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I routinely check http://rx2.wa2zkd.net:8073/ in the late afternoons to see what propagation from Europe will be like later on here, for pirates in the 5-7 MHz range. Your KiwiSDR is about an hour or so ahead of mine here in Maryland for those signals
I am willing to try writing an extension to do this, if someone can point me to information on making your own extension. I found the example code at https://github.com/jks-prv/Beagle_SDR_GPS/tree/master/extensions/example but need a few more scraps here, like where code goes, how it's built, etc. I'm even willing to document my steps into some sort of a tutorial, if one does not already exist.
It's quite difficult to come up with an automated way to measure SDR performance. Been there, got the t-shirt
I looked at the characteristics of known "good" KiwiSDRs vs "bad" ones. And wow, there are a lot of bad ones. Good SDRs have a few strong bins, and lots of low noise bins. Bad SDRs have a lot of high noise bins. This is one of those cases where you can look at the histogram plot and immediately determine if an SDR is good or bad, but it's tricker to do it in code. It boils down to distinguishing signal from noise.
It was a few months ago that I wrote my code, so I would have to look over it again to remember all the details. Basically I built a histogram of signal levels over the spectrum, and find the amplitude of the 98th percentile bin. Then I find which bin is equal to 0.4 of that value. I call that the quality factor. Larger is better. These numbers were all empirically derived of course, not based on any theory. Just what seems to work. As with all things software, it could certainly be improved, I got it to the "good enough" stage. I mostly use it to test my own setup and various antennas, and a few others privately use it to evaluate their own KiwiSDR.
This is what a "good" one looks like:
And a bad one (some are even worse than this, this one is still bad enough to be effectively unusable IMHO):
A smarter improvement might be to look at the frequency when examining the signal level to decide if it is real or noise. High signal levels are OK in the MW and SWBC bands. Maybe the ham bands Not good if all over the spectrum. Of course the time of day is going to affect what signal levels you expect on a given band. And of course the antenna type which may favor certain bands. And.... See, it's complicated