An E-field probe with perfect isotropic response will give exactly the same electric field strength reading, independent on how the probe is rotated in the field. In other words, if the probe is rotated over all its axis in a random way, the readingof the probe would not change at all. In this case the measured E-field diagram would be perfectly spherical, 360 degrees around. In practice, every E-field probe will have non-isotropic behaviour due to many reasons, like differences between the detectors of the x, y and z axis, sharp edges of the probe housing (causing eddy currents), probe support (mounting), fiber optic connections etc.