  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. Among the electromagnetic modes trapped at the
intersection region of the two waveguides, the TE011 one appears as the most
appropriate for high-frequency applications. The experimental characterization
of this mode is described in detail, investigating the response of a millimeter
wave configuration resonating at 281 GHz, which shows 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, in which one of the component
waveguides is filled with a low-loss dielectric material. The TE011 mode
reveals in general remarkable merit figures, as well as a significant stability
with respect to the geometrical imperfections and to the insertion of a sample
holder. The obtained results show that the proposed single-mode resonator
competes with the standard cavities in terms of performance, versatility, and
simplicity.
