Microwave planar sensors have a great interest in the medical environment due to their ability to measure the bulk dielectric parameters of biological tissues through non-invasive and contact-less sensing properties. Changes of these parameters, which are frequency dependent, can be representative of the pathological state of biological tissues. In this work, an improved prototype of planar sensor based on a microwave ring resonator operating at 1 GHz for the fundamental mode is presented. The objective is to obtain a better sensitivity for measuring high complex permittivity values of materials such as biological tissues, and to obtain higher precision in parameters determination. The performances of two sensors optimized on two different substrates were measured in a frequency range 1–10 GHz; an accurate equivalent electrical model is proposed to reproduce the frequency dependence of the resonators. Characterization of liquids and ex-vivo animal tissues is achieved to evaluate the effectiveness and the performances of the resonator sensor, and results are compared with electromagnetic simulations.