AN OPEN RESONATOR FOR PHYSICAL STUDIES
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AN OPEN RESONATOR FOR PHYSICAL STUDIES I.K. Kuzmichev, P.N. Melezhik, and A.Ye. Poyedinchuk The A.Ya. Usikov’s Institute for Radio Physics and Electronics of the NAS of Ukraine 12, Ak. Proskury st., Kharkov, 61085, Ukraine e–mail: [email protected] Received 26 August 2005 Abstract The excitation efficiency of the ɌȿɆ
01q
oscillation of an two-mirror
hemispherical open resonator (OR) is studied. The resonator is excited by the TE 01 wave of a circular waveguide joined in the middle of the OR plane mirror. Given the waveguide optimum size, the ɌȿɆ 01q mode excitation efficiency reaches 78%. Analysis of the resonant system spectrum in the 4-mm wave region shows that this waveguide - OR system offers a single mode resonance curve across almost a 10-GHz tuning range. The ɌȿɆ0110 mode field distribution with and without the circular waveguide in the middle of the OR plane mirror is available due to the small scatterer method. It is shown that the considered open system is suitable for measuring electromagnetic characteristics of high-loss substances and metamaterials in the short-wave end of the millimeter (mm) region as well as in the submillimeter (submm) wave region. Keywords: millimeter waves, open resonator, circular waveguide. 1. Introduction An open resonator (OR) is a highly sensitive instrument for the mm-wave measurement of electromagnetic characteristics of materials >1,2@. In our method, a plane sample is put inside a resonator excited at the lowest ɌȿɆ 00 q mode, where q is the longitudinal index. The resonator hemispherical geometry allows one to avoid the errors originating from the angular measurements of the sample, which is placed on the OR plane mirror [3, 4]. Besides, a care is taken to assure the sample location in the maximum of the electrical component of the OR standing wave, to provide the highest in accuracy of measurement. However, the suitability of the OR method for measuring electromagnetic characteristics of materials is specified by the 857
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requirement of a small power dissipation in the specimen because only then does a specimen-carrying OR remain a high-Q resonant system providing benefits of the method. It means that strongly absorbing substances, among which are water and water-based solutions, can be analyzed only it the sample thickness is smaller then the skin depth. In practice, however, we often deal with cylindrically shaped object, and a technical problem arises how one can put it in the domain with the uniform electric field. A high-loss sample can spoil an OR oscillation since the specimen diameter is usually greater than the skin depth. In the centimeter-wave region, cylindrically shaped samples of high-loss materials can be easily measured using a cylindrical resonator with the TE 01n mode, where n is the number of half-wavelengths along the resonator axis [5]. In the shorter wavelength mm-wave region, the OR geometric size gets too small while the surface resistance of the mirror metal grows high. The oversized resonator is unsuitable either
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