Changes in propagation give huge changes in signal strength. When the propagation get betters the signal strength and the value of the signal to noise ratio (SNR) will increase.
So the higer the SNR the better the propagation.
To compare WSPR spots I calculate the minimum power that is required the make a WSPR spot.
The SNR of a WSPR spot can be as low as -33 dB. But when the SNR is -29 dB the WSPR will be correctly decoded. So the minmal SNR to calculate the lowest possible power is -29 dB.
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