HF: Doppler, Signal Level and Time

Two views of the carrier of Sofia-Kostinbrod on 9400kHz from 15:30 to 18:30 UTC: On top the frequency within a window of 2Hz height only, at the bottom the synchronized HF level of this carrier; see text. [Click onto the picture for a better view.]

What you see in the picture at the top, is a mostly hidden gem of HF propagation. I took the carrier of Sofia-Kostinbrod transmitter form Bulgaria (250kW) on 9400kHz and observed it for three hours. In the upper window you see the frequency wihtin a window of 2Hz height only. You see two strong carriers: one nearly in parallel to the x-axis, the other snaking some fraction of one Hertz below it.

With one transmitter only on this frequency: How does this happen?

It’s multipath propagation. The signal takes one way via a groundwave-like way, the upper trace. It reveals a very slight drift downwards. As I use a GNSS-controlled receiver, the FDM-S3 from Elad, this miniscule drift should be happen within the transmitter, not the receiver.
The snaking trace stems from a second way, most likely via the F2 layer of the ionosphere. As the ionosphere is prone to winds and an ever dynamic change of its ionization, it is moving. And with all moving objects, also this causes a Doppler effect to waves. This is exactly what we see – the angular speed of the ionosphere, relative to the “groundwave-like” signal.
You may also see at least two weaker traces, caused by two further ways, hence showing other Doppler shift.

In the diagram at the bottom, you see the combined level of all traces. Because they reach the reeiver at different time and, hence, different phases, their addition leads to an ever changing signal level, called: fading.

I hope to continue this work with some other examples in the future, also taking fade-in and fade-out into account.


  • Very interesting ! Thanks LU2DKN

  • Food for thought as always

  • A plot like this is raising a whole bunch of (to me) interesting questions and thoughts:

    1. Why is the “groundwave looking” path looking so “groundwave-ish”? Sofia->Hannover is ~1,500km and for all I know there should be no groundwave signal at this distance on 10 MHz. If it’s not groundwave, what does that say about the ionosphere layer that refracted it? Could this just be indicative of a quiet F-layer? Is the slight drop in frequency caused by the transmitter or was it just the ionosphere resting its foot on the brake pedal?

    2. Why is the Doppler shift of the other paths fluctuating so wildly? That looks like an indicative of varying layer velocity of course, but of what layer(s)? if the whole height of the window is 2Hz, that would be low velocity changes <30m/s=108 km/h. AFAIK velocity changes in the thermosphere are a matter of several hours, the plot indicates that the changes are much faster, almost atmospheric. The E-layer is just above the mesosphere, are there wind conditions like that up there?

    This is all utterly fascinating (cue Spock's eyebrow rising here), just as fascinating as the fact that hobby equipment for – in comparison – mind-blowingly low prices allows for such observations and so much food for thought. 🙂

  • ,,, well, Ollie – all interesting. A “stable” ionosphere behaves similar to a groundwave. Bear in mind, what we see: the relative (!) velocity of reflecting patches. If there are no high winds, or the are not causing any relative (!) velocity from my point of observation, the spectrogram seems stable – wihtin the limits of its resolution. You often experience this picture under one hope/one path condition under a quiet ionosphere.
    The wildly fluctuation can be caused by high winds and/or changing paths or fast (re-)combination of molecules, resulting in an upward or downward movement of the ionosphere; hence again causing Doppler.
    It would be interested to triangulate such observations: samer station/frequency/time, but different locations and/or same location, but different stations in the same band and/or same station over different bands.
    All this is possible – as you said – by relatively cheap SDRs, and can be made off-line.
    These observation must be enriched by the freely available data of ionosondes (among other data). A raytrace simulation via e.g. PropLab 3.0 also helps.
    Sorry to say that there is no – repeated: no – interest of the leading German magazines to publish an article on this (and many other …) technical topics which, last but not least, would show some application of SDRs plus some interest of amateurs in the vital field of propagation. Trust me: I’ve tried that many times over years and eventually give it up … 73 Nils, DK8OK

    • Well, Nils…in my brief internal brainstorming, I obviously didn’t consider that the ionosphere isn’t a lasagna, at least not a neatly layered one. 🙂 Maybe this is just some form of “spread-F” diffusion at work? Whatever it is, triangulation of such paths would be interesting indeed, even more if there would be a way to assess at what height (ballpark numbers) that happens. Now I’m not a physicist, much less an expert in ionosphere physics but of course that’s exactly the point of an article like this: Despite I’m not any of those, I’d have a chance to do my own experiments and observations to get a better understanding on what that “not-a-lasagna” ionosphere may actually look like. Coherent receivers? Wide aperture radio interferometry? All within reach, with the “logistics” more of a problem than the price of the gear. BTW, you probably know that already but I found this paper really interesting in this context:


      Re magazines publishing stuff like that… maybe you have to cut them some (or even more) slack and take their readership demographics into account? It’s likely the part of the radio community that’s less inclined to use the internet extensively to stay on top of the developments, maybe not the most neophilic part either… age is certainly a factor in it, too. Now this was said with the international media in mind, but German magazines? Pardon my ignorance but is the plural “magazines” even applicable anymore? I wanted to add a smiley to the last sentence but I choked on that: For an author, this isn’t funny or ironic, not even remotely and what that means for German readers without an above-average fluency in English, who are severely restricted to a trickle of information that makes it into the sad remnants of the German radio-related media landscape would be too much to comment on here.

      Anyway…in the years reading the international media outlets / blogosphere I think I’ve gained some idea on what’s getting the attention of the crowd, which is not that much different in its demographics from what I described above anyway: 1. new gear, 2. stuff to buy, 3. accessories to spend money on, 4. purchasing advice, 5. high-end gear to dream of, 6a. nostalgia stories, 6b. nostalgia to buy (vintage gear), 7. simple DIY stuff, 8…398. everything else, 399. ionosphere physics observations. That may sound a bit more dim than it actually is, I think a sizeable (but likely less vocal) part of the community is still interested in very technical or educational content and generally embraces the explosion-like extension of possibilities and hardcore-nerdy stuff SDR technology brings. But I can imagine how this is some difficult matter in a print media publisher’s mind when I consider how much protest a single article beyond the usual scope, a one-off deviation from reader’s expectations has created in the past. As if anyone would force them to read that, print media or other, they rather spew threats to cancel their subscriptions even if it’s subscription-free and online (go figure!) than just flipping the page or hitting the forward button, because that would’ve deprived them of a chance to steam off completely unrelated frustration. Well, a tough audience at any rate. 🙂


      • … thanks again, Ollie. Yes, and there are many papers around, plus many (professional) projects like FORMOSAT/COSMIC, freely available also for us mere laymen. Not the slightest interest by German magazines. Whereby the age of the editors as well as readers does not necessarily mean disinterest in modern developments and stupidity to describe them. This is, at least outside of amateur radio, not a law of nature. But maybe inside? Some say this, others say that …
        I tried it many times to get these lavishly payed editors attracted to these things, alas, in vain. RIP. After 50 years of publkishing in this field (not my real job, mostly done in my free time), I resign myself to this obtuseness.
        BTW: The clock of the SDR is controlled by GPS, so any visible (!) (absolute) deviation and drift originates from the transmitter. Even renowned broadcasters do feature considerable unintentional offsets from their assigned channel, plus warm-up drift. That’s another topic. But not mine, anymore …
        73 Nils, DK8OK

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