Resonance systems of millimeter wave oscillators
An analysis of open resonators specially designed for millimeter wave sources which are characterized by high frequency stability is presented. The obtained numerical results and experimental data have been used to improve the characteristics of oscillating systems formed by a two-mirror resonator with metal rod inside. To proceed the preselection of modes in the given frequency region the flat mirror of a conventional Fabri-Perot hemispherical open resonator have been replaced by a specially constructed mirror with a stepwise cross-section. The further tune axial mode filtration and the complete transverse modes suppression have been achieved by the insertion of a metal rectangular rod into the resonant cavity. The determining of parameters of the stepwise mirror and the rod is based on a detail theoretical investigation of the problem of wave scattering from these structures. Experimental results are in good agreement with numerical data. The designed open resonators can be used for generation of electromagnetic waves in the frequency region from 30 to 150 GHz. 1. The analysis of electromagnetic fields in open quasi-optical resonators has been extensively investigated because of its importance for solid-state sources design and for measurement problems in the millimeter wave band (Kotserzhinsky et al (1)). An introduction of GaAs and InP active elements into high Q circuits led to the development of millimeter wave oscillators characterized by rather high output power (about 100 mWt) and by a high frequency stability. The quality of a signal depends on the Q-factor of a circuit used in oscillators.
Open resonators with inhomogeneities
An analysis of electromagnetic properties of an Open Resonator (OR) with perfectly conducting cylindrical inhomogeneities is presented. The inhomogeneities of three types were studied. In the first case the conducting thin circular cylinder with a longitudinal slot is considered. This cavity-backed aperture possess the resonance features. It is shown that such cylindrical screen may be used in order to control the OR properties. The case of the conducting cylinders with rectangular and circular cross-section are considered too. It is shown that the inhomogeneity of small cross-sectional wave dimensions can both be imperceptible for the OR-field and efficiently scatter it, depending on location of the inhomogeneity in the OR. The theoretical analysis and experimental results are presented. The stationary wave model of the OR field is used in the theoretical analysis of scattering from cylindrical inhomogeneities. The generator of the cylindrical obstacles is parallel to the H vector of the incident field. The experiments were carried out in the millimeter wave band with the OR having spherical and flat mirrors. The numerical results are in a good agreement with the experimental data.
The absolute determination of extinction cross-sections by the use of an open resonator
A new experimental technique for determining the extinction cross-section of an obstacle at microwave frequencies is described. This method does not involve the measurement of fields but requires only the determination of the Q -factor of an open resonator, which can be done very accurately. By measuring the Q-factor of the resonator with no obstacle present, and then with the obstacle in two different positions a quarter wavelength apart along the axis of the resonator, the extinction cross-section can be deduced. The methodis, however, restricted to a class of obstacles possessing reflexion symmetry in a plane perpendicular to the direction of propagation of the incident wave. Experimental results with brass spheres for which the absorption is negligible (and therefore the extinction cross-section is nearly equal to the total scattering cross-section), give excellent agreement with Mie’s theory. Results are also given for steel spheres.