longer paths are smoother and more stable. Importantly, these groups don’t evolve together.

So I think your point about hop structure is probably part of the explanation in terms of geometry, but it doesn’t seem to fully account for the differing temporal behaviour between the distance groups.

I may be over-interpreting it, but it feels like there’s something slightly more going on than hop count alone

(Updated plot below shows the same dataset after per-reporter normalisation — structure persists, but now within a much smaller and more plausible ~±7 Hz range.)

Steve (M7SQI)