RADIO PHYSICS AND RADIO ASTRONOMY

Purpose. The excitation efficiency is investigated of the first higher-order axially asymmetric oscillation mode (TEM_10q) excited in a hemispherical open resonator (OR) at the frequencies of the fundamental and second-order harmonics of the Gunn diode in the 4-mm and 2-mm wavelength ranges. The hemispherical resonator is coupled to its input waveguide via aperture-type coupling elements. The diameter 2a of the OR mirror apertures is 38 mm, while the curvature radius of the spherical reflector is R = 39 mm and the normalized distance between the mirrors is L/R = 0.593. Two aperture coupling elements of dimensions a × b = 6.9 × 9.6 mm are used to excite the OR. They permit controlling separately the functions of field-to-field matching (modes in the resonator and in the waveguide) and volume-to-volume coupling of the structural elements (the resonator and the waveguide). They are located at the center of the planar mirror. The field matching is determined by the geometric dimensions of the coupling elements, whereas the coupling matching is determined by the period of the one-dimensional E-polarized grating in their apertures. The Gunn diodes are used as generators, operating at the frequencies of the fundamental (75 GHz) and the second-order (150 GHz) harmonics. The excitation efficiency of the TEM₁₀₁₁ oscillation in the OR of the geometry specified here, using aperture-type coupling elements as described, is 81.5%.

Design/methodology/approach. The excitation efficiency of higher-order oscillation modes TEM_10q in the OR being driven by an incident TE₁₀ mode that arrives via two rectangular guides, is evalua-ted using the antenna surface utilization factor. The reflection coefficient from the OR and the loaded Q-factor are estimated in the familiar technique of partial reflection coefficients summation.

Findings. As has been shown, in an OR of parameters 2а = 38 mm, R = 78 mm, and L/R = 0.287 TEM₁₀₂₂ oscillations are excited at the frequency of the Gunn diode’s second-order harmonic (i.e., 150 GHz) with an efficiency of 84%. In that same resonator, the excitation efficiency of the TEM₁₀₁₁ mode at the fundamental Gunn diode’s harmonic (frequency of 75 GHz) equals 54%. By placing one-dimensional (E-polarized) wire gratings in the aperture of the coupling elements it proves possible to match the resonator with the waveguide. It has been found that in the case of a l = 0.2 mm spatial period of the wire grating and matched excitation of the resonator at f = 150 GHz (i. e. Г150 = 0), the reflection coefficient Г75 from the OR at f = 75 GHz equals 0.637. Upon excitation in the OR of oscillations in the TEM₁₀₂₂ mode, the total loss at f = 150 GHz is –1.23 dB. With TEM₁₀₁₁ oscillations excited in the same resonator at a frequency of 75 GHz, the total losses increase up to –5.4 dB.

Conclusions. The analysis has shown that an OR implementing the proposed method of excitation of higher-order axially asymmetric oscillation modes can be used for constructing a subterahertz range local oscillator. Moreover, such a resonant system may be considered both as a power combiner and a diplexer (filter).

Keywords: open resonator, aperture coupling element, rectangular waveguide, excitation efficiency, wire grating, oscillation Q-factor

Manuscript submitted 02.12.2021

Radio phys. radio astron. 2022, 27(1): 064-074

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