Two unique definitions of mutual coupling between a pair of antennas are described in . In the “transmit mode” coupling definition, anexcitation source is placed at the feed of one antenna, and the other antenna is terminated ina load. Coupling between the two antennas is then calculated by
Ctr = PL/PD, where PD is power delivered by excited antenna and PL is power delivered to the load of un excited antenna.
After calculating this, you can use S11 and S21 using CST microwave studio for the formula,