Active S parameter reading of MIMO antenna in two port VNA

please help me ,
i want to measure active S parameter of a 2 port MIMO antenna. i know the equation to calculate active s parameter
"Active SParameters
is (S11*a1 + S12*a2)/a1 where a1 is the complex
excitation in volts incident on Port 1 and a2 is the complex
excitation in volts incident on Port 2."
my network analyser transmitting 0 dbm power .
please tell me how can i convert power to voltage so that i will get the values of the a1 and a2 in the equation?

if terminal impedance of your antennas is 50 Ohm +j0 then P = U*U/R

thank you Andrey