  In the present paper we discuss the effect of artificial magneto-dielectric
substrates on the impedance bandwidth properties of microstrip antennas. The
results found in the literature for antenna miniaturization using magnetic or
magneto-dielectric substrates are revised, and discussion is addressed to the
practically realizable artificial magnetic media operating in the microwave
regime. Using a transmission-line model we, first, reproduce the known results
for antenna miniaturization with non-dispersive material fillings. Next, a
realistic dispersive behavior of a practically realizable artificial substrate
is embedded into the model, and we show that frequency dispersion of the
substrate plays a very important role in the impedance bandwidth
characteristics of the loaded antenna. The impedance bandwidths of reduced size
patch antennas loaded with dispersive magneto-dielectric substrates and
high-permittivity substrates are compared. It is shown that unlike substrates
with dispersion-free permeability, practically realizable artificial substrates
with dispersive magnetic permeability are not advantageous in antenna
miniaturization. This conclusion is experimentally validated.
