Optimal Deployment of Antenna for Field Coverage in Coal Mine Tunnels

A deployment strategy for antennas is proposed to enhance the field coverage of wireless devices in underground coal mines, which uses the multimodal approach. Based on the attenuation coefficient of each mode, the propagation loss of the vertically polarized modes with n = 1, 2, and 3 and that of the horizontally polarized modes with m = 1, 2, and 3 are less. For the signal propagation in the near-field region of the source, these modes play significant role. According to the coupling of the antenna to the modes, the significant modes can be effectively excited by mounting horizontally polarized antenna on the ceiling or the vertically polarized antenna on the sidewall. For the signal propagation in the far-field region, the (1, 1) mode is significant, and the horizontally polarized excitation is preferred due to smaller attenuation. By comparing the total coupling efficiency of the antenna-radiated power, stronger excitation power can be obtained by mounting the antenna near the center of the ceiling or that of the sidewall. The field strength in the near-field and far-field region coverage can be maximized by optimizing the antenna polarization and position. For verification, a grid-based test system was designed and a series of measurements inside an underground coal mine tunnel were conducted. The measured results have a good agreement with the theoretical ones. In addition, the EM interference caused by the infrastructure in real mines on the signal received by the antennas at different positions was also measured and compared. Results show that the influences of cables with long-term stable loads, such as power cables and communication cables, can be neglected.