  SQUID magnetization measurements in oriented powders of Y$_{1-x}$Ca$_{x}$Ba$%
_{2}$Cu$_{3}$O$_{y}$, with $x$ ranging from 0 to 0.2, for $y\approx 6.1$ and
$y\approx 6.97$, have been performed in order to study the doping dependence of
the fluctuating diamagnetism above the superconducting transition temperature
$T_{c}$. While for optimally doped compounds the diamagnetic susceptibility and
the magnetization curves $-M_{fl}(T=const$) vs. $H$ are rather well justified
on the basis of an anisotropic Ginzburg-Landau (GL) functional, in underdoped
and overdoped regimes an anomalous diamagnetism is observed, with a large
enhancement with respect to the GL scenario. Furthermore the shape of
magnetization curves differs strongly from the one derived in that scheme. The
anomalies are discussed in terms of phase fluctuations of the order parameter
in a layered system of vortices and in the assumption of charge inhomogeneities
inducing local, non percolating, superconducting regions with $T_{c}^{(loc)}$
higher than the resistive transition temperature $T_{c}$. The susceptibility
displays activated temperature behavior, a mark characteristic of the
vortex-antivortex description, while history dependent magnetization, with
relaxation after zero-field cooling, is consistent with the hypothesis of
superconducting droplets in the normal state. Thus the theoretical picture
consistently accounts for most experimental findings.
