WWV 10MHz Interference (March 21, 2026)

[Joshua Lee]

Hi all,

K3GCC is currently doing some analysis on the G3 geomagnetic storm that took place from March 20-21, 2026.  Our plots of the day seem to indicate some pretty large and varied Doppler shift values but I ran into something interesting when looking at other node's plots.  On March 21, many west/central nodes had what looks like a flatline starting around 1500 UTC.  Attached are some plots below of other nodes' data.  The plots were plotted in MATLAB and run through some filtering. 

Any thoughts on the reason for the flat-lining? Our base station's (K3GCC) plot is also attached and there is a notable difference between our plot and other station's plots.  The storm seemed to affect mostly the northern part of earth, but it would not seem to explain the flatlining in other areas of the US. 

Thanks and 73s,

Joshua

KN6GFZ

[Robert VA3ROM]

This is what my Raspberry Pi 3 Grape1 receiver clone reported. I can't build the SMD version, so I made a TTH version using the TLV2462CP opamp. Antenna is a ground-mounted 10 m tall untuned vertical (self-resonant at 7 MHz). This is what I recorded. I'm just inside the subauroral zone at ~54 degrees ILAT. My Doppler shift flatlining after 1330 UTC or so is probably due to my mid‑latitude position within the storm‑depleted F region. This is actually a known signature in ionospheric physics — it’s called a post‑storm F‑region stabilization period. But I don't have a PhD in radio science to explain why other stations have the same signature or don't. My local ionosphere is not your local ionosphere. I just put my real-time WWV Propagation Monitor online at http://va3rom.com/WWV/WWV.html. It's path-specific for Fort Collins to Thunder Bay (EN58jk), and I run a simple prediction path from Fort Collins to Thunder Bay. I'm monitoring 5, 10, and 15 MHz for comparison, but my path isn't yours, so results can vary. In this case, I'm using a W6LVP receiving loop aimed at Fort Collins, which helps kill WWVH that occasionally shows up now and then. But I've compared the results with the Grape1 clone on the vertial to the RX888 MKII on the loop and the rsults are eerily close to each other. Not bad for a simple DCR v. a complex SDR, eh?!

http://va3rom.com/WWV/WWV.html

Anyway, just my 2 (Canuck) cents worth. I know just enough to be dangerous

!

73,

Robert

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[Robert VA3ROM]

Hi Joe,

I've never had that kind of problem, no matter which types of receivers I use to receive WWV signals.

By frequency standards, do you mean Leo Bodnar GPSDOs? I only use one GPS puck antenna to feed my four-port active GPS distribution system (from https://sv1afn.com/), feeding three side-by-side Bodnar units with short pieces of coax. I then run 1 m coax lines to my Grape1 clone, my two RX-888 MKII's. My Web-888 is on the main dedicated/separate GPS feed. My outdoor vertical and W6LVP are both within 30 m of the Bodnar units and receivers inside the shack, but I've never experienced any kind of RFI issues. I also use a 5-port active antenna splitter, so my one vertical antenna feeds my five different receivers, and my W6LVP has its own three-port active antenna splitter. I'm not sure whether the active GPS or antenna splitters provide any isolation or suppress any RFI issues. The Grape DCRs do leak some LO into the immediate area inside the shack, unless they are put into metal cases. But then you'd have the problem of RFI or desense 24/7, not just intermittently. And if the signal strength isn't affected and it's only the Doppler shift plot, then that means the 1000 Hz audio tone feeding fldigi is changing in audio level but not in tone, which is impossible. There's always some ripple of a few millihertz. If you look at the raw fldigi analysis_yymmdd.dsv file with Notepad++, you will see if there are any NaNs or perhaps data that is not formatted properly or a lot of 0's or near 0's for Freq Err, which is impossible. First, I would replace the audio cables with premium ones. 90% of the time, it's a cheap audio cable with an intermittent issue. Otherwise, I haven't got any other ideas, Joe.

73,

Robert

On Mon, Apr 20, 2026 at 7:35 PM Joe Hobart <no...@npgcable.com> wrote:

Hello Robert,

I operate GRAPE 1 and GRAPE DRF receivers here, and I also frequently get flat
line Doppler traces, which seems to correlate with low signal strength from WWV.
 I have seen these flat line displays off and on since April 2021.  The puzzling
part is there are no flat lines at night when signal strength can also be low.

I also operate several 5 and 10 MHz frequency standards, and John, who designed
the GRAPE receiver, thinks leakage from those standards is to blame.

My question is do you also operate a high accuracy standard within about 100
feet of the GRAPE antenna?

Thanks,
Joe, W7LUX
Flagstaff, Arizona

>     <https://www.swpc.noaa.gov/news/g3-strong-geomagnetic-storm-alerts-issued>

[gxgri...@virginmedia.com]

Hello all,

In addition to the plots shared I've looked at the 21 March 2026 spectrogram on the PSWS database from Joe W7LUX Grape DRF and I strongly suspect that the flat line Doppler is during periods of propagation via the normal (i.e. not sporadic) E region. The very small diurnal variations in height of the E region together with the geometry from its lower height  result in minimal and consistently near-zero Doppler. 

Bob Mattaliano N6RFM and I have a project to document typical features and events seen on Grape spectrograms, you can see the entry for E region propagation on readthedocs here. On the left of that page you will see the list of features and events documented so far. Click on those of interest. Feedback and other examples welcome.

As for Joe's comment from John Gibbons then yes, at some sites leakage is a problem, but it tends to be there 24/7 unless overwhelmed by a real signal with much higher level. See this example on our readthedocs.

73

Gwyn G3ZIL

[Robert VA3ROM]

Good morning to the group,

Joshua, KN6GFZ, noticed that after the March 21 G3 storm, "many west/central nodes had what looks like a flatline starting around 1500 UTC." He asked for thoughts on this specific case of Doppler shift flat-lining post-storm. We had a G3 storm, F region depleted and disturbed, and post-storm recovery during the day that will produce what Joshua asked about. But, yes, a stable E region with small height variation can produce similar results, but after a G3-level storm, a now stable F region will also produce the same results. However, in my very specific case, I can say that E-region propagation would never carry the 10 MHz WWV signal up to Thunder Bay (EN58jk) because my foE is typically 2–4 MHz. Even with oblique incidence, MUF(E) rarely exceeds 6–8 MHz at invariant mid-latitude (ILAT). In my case, it's ~54 degrees ILAT and in the subaural zone. So I can 100% say for sure that it was the post-storm recovery of the F region that caused my Doppler flat-lining during the day and probably for most other stations. But we need the specific geophysical context involved: (Kp index, local foF2, local MUF(E), ILATs involved, Proplab model, et). Could I have recorded one effect and someone else another? Sure, why not, because my local ionosphere is not your local ionosphere. But, in physics, effect usually follows cause. So, for March 21, 2026, it's more likely to be a post-storm F region recovery. But I'm self-taught, so I have a limited understanding of all the ionospheric dynamics involved. And now I will go back to being "a fly on the wall".

73,

Robert

 

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