  Several basic properties of homogeneous planar Whispering Gallery dielectric
resonators are investigated through a general approach. In particular the
values for the minimum and maximum allowed radius, defined by irradiation and
dielectric losses, respectively, are obtained in terms of the complex
dielectric constant of the involved media. The same approach is employed in the
analysis of the limit allowed frequencies of a fixed family of mode, leading to
the definition of the "ideal" frequency band. The role of the transverse mode
is then discussed, and the "effective" frequency band is introduced and
determined both in simple disc and circular ring resonators. The extension of
the effective band is limited at least by a factor 3, in terms of decades, in
comparison to the extension of the ideal one; this limitation, due to the
presence of transverse modes, can be overcome using suitably designed
non-homogeneous resonators, as discussed in a following companion paper.
