Possibility of excitation of the lowest mode oscillations in a semi-symmetric open resonator (OR) using 4 element microstrip array (MSA) is examined. MSA properties at 10 GHz frequency are calculated. Experimental curves of loaded Q-quality and resonant gain as a function of distance between the mirrors of the OR, excited by the MSA, and an aperture coupling component are shown. Promising application of the MSA in creating active high-Q quasi-optic wave generator in the millimeter (mm) range is shown. The aim of this paper is the calculation and analysis of the electrodynamical characteristics of an OR with MSA, consisting of four identical rectangular patches excited in phase.