Timeline0645Z: I'm bouncing between 40M and 80M following RBN spots.
0648Z: I work 4 stations S&P on 80M, and don't hear any echoes.
0700Z: I pop up to 40M to make a couple of S&P QSOs, then call CQ for a bit with no takers.
0703Z: I drop back down to 80 to call K6JS and hear the echoes for the first time. I can hear them so distinctly in between QSK breaks that I think I'm losing a jump ball (that's when 2 or more stations respond to another station's CQ).
(Direct link to mp3, if your browser isn't showing an audio player.)
0706Z: I find a clear frequency at 3571 and start calling CQ. Again the echoes are so pronounced I think I'm doubling with another station.
0730Z: I pop back up to 40M for a couple of S&P QSOs. No echo there.
0746Z: I send a series of dits on my CQ frequency so I can measure the delay later using Audacity.
Here's one echo, zoomed in. I've selected the time period from when I send the dit to when the echo is heard. That's reflected in the "Selection Length" text at the bottom on the window, where you can see that the delay is 0.157 seconds (157 milliseconds). 186,000 mile/sec * 0.157 sec ~= 29,000 miles, roughly the circumference of the earth.
0752Z: I work N6RO and the echoes are starting to fade.
0802Z: I hear K6MMM (about 15 miles directly south of me) and hear echo on Rich/Anna's signal.
0804Z: I head to bed.
What was happening?I'm not sure if it was related, but there was a coronal mass ejection late last week that impacted the earth. If I recall correctly, the A index was about 42 and the K index was 4 at the time I was hearing the echoes. Those numbers represent geomagnetic activity in the unsettled to minor storm range.
I did find another blog post from W2PA about a similar phenomenon (with links to other QST articles about echoes like this, as well as longer-delayed ehoes). My best guess is this was some sort of magnetospheric ducting. I can't imagine around-the-world multi-hop F2 propagation being this strong. The delay of the echoes is consistent with an around-the-world trip, although from reading the articles, it looks like magnetospheric ducts can take the signal away from the earth and back.
The effect seems to have been localized to stations in the immediate area. One station in Sonoma County (K6SRZ), about 70 miles NNW, reported the effect, as well a few stations just to the south of me in Campbell (W6FB), Cupertino (N3ZZ) and Los Gatos, CA (K6MMM). I did not hear the echoes on stations in SoCal, nor did I see anyone other than N3ZZ reporting it in soapbox comments on the 3830 reflector.
My big regret is that I didn't think to hook up my SDR and capture the entire band data. Then I could have gone back and listened to all the other local stations on 80m to see which ones were experiencing the echo. But what a reminder about how cool mother nature can be!
I 've experienced this several times before. One time, on 40m, I was getting multiple echoes with a fluttery sound, and I was running 5 Watts to a dipole! I thought I was hearing my signal circumnavigation multiple times. Wild!
This could easily be a magnetospherically ducted signal. Assuming Lat 37.31, Lon -121.96, then this gives a geomagnetic latitude of about 42.5 degrees. According to my software for such ducts, a path in the magnetosphere should give a delay of 126 ms +/- 5 ms or so.
The delay value is slightly less than 138 ms and easy to confuse with a round-the-world path.
The challenge with estimating delays like this from the signal is that amateur transceivers may have an unspecified delay between start of transmission and start of sidetone. Measuring on the audio output measures the sidetone, not the actual RF. I
discussed this in my 2009 QST article "Magnetospheric ducting as an explanation for delayed 3.5 MHz signals," .
Therefore the measurement shown above may fit with 138 ms just as well as with 126 ms, it depends on the actual transceiver's delay.
More stuff about such echoes can be found in my blog: Medium delayed echoes
Thanks, Sverre! The additional internal delay between sidetone rising edge and actual RF output is something I didn't think of.
I plan to measure this delay some day for my K3. But every make of transceiver probably has different values.
After I wrote my first comment, I have discussed your echo on my blog: Magnetospherically ducted echoes in the San Francisco area including the predicted path for such a signal along a duct in the magnetosphere.
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