Medium with high resistivity can enhance the absorption and reduce the reflection, but meanwhile, it also increases the skin depth, resulting in an increase in the requirement for thickness. However, for simplicity fabrication of high-frequency microwave absorbers, only a few ten of microns lossy material is needed and the higher the operating frequency is, the thinner the required lossy material is. Thus, based on these two conditions, a minimum resistivity of material or the thickness of lossy layer can be found to absorb a specified frequency band. The simulation and experiment results both demonstrate that such wave absorbers with a lossy conducting layer can accomplish the absorption for Ka-band microwaves and also imply this method is certainly applicable at higher frequency bands.
The simulation results also indicate that the shape is the most predominant factor in absorption performance. The geometrical advantage of the pyramidal absorber is its tip, a dot basically, which provides a good impedance match at first contact position and produces the minimum reflection of wave. Moreover, in terms of wedge-shaped absorbers, field polarization is also an important key to the issue. The power absorbed is proportional to the square of tangential magnetic field. Except for exactly 0o and 90o, the parallel polarization possesses the better performance than perpendicular polarization at any incident angles. The simulation results corroborate this argument and are consistent with the theoretical calculation of reflection coefficient in chapter 3.
This information has great help for the fabrication of wave absorbers.
Based on this study, it has been well known what kind of materials and shapes are applicable to the high-frequency microwave absorber. But, we have only simulated wave absorbers in the waveguide; in fact, this is barely a fraction of an anechoic chamber. To understand the absorption performance of an anechoic chamber with many wave absorbers lined on the walls, it is necessary to measure under practical condition. The manner presented in this thesis makes the manufacturing of anechoic chambers easier to implement, serving as a convenient tool for high-frequency research and applications.
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