We present an analysis of electro-optic interaction of a fundamental WGM in a dielectric microdisc with a radio frequency mode of a half-wave microstrip resonator placed on the disc circumference. 2D and 3D numerical models of the system are developed. Both optical and radio frequency modes are simulated using finite element method with Comsol Multiphysics software. The comparison between the models is presented, and a theory of modulation in the multi-mode system is discussed. The magnitude of electro-optic effect is calculated depending on optical and radio frequency mode numbers and the microstrip length. The magnitude of the effect in such system is shown to have an extremal dependence on the microstrip length for a given set of mode numbers.