Đang chuẩn bị nút TẢI XUỐNG, xin hãy chờ
Tải xuống
Bảng 4.4 liệt kê các giá trị SNR cho khoảng cách từ 0 đến 45 mét với góc định hướng máy tính xách tay 0, 90, 180, và 270. Các giá trị SNR thu được thông qua chương trình khách hàng mạng WLAN IBM Configuration Utility kiểm tra được. Khoảng cách được đo từ AP đến máy tính xách tay. | 104 RFID Tag Antennas -20 -5 -10 -15 850 900 950 1000 -30 4 800 in w E w -25 Antenna Free space d 1 mm d 5 mm d 10 mm d 15 mm d Metal plate Frequency MHz Figure 3.32 Transmission coefficient of the antenna as a function of distance from metal plate calculated by IE3D . Table 3.6 Effect of metal on the tag at 915 MHz. Directivity dBi Radiation efficiency Gain dBi Input impedance H Transmission coefficient dB Reading range m R power-link Free space 1.81 81.84 0.94 33.65 j427.00 -1.32 5.00 d 1 mm 7.79 0.67 -13.97 3.20 J336.00 -17.17 0.15 d 5 mm 8.08 6.47 -3.80 3.45 j372.50 -12.27 0.82 d 10mm 8.11 17.48 -0.53 4.49 j404.30 -4.40 2.96 d 15 mm 8.10 30.01 2.88 7.08 j423.80 -0.86 6.60 a The system parameters were described in Section 3.3.1.6. the reading distance of the tag may be enhanced because the metal object functions as a reflector. 3.4.2.2 Effects of Water on Tag Antenna Figures 3.33-3.36 show the characteristics of an RFID tag antenna which is placed close to a water cuboid. As in the case of the metal plate the antenna used is a folded dipole antenna and is positioned parallel to and above a water cuboid measuring 250 mm X 80 mm X 80 mm sr 77.3 and tan Ỗ 0.048. The directivity radiation efficiency gain and input impedance are investigated for the different distances away from the water cuboid. When the antenna is placed close to water d 1 mm the directivity of the antenna increases while the radiation efficiency decreases significantly which results in a reduction in the antenna 3.4 Effect of Environment on RFID Tag Antennas 105 Figure 3.33 Gain of the tag antenna as a function of distance from water calculated by IE3D . Frequency MHz Figure 3.34 Real part of the input impedance of the antenna as a function of distance from water calculated by IE3D . 1200 1000 800 600 400 200 Antenna 800 850 900 950 1000 Frequency MHz Figure 3.35 Imaginary part of the input impedance of the antenna as a function of distance from water calculated by IE3D . Free space d 1 mm d 5 mm
