The impedance of a sphere in a plasma is calculated for four different models. There are important differences between the four results. In particular, the real part of the impedance is shown to vanish at several frequencies above the plasma frequency for a spherical grid if the plasma is described by hydrodynamic equations. If the kinetic (Vlasov) equation is used, the nulls are changed into minima that are only noticeable if the radius is much greater than the Debye length and the frequency is not too far above the plasma frequency. If the hydrodynamic equations are used but the normal component of the fluid velocity is assumed to vanish on the (solid) sphere, there are no nulls or minima. The relationship of these results to previous work on antennas in plasmas is considered.