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The confinement properties of the open structure formed crossing a circular waveguide perpendicular to a parallel-plate waveguide are discussed, highlighting the fundamental differences with respect to the common high-frequency resonators. The structure supports a TE011 mode that appears suited for millimeter- and submillimeter-wave applications. The experimental characterization of this mode in a configuration resonating at 281 GHz is described in detail. The observed resonance exhibits state-of-the-art performances. The properties of the TE011 mode are studied in terms of the geometry, calculating the mode chart and the related quality factor and power-to-field conversion efficiency. The mode chart is then determined for configurations including a sample holder, where one of the waveguides is filled with a low-loss dielectric material. In addition to excellent merit figures, the TE011 mode shows a relevant stability with respect to the geometrical imperfections and to the insertion of a sample holder. The obtained results demonstrate that the proposed single-mode resonator competes with the standard cavities in terms of performances, versatility, and simplicity.