Why do you say it's not? Which one of those elements do you want to define as being at zero? Which ever one it is, just subtract it from all the other elements. For example, you want Antenna_Z_Pos(3) to be defined as z=0. So just do
Antenna_Z_Pos = Antenna_Z_Pos - Antenna_Z_Pos(3);
Same for the other two arrays, Antenna_X_Pos and Antenna_Y_Pos. You don't need to use the same element number (3 in my example) for each one. For example you can do
Antenna_X_Pos = Antenna_X_Pos - Antenna_X_Pos(1);
Antenna_Y_Pos = Antenna_Y_Pos - Antenna_Y_Pos(3);
Antenna_Z_Pos = Antenna_Z_Pos - Antenna_Z_Pos(5);
