Well, when looking head on, a dipole will have a phase center right at the central feedpoint. If you go off angle, the phase center will move a little--especially if there is some nearby metal or a ground plane. If you then put an array of these suckers in a sphere, obviously there will be a different phase center depending on where you are looking at the sphere from. I would guess that since you will be seeing energy coming from the closest dipole, but also from other dipoles that you are seeing "on edge", that the phase center will actually look to be a little inside of the sphere's surface. Just a guess, as i am not an antenna guy, but have played around trying to measure dipole and monopole phase centers empirically for a project once. In that project I had monopoles distributed along a 3D shape also, but used a switch to only turn on one radiator at a time.