characteristic impedance calculation s11

How would you obtain the characteristic impedance (Zo or Zc) of a microstrip (or any 2-port network for that matter) from its scattering parameter 2x2 matrix ( [s11 s12;s21 s22] ) alone ?

s11 is the reflection coefficient and reflection co efficient in z paramets is Z0-Zc/Z0+Zc.where Z0 is the primary impedance which is most likely to be 50 ohms.from there you can find the characterisitic impedance.

regards
sajid mohammed.

Thanx for the reply :)