RADIO PHYSICS AND RADIO ASTRONOMY

Subject and Purpose. Investigations on axially symmetric oscillations excited in a hemispherical open resonator (OR) are presented with a specific focus on the effects exerted by internal inhomogeneities in the OR structure. In this context, a waveguide section is inserted in the center of one of the OR mirrors, and the field distributions of axially symmetric oscillations and the OR oscillation spectrum selection are examined. The motivation behind this study is to minimize the geometric size of the inserted waveguide while still accommodating a small-diameter dielectric sample, thereby ensuring that the measurement results are as little affected by diffraction loss as possible.

Methods and Methodology. The electric-field distribution of OR eigenoscillations is studied in the framework of quasi-optic methods of probe perturbations. The OR resonance characteristics and the physical phenomena occurring within the OR are examined using established and validated techniques to measure transmission coefficients along the EHF path.

Results.It has been established that in a circular waveguide with radius ɑ = 0.6042w₀ (w₀ is the TEM_00q mode field spot radius), the TE₁₁ mode is excited with an efficiency of 0.8993 by the central spot of the TEM_10q mode. The oscillation amplitude distribution was measured at a frequency of 74.98 GHz. The axial symmetry of the OR structure is broken, but axially symmetric oscillations are still excited due to the flat insert. The circular waveguide section provides angular selection of the oscillation spectrum. The loss introduced by this waveguide to the OR does not exceed −2 dB.

Conclusions. A hemispherical open resonator incorporating specific inhomogeneities that facilitate axially symmetric OR oscillations has shown potential for measuring the electrophysical parameters of materials. For this, a disk-shaped sample is placed at the bottom of a circular waveguide section inserted into the OR flat mirror in its center.

Keywords: extremely high frequency (EHF) range, open resonator, axially symmetric  oscillations, circular waveguide, coupling element, transmission coefficient

Manuscript submitted  07.02.2026

Radio phys. radio astron. 2026, 31(1): 051-064

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