Perhaps an example would be helpful with
Isotropic.
A theoretical antenna that can radiate radio frequencies equally in all directions, just like an ever-expanding perfectly spherical ball of energy, is an Isotropic Antenna. In practice, there can not be a physical isotropic antenna because you would have to take into account the feed cable to get the signal to the antenna (which adds electrical impedance, distortion of the field, etc.), the way the earth's surface distorts the radiation pattern, and so forth.