A microwave resonator for operation in the whispering-gallery mode is
constituted by a resonant element included in a flat disk having a
diameter (2a) which is considerably larger than its thickness (2d). An
electromagnetic wave which propagates within the disk is confined between
the periphery of this latter and a so-called caustic surface having a
smaller radius (a.sub.c). The wave does not radiate to the exterior and
the resonator can be placed on a dielectric or metallic substrate. The
disk can be hollowed-out within the caustic surface. A resonator can be
simulated within the thickness of a dielectric substrate by at least one
metallic ring which forms a magnetic short-circuit with the ground plane.
Excitation is produced by microstrips or by dielectric image waveguides.
