engleski

Artififical Material with Negative Effective Permeability based on Capacitively Loaded Loops

Recently there has been a great progress in development of the artificial composite electromagnetic materials (meta-materials). These materials comprise small metallic scatterers (implants) periodically embedded into a dielectric host [1], at the mutual distances much shorter than a wavelength. The implants may alter both the dielectric and magnetic properties of the original (host) material yielding new values of the macroscopic effective permittivity and permeability. Very recently, the negative value of effective permeability was achieved using an array of the metallic implants called &#8216 ; ; split-ring resonators&#8217 ; ; [2, 3]. These resonators locally decrease intensity of the magnetic field of the incoming plane wave yielding negative permeability within a finite frequency band. Following this basic idea, a simpler structure which comprises a periodic array of the capacitively loaded loop antennas is proposed in this paper. If a plane wave with magnetic field normal to the loops impinges the structure, the local magnetic field intensity across the loops will be different from the intensity of the incident magnetic field. Analysis of the simple equivalent circuit of the loaded loop shows that intensity may be either increased or decreased, depending on the phase of the loop current. Thus, the distribution of the local magnetic field across the structure is inhomogeneous. Macroscopic averaging of the local magnetic field intensities yields effective permeability of the structure. Below the resonance of the loaded loop, the effective permeability may exhibits positive values higher than unity, thus behaviour analogue to ferromagnetism. The value of the effective permeability diverges at the resonance, may have negative values at the frequencies slightly above the resonant frequency and approach unity at higher frequency. The experimental structure was built in 10 GHz frequency band and comprised 7 layers in the direction of the wave propagation. Each layer is consisted of the 9 x 9 metallic ring array laid on the foam. The structure was placed between two horn antennas and amplitude and phase of the transmitted signal measured by network analyzer. The measurements results show stop-band centred at 8.5 GHz with attenuation between 30 and 50 dB. The attenuation was limited by diffraction on the structure due to its small dimensions (cross section of approximately 3 x 3 wavelengths). In order to avoid this problem, a separate small structure was fabricated and placed into a rectangular waveguide. The waveguide structure showed very pronounced stop-band with negative effective permeability (attenuation> 80 dB).

negative permeabilty; loop; meta-material

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