The design and measurements of a resonator operating near 100 GHz and intended for use in a tandem FEL are presented. The designed resonator employs two parallel curved plates as a waveguide. In FEL operation the TE01 mode is excited. The resonator employs two wave splitters as reflectors. The wave splitters are segments of an overmoded rectangular waveguide which is connected at one end to the waveguide as described above, and is shorted at the other end by a metal plate with an aperture in the center for e-beam passage.Gain calculations were made in the low gain regime. At the operating frequency the curvature of the plates and the gap size were chosen so as to maximize the gain. A multimode analysis of the wave splitter was made. Calculations show, that the optimal splitter width and length allow achievement of very low diffraction losses at the aperture (∼ 2%). This means that the aperture can be made sufficiently large to allow efficient beam entrance into the resonator without degrading its Q-factor. A resonator prototype was constructed and its performance was evaluated experimentally.