Resarch on the FDTD analysis method of wireless transmission characteristics in underground mine

In order to reduce the number of underground wireless communication base stations and increase the coverage and communication quality of wireless signals in underground mines, it is necessary to study the influence of the installation position of wireless communication base station antennas and fixed wireless communication equipment antennas on the propagation characteristics of the mine electromagnetic waves. A three-dimensional finite-difference time-domain tunnel model composed of air medium, tunnel walls and perfectly matched layers is established, and the iterative equation of finite-difference time-domain (FDTD) is derived based on the model. A variable internal resistance excitation source is proposed to decrease the calculation error caused by the induced current between Yee cells, and the explicit FDTD iterative equation of the excitation source is derived. The electromagnetic wave signal strength radiated by the transmitting antenna at different positions of the tunnel section was measured. The results show that the simulation value of electromagnetic wave signal strength calculated by the proposed variable internal resistance excitation source is consistent with the attenuation law of the analytical value calculated by the analytical equation of the Hertz dipole antenna in 300 m free space, and the performance of the excitation source is better than that of the transparent excitation source and the hard excitation source. When the excitation source and receiving point are located in the center of the tunnel section, the simulated signal strengths of 580, 740 and 900 MHz electromagnetic wave are consistent with the measured values, indicating that the proposed calculation method can be used to study the propagation characteristics of electromagnetic waves in the tunnel. When the transceiver antenna is equidistant from the same tunnel wall and the two antennas move synchronously from the center of the tunnel section to the vicinity of the tunnel wall, the attenuation intensity of the electromagnetic wave signal gradually increases. The average attenuation gradient of electromagnetic wave signal is 217 dB/m within the range of 0-0.02 m from the transceiver antenna to the tunnel wall, which is 6.2 times of 0.04-0.30 m and 1.6 times of 0.02-0.04 m. Accordingly, under the condition of not affecting pedestrians and traffic, the antenna of wireless communication base station should be as far away from the tunnel side as possible to obtain the best communication effect and communication distance.