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The microstrip-lines microwave interferometer and investigation of the dispersion characteristics of polycrystalline ferrite plates


I.N. Antonov, A.G. Lavkin

Polycrystalline structures of yttrium iron garnet (YIG) and ferrite ceramics are widely used in microwave-components technology. However, the available data on the frequency dispersion of these materials in a centimeter-wave band in different magnetizing fields are in-sufficient for developing adequate theoretical models of these structures and for their applications. This work deals with the development and manufacturing of an appropriate probe installation on the basis of a fringe pattern analysis under superposition of the electromagnetic microwaves propagating in the studied samples. The studied sample (a ferrite plate) is mounted on the probe consisting of a radiating and a receiving antenna. The antennas are short-circuited microstrip lines of 5 mm in length based on the Polycore substrate. The receiving antenna can travel within 0.4-3.0 mm along the sample relative to the radiating antenna by means of a motor. The probe with the sample is placed in the magnetizing field produced by an electromagnet in the normal direction to the sample. The circuit of the voltage standing-wave ratio analyzer is based on the principle of separate extraction and detection of signals of incident and sample-reflected electromagnetic waves. A directional coupler extracts a signal proportional to the power incident on the radiating antenna. The signal from the radiating antenna is picked up by the receiving antenna (due to a strong coupling between the antennas). Then it excites a forward traveling magnetostatic wave in the sample, which is also picked up by the receiving antenna and extracted by the directional coupler of the reflected wave. The microwave signal, arriving at the studied sample, is modulated with the 100-kHz frequency. Thus, at the outputs of the detectors of the signals, which are proportional to the powers of the reflected and incident waves, there are 100-kHz voltages. These voltages are used in the VSWR-indicator to determine the relation between them. The VSWR-indicator amplifies the voltages of the incident and reflected waves (at the microwave-signal modulation frequency) and performs their division, detection, and visual indication. As a result, it is possible to observe and fix the spatial distribution pattern of elec-tromagnetic waves in the polycrystalline ferrite sample at a given magnetizing field and a given frequency of the sweep generator. Further, the microwave-signal frequency is altered (at a given magnetizing field) and a new pattern is fixed, etc. The investigations were performed at frequencies of 2-7 GHz and magnetizing field strengths of 2-3.2-kGs. The installation was aligned with a computer with the help of an interface unit. The electromagnetic waves distribution data allow one to determine the electromagnetic wavelength, detect the value of a wave vector K of the electromagnetic wave in the sample using the fringe pattern, and restore the dispersion characteristic ω(К) of the sample. Experimental and theoretical dependencies are in a good agreement to each other.

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