Characterization of wireless signal propagation in coal-rock media

To enable wireless monitoring and early warning of collapse zone disasters, reliable long-distance electromagnetic (EM) wave transmission is essential. This study investigates how coal’s electrical properties influence optimal EM frequency selection for underground monitoring. Dielectric properties (permittivity, conductivity, loss tangent) of coals with varying ranks were measured at 30–90 °C across 1–3000 MHz. Results show permittivity and loss tangent decrease with frequency, while conductivity peaks near 2 GHz; anthracite exhibits lower permittivity but higher loss than lignite. An experimental platform and propagation model for porous collapse-zone media were developed. Combined physical and numerical analyses reveal that signal attenuation increases with distance and conductivity but decreases with porosity. Attenuation rises with frequency initially but declines above 2 GHz. Among tested bands, 400 MHz shows the lowest attenuation, while 1.6 GHz and 2.4 GHz suffer severe loss (>20–30 dB/m in lignite) and multipath interference. Thus, 400 MHz is recommended for wireless sensor networks in collapse zones.