Piezoelectric ZnO coatings on optical fibers are of interest for active optical-fiber devices such as phase and wavelength modulators. Reactive magnetron sputtering has been used to prepare high-resistivity  radially oriented ZnO coatings on 85 mm long sections of Cr/Au-coated optical fibers. The impedance spectra of 2 and 6 mm long transducers are analyzed between 1 kHz and 100 MHz by applying an electrical potential across the thickness of the ZnO coating. The capacitance of these devices exhibits a logarithmic frequency dispersion and a nearly constant dielectric loss of 0.006 +/- 0.002 between 1 and 100 kHz. Two radial-mode piezoelectric resonances, the first at approximately 22 MHz and the second at 66 MHz, are identified. The thickness distribution of the ZnO coating, which results from the magnetron sputter-deposition process, introduces parabolic dependencies of the capacitance and resonance frequencies of elements placed at different positions along the length of the fiber. Identification of the radial-mode resonances and the effects of ZnO thickness gradients on the piezoelectric resonances are made possible by the occurrence of the ZnO thickness distribution. Thickness-induced changes of the piezoelectric resonance frequencies also allow the observation of an 'inversion' of the resonance response for resonances that occur above the LCR resonance. (C) 1997 Elsevier Science S.A.