measure antenna impedance
you can measure the antenna impedance using the value of s11, simply by determining the input impedance from s11, normalizing, placing the normalized impedance on smith chart then moving toward the load by a length equal to that of the feeding line. the point you will reach is the normalized input impedance of the antenna.
you can do this algebraicaly on matlab using the formula
zin = z0 * (zl-z0*tan(beta * L))/(z0-zl * tan(beta * L))
(but note here that L should be negative since you are moving toward the load.
Hope this might help.
best regards,
Adel