Voltage Open Circuit of an Antenna

Hello !

I have to make a project and one of my tasks is, to calculate the open voltage of the antenna ! If we can say that the equivalent circuit of an antenna in transmit mode is a Voltage source with a Zantenna in a given frequency, i undertand that if i have a Zload=0. this is the open voltage circuit. So I) why is this important? what information i profit from this ? and II) how can i calculate that ? I run CST and i have a printed dipole

Thanks very much,