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of modes in the given frequency region the flat mirror of a conventional Fabri-Perot hemispherical open resonator have been replaced by a specially constructed mirror with a stepwise cross-section.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.
In search of optimum chest radiography techniques.
A nation-wide study of chest radiography in Sweden had previously revealed a large variation in the physical and technical factors involved, in particular the radiation dose. In this study the image quality could not be assessed unambiguously. The aim of the present study was to try to establish a correlation between visual grading of radiographs and physical and technical factors in order to find the optimum chest X-ray system. The physical and technical performance of 24 chest units was evaluated. Radiographs were taken of an anthropomorphic chest phantom supplied with test structures simulating various pathologies. Image quality was assessed by visual grading analysis of the radiographs. The physical and technical parameters of the units rated best were used to exemplify good radiographic practice. The results were in agreement with the recommendations issued by the Commission of the European Communities (CEC). It was also shown that low radiation dose is compatible with high-quality radiographic imaging of the chest.