Rather Odd-Looking Plot from 21 Dec 2024

[Gary Mikitin, AF8A]

My Grape 1 plot from Saturday, 21 Dec shows a wild 'oscillation' in the daytime received signal strength (after 1300z).  I've never seen a plot remotely similar to this one - did anyone else have similar results on that date, or have you seen anything similar in the past?

My first thought is there's a system problem here at my QTH, but before I start troubleshooting, I'd like to hear from others.  (This is a 15MHz plot - I was operating CW on 20 meters yesterday, but only for ~two hours mid-day, and the band sounded pretty much normal when I was on the air.)

Tnx es 73 de Gary, AF8A

Cleveland, OH, EN91gn

[Normand Juneau]

Hi Gary,

By doing a simple graphic analysis, the rapid changes of the power levels display periodicity. I have measured the distances between the lower peaks. If I take out 2 outliers out of the set, the period mean would be around 20 minutes.

I suspect that these power level rapid changes could be related to a man made noise.

73,

Normand

VE2DVG

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[Gary Mikitin, AF8A]

That is good advice, Normand.  I am monitoring 15MHz WWV on my transceiver while in the shack today.

I can pretty much say with certainty that there is a local problem, local to my Grape system.  The fldigi screenshot is rather unusual - the 15MHz signal disappeared entirely a few minutes ago while other signals appear outside the sampled audio passband.  Now to figure out the cause.  I expect it is something simple, like the cabling.

[Gary Mikitin, AF8A]

Doh!  This is embarrassing:  My WSPR transmitter was overloading the Grape system (the antennas share space in my attic).

I figured it out by splitting the Grape receive audio between the D/A and a powered speaker.  I heard the problem right away.

As Emily Litella,  the fictional character on Saturday Night Live used to say, "Never mind."