Testing with a touchstone file.
# kHZ S RI R 50 1000 0.0000 0.0000 2000 0.0099 0.0990 3000 0.0384 0.1923 4000 0.0825 0.2752
The S parameters are artificially generated as Z=50.0 + jX [ohm], and the reflection coefficient, S11, is obtained as S11=(Z-Z0)/(Z+Z0).