無線LAN対応繊維強化セメント板3層型電波吸収体に関する検討

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Recently, wireless LAN is widely used around/inside building, and some problems have been appeared, such as multi path fading or security. Some EM-wave absorbers are proposed to solve above problems. In this situation, three layer type absorber by using common building materials is presented. In this paper, first three steps of design procedure are introduced. Secondary, the frequency of the temperature and relative humidity for the experimental room are investigated in order to estimate the condition of setting environment, and the variation of the water contents for the fiber reinforced cement boards are estimated. Lastly, comparison of measured absorption and calculated one for experimental absorber to confirm the validity of the design procedure and effectiveness of the absorber. Recently, wireless LAN is widely used around/inside building, and some problems have been appeared, such as multi-path fading or security. Some wave absorbers are proposed to solve these problems as a one of the counter plan. In this situation, we presented the three layers type absorber by only common building materials (the Absorber) to propose the better wireless communication environment, while other absorber is expensive and difficult for handling, comparing common building materials. The target of the absorption is 12dB for two major wireless LAN frequency ranges up to 30 degree of TE and TM polarization wave including normal incident. The Absorber has advantage of cost and handling, and absorber with two layers is widely known to have two absorption peaks. Most of building materials, however, have very low absorption, especially for oblique incident, even with two layers structure. So, we try to insert air layer into two building materials to increase the performance. Fig.1 shows the basic structure of the Absorber. Fig.2 shows the design procedure of the Absorber. The absorption of the Absorber could be calculated by gradually electric circuit equivalent equations with 9 unknown factors (3 thicknesses, 3 real parts and 3 imaginary parts for complex permittivities). By linear relationship between complex permittivity (real and imaginary) and water content in certain range (typically 5-10%) shown in Fig.3, 9 factors can be reduced to 5 unknown factors (3 thicknesses and 2 water contents). Table 1 shows the combination ranges for each layer and water content for type A, which can be over 12dB absorption, as results of the calculation for the Absorber by fiber reinforced cement board. As the first step, the course calculations are conducted in the range of 0 to 30mm, 0.1mm step for each layer and 5% to 10%, 0.1% step for water content. Selected number in the table means the number of combinations of three thicknesses and two water contents satisfied the target absorption (12dB for up to 30 degree TE and TM oblique incident). The results of course survey indicated only two combination ranges for type A could be over the 12dB among wide survey ranges for two types of fiber reinforced cement board, and case 1 have narrow ranges for d_1 and d_2, comparing that for d_3, and also have narrow ranges for wc_3 comparing that for wc_1. Next, fine calculations are conducted to fix the thickness of each layer for the design of the Absorber. Table 2 and Table 3 show the fixing procedure for the thickness of the layer in the order of narrow range. The results of the fine calculation indicate fixed combination for case 1 has wide range for wc_1 comparing that for case 2. This property is suitable for the Absorber, because layer 1 is more affected to the variation of temperature and humidity in the setting environment than layer 3. Fig. 4 and Fig.6 show the area of water content of each layer over 12dB and 10dB, respectively, and Fig.5 shows frequency characteristics of the Absorber. The frequency of the temperature and relative humidity for the experimental room are also investigated in order to estimate the condition of setting environment at Fig.7, and the variation of the water contents for fiber reinforced cement board are estimated in Fig.8. Based on the above investigation, 3mm thick fiber reinforced cement board and three 8mm thick fiber reinforced cement boards + 5mm thick fiber reinforced board are selected for layer 1 (3mm) and layer 3 (29mm). These boards set in humidifier to control the water content to fit one in the Table 1 (5.8% to 8.5% for layer 1 and 9.3% to 10% for layer 3). Fig.9 shows the comparison of measured absorption to calculation, which is conducted by measured thicknesses and water contents by each layer for normal incident Calculated characteristics have very good agreement of the measurement for all frequency range and are confirmed over the target absorption including for oblique incident. Dot line shows another absorption characteristic in changed wc_1 by humidifier, to confirm the Absorber is capable to keep the certain absorption for varying water content in the range of 1%. The dot line (wc_1=6.9% and wc_3=9.4%) is close to the bold line (wc_1=5.9% and wc_3=9.5%). The absorptions of these two lines are confirmed above 15dB in 2 wireless LAN frequency ranges (2.4Ghz-2.5GHz and 5.15GHz-5.25GHz). The calculation of the absorber with the experimental equation of water content and complex permittivity is conducted few combinations (layers and water content) satisfying the target absorption 12dB by wide range of survey. The absorption for the selected combination is confirmed over the 15dB, and the comparison of the measurement of experimented three layers type absorber and calculation with measured thickness and water content is conducted the validity of the design procedure presented this Letter, and effectiveness of the absorber even water content movement.
2006-01-30

無線LAN対応繊維強化セメント板3層型電波吸収体に関する検討

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