  Electric and magnetic currents are essential to describe electromagnetic
stored energy, as well as the associated quantities of antenna Q and the
partial directivity to antenna Q-ratio, D/Q, for general structures. The upper
bound of previous D/Q-results for antennas modeled by electric currents is
accurate enough to be predictive, this motivates us here to extend the analysis
to include magnetic currents. In the present paper we investigate antenna Q
bounds and D/Q-bounds for the combination of electric- and magnetic-currents,
in the limit of electrically small antennas. This investigation is both
analytical and numerical, and we illustrate how the bounds depend on the shape
of the antenna. We show that the antenna Q can be associated with the largest
eigenvalue of certain combinations of the electric and magnetic polarizability
tensors. The results are a fully compatible extension of the electric only
currents, which come as a special case. The here proposed method for antenna Q
provides the minimum Q-value, and it also yields families of minimizers for
optimal electric and magnetic currents that can lend insight into the antenna
design.
