  In this paper we study microstrip antenna miniaturization using partial
filling of the antenna volume with dielectric materials. An analytical
expression is derived for the quality factor of an antenna loaded with a
combination of two different materials. This expression can be used to optimize
the filling pattern for the design that most efficiently retains the impedance
bandwidth after size reduction. Qualitative design rules are given, and a
miniaturization example is provided where the antenna performance is compared
for different filling patterns. Results given by the analytical model are
verified with numerical simulations and experiments.
  Key words: Microstrip antenna, miniaturization, partial filling, impedance
bandwidth, quality factor.
