|I'm using an SDR (the Andrus MK 1.5 in this case) to monitor a carrier at 21.750 MHz. When the sun gets all crazy like it did today (three X-class solar flares), the earth's ionosphere changes characteristics radically, becoming a "hard mirror" for signals such as this one. So what you see is a sudden increase or decrease in signal strength as the wavefront from the flare arrives at the earth's upper atmosphere.|
I've set up SdrDx in a pretty specific way to do this. First, I am using upper sideband, which allows me to narrow the receive passband right around the carrier. I've got it set for 600 Hz (1200-1800 Hz.) This creates a heterodyne (a squeal) at 1500 Hz, at the same time it allows me to measure the signal strength of pretty much just that carrier. So, to get rid of the squeal, I turned on a notch, N2, and set it for exactly 1500 Hz; blessed silence (well, almost... there's a little hiss.) Finally, I used the graphing S-meter setting to create a time-wise graph of the signal strength so I can just glance at it now and then to see if there's been a significant change in signal propagation. I've also set the FFT to 8192 points across 50 kHz, which gives both the waterfall and the spectrum very sharp resolution; then I set the FFT to average across ten measurements, which in turn reduces noise to a minimum and just leaves the signals showing peaks. That's because noise is random, and over time averages out to zero, while a signal is not random, and averages out to its signal strength.
So what's it all mean? Well, the TL;DR version is I'm watching for a measurable, visual effect that will arrive if and when we have another significant solar flare; we've had three already today, and I am, late as usual, trying to catch one in the act. The science folk are saying there's about a 40% chance of same. Sun's almost over the horizon here, so I've only got a little while longer to catch it, at least until tomorrow.
Just one more example of the silly fun you can have with radio!
EDIT: At 0136 GMT, a moderately slow onset flare erupted. It reached X1.2; at M5, a flare alert is issued, so this is VERY notable; Noise levels have risen significantly, as has my reference signal strength, which had earlier faded to zero.
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