Spectrum of OR Oscillations With a Segment of a Circular Waveguide

The paper analyzes the oscillation spectrum of a hemispherical open resonator (OR). The resonator is excited by a slotted coupling element located on the periphery of the spherical mirror. The measurements were carried out in the four-millimeter wavelength range. It is shown that the oscillation spectrum of such a resonant system is dense. The presence of a segment of a round oversized waveguide at the center of one of the mirrors leads to a significant rarefaction of the spectrum. At the resonant length of the waveguide segment, the axially asymmetric TEM
3012
and TEM
3011
modes are converted into axially symmetric ${\text{TEM}}_{1112}^{\ast}$ and ${\text{TEM}}_{1111}^{\ast}$. It has been established that with this method of powering, axially symmetric modes are excited in the resonator, which do not fit into the known oscillation classification system. They are characterized by a large area in the center of the mirrors with zero electric field strength. Therefore, the presence of a circular waveguide segment does not affect the structure of the field of such modes.