Difei Liang

Resistance Selection of High Impedance Surface Absorbers for Perfect and Broadband Absorption

High impedance surface absorbers comprising of lossless dielectric substrate and resistive pattern were studied based on equivalent circuit method. Resonant conditions in terms of potential resonant frequencies and corresponding equivalent resistance were deduced. Theoretical calculation, numerical simulation and experiment showed that expected absorption was obtained by matching the capacitance, inductance and the resistance. We demonstrated that the optimal resistance of frequency-dispersive was between 0 and 377 Ω. For square, dipole and square ring pattern, a mean resistance value was required to achieve broadband absorption. It offered a way to create broadband and perfect absorbers by controlling the surface resistance of sub-wavelength structures.

A Study on the Effective Permittivity of Carbon/PI Honeycomb Composites for Radar Absorbing Design

Radar absorbing honeycomb composites with different coating thicknesses are prepared by impregnation of aramid paper frame with solutions containing conductive carbon blacks (non-magnetic) and PI (polyimide). Expressions for the effective permittivity of the composites are studied and validated both in theory and experiment. It is found that a theoretical equivalent panel with given permittivity can be obtained to represent the honeycomb structure in the quasistatic approximation, which provides a feasible way to optimize the design of radar absorbing honeycomb structure by connecting the effective electromagnetic parameters with the unit cell dimensions. The effective permittivity is measured by a network analyzer system in the frequency range of 8-12 GHz and compared with the theoretical result.

Electron Structure and Microwave Absorbing Ability of Flaky FeSiAl Powders

Soft magnetic metallic materials have been widely used for absorbing electromagnetic wave. Flaky Fe 86- t Si t Al 14 (t=9, 11, 14 and 16) alloys powders were prepared from melt-quenched ribbons by annealing and milling. In a previous report we discussed the order-disordered structure of this alloys. In this article, we studied their electron structure. Covalence electron numbers of (111) and (100) plane increase with increasing Si content but Bohr magneton decreases. Complex permittivity and complex permeability are both decreased with increasing Si content t . The lowest value of reflectivity among the four alloys is originated from Fe 70 Si 14 Al 14 . The peak values of reflectivity are all lower than −10 dB, and the absorbing frequency range ( R t reaches 16.

Prediction of Microwave Absorption Behavior of Grading Honeycomb Composites Based on Effective Permittivity Formulas

Microwave absorbing honeycomb composites with grading coating thickness are prepared by impregnation of aramid paper frame within water solutions of conductive carbon blacks. Based on effective medium method, grading impedance in the composites can be theoretically expressed by a linear approximation of the inversely tapered effective permittivity at wave propagation direction, which provides a feasible way to optimize the design of microwave absorbing honeycomb structures. The consistent result of calculations and experimental measurements for positive impedance grading design confirms the broadband absorption effect in high frequencies.

Low frequency (100 kHz–1 MHz) permeability measurement method in magnetic material

A low frequency (100 KHz~1 MHz, mainly use in magnetic resonance wireless power transfer system ) permeability measurement method is designed and tested in this paper. In this method, a coaxial cavity and a low frequency band (30 KHz~8 GHz) Vector Network Analyzer (VNA) are used as measurement equipment. Comparing the impedance change of coaxial cavity, with magnetic material in it or with air, we can calculate the permeability of the magnetic material. This is a mature method of low frequency permeability measurement. The original measurement method is well developed while has a critical disadvantage in previous works. It is because that the equipment will bring in a strong nonlinearity error and cause the measurement data inaccuracy when the measurement frequency is as lower as VNA work range. Our sample work is attempting to modify the nonlinearity error and attain more accuracy test data. Compared with professional Impedance Analyzer, this measurement method takes lower costs and obtains more accurate permeability parameter by common VNA equipment and corrected data respectively.

Magnetic Substrate Folded Dipole Antenna for UHF RFID Metal Tag

In this paper, magnetic material was applied in the design of a UHF-RFID metal tag to increase the reading distance. The in∞uence of the electromagnetic properties, especially the electromagnetic loss, of a magnetic substrate on the gain of the tag antenna is discussed by analyzing the re∞ection and the surface current distribution. As to folded antenna, the loss of energy caused by the magnetic substrate tends to occur in the folding edge of the antenna. Both simulations and experiments indicate that electromagnetic loss markedly reduces the gain rapidly when both the dielectric loss tangent (tan-) and the magnetic loss tangent (tan-m) are between 0 and 0.3. The reading distance drops from 3m to 1.5m when the tan-m of the magnetic composite substrate increases from 0.009 to 0.023. And tan-m should be less than 0.1 for the antenna working excellently. The conclusion ofiers meaningful guidance for future studies of magnetic substrate folded metal tag.