| Research Abstract |
The electromagnetic properties of finite-length metal-rod array sheets are discussed as metamaterial, or artificially designed material, and as their chief uses, frequency selective shielding screen and radome materials are investigated.
The metal rods are periodically aligned on a thin dielectric sheets. The resonance of the rods occurs at specific frequency whose wave-length is approximately equal to double the rod length. The metal-rod array sheet permittivity,ε_r=ε_r'-jε_r", which is a average permittivity when the sheet is supposed to be homogeneous, shows frequency dispersion by the metal rod resonance. At frequencies lower than the resonant frequency, f_0,ε_r' becomes high due to the polarizations of the metal rods. The permittivity decreases rapidly at f_0, and at frequencies higher than the resonant frequency, ε_r' becomes negative. The imaginary part, ε_r", becomes maximum at f_0. As magnetic materials, hybrid materials of fine particles of magnetic metals are considered, and
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experimentally their permittivity was confirmed to be almost zero.
The transmission coefficient of the metal-rod aligned sheet becomes minimum at f_0. The frequency selective EM wave screens are made with two or more metal-rod aligned sheets with spacing materials between them. Their shielding properties are experimentally confirmed.
As one of the another applications of this material, absorber is discussed. As absorbing material, one of the key properties of the metal aligned sheet is the rapid decrease of ε_r' around f_0, which leads to the opposite frequency change in reactance X of the sheet input impedance, that is, dX/dw< 0. In conventional material, the frequency property of X is always dX/dw> 0, which is known as Foster's reactance theorem. Since the frequency changes of the metal rod aligned sheet and conventional dielectric sheet are opposite each other, it is possible to minimize the frequency change of input impedance, when these sheets are laminated. Based on this idea, the wide band absorber is achieved.
The frequency selective shielding screens and the wide band absorbers, both made of the rod aligned sheet, are now in process of coming into use.. Less
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