"Isotropy" and antenna gain

Antenna systems with isotropic radiation pattern will often be required for aircraft, rockets, and satellites. However, for characterizing the "roundness" or better the "isotropy" of the pattern, which is the most important factor of such an antenna, there is no weldle fined number, as we have the gain for directive antennas. In many cases, e.g. in telemetry systems, a deep, but very narrow minimum in the pattern of the on-board antenna does not affect the radio link condiserably. Therefore, the deepest minimum cannot be used for characterizing such a pattern by one numerical value.