Is there any software available for measuring S/N and signal strength,
through the soundcard? My idea is to analyze signals arriving from my
antenna, through the receiver and the soundcard, and thus compare
antennas numerically in terms of signal strength and noise.

This capability is already present for the most part in the Faros
beacon monitoring software, but it is tailored to monitoring only the
IARU/NCDXF beacon signals.

73 - Kristinn, TF3KX