  In the present paper we discuss antenna miniaturization using
magneto-dielectric substrates. Recent results found in the literature reveal
that advantages over conventional dielectric substrates can only be achieved if
natural magnetic inclusions are embedded into the substrate. This observation
is revised and the physical background is clarified. We present a detailed
discussion concerning magnetic materials available in the microwave regime and
containing natural magnetic constituents. The effects of magnetic dispersion
and loss are studied: constraints on the microwave permeability are used to
estimate the effect of magnetic substrates on the achievable impedance
bandwidth. Microwave composites filled with thin ferromagnetic films are
considered as a prospective antenna substrate. We calculate the impedance
bandwidth for a $\lambda/2$-patch antenna loaded with the proposed substrate,
and challenge the results against those obtained with conventional dielectric
substrates. It is shown that the radiation quality factor is strongly minimized
with the proposed substrate even in the presence of realistic losses. Estimates
for the radiation efficiency are given as a function of the magnetic loss
factor.
