A MODELING OF RF PROPAGATION FOR LOW FREQUENCY IN NATURAL CAVES WITH 3D MODEL SIMULATION

Abstract

The study of the RF propagation model within natural caves remains challenging. In the past, the propagation model in the tunnel was compared using waveguide theory because of the electrical conductivity of tunnel walls. However, waveguide models have limitations because a cut-off frequency from the tunnel dimension does not allow frequencies lower than the cut-off frequency to propagate. Although the frequency wave lower than the cut-off frequency cannot theoretically propagate, in practice, it can propagate through the cave. Therefore, this paper presents the modelling of RF propagation at low frequencies in natural caves using empirical methods by measuring the waves propagated within cave passages at the Chiang Dao Cave (limestone) and the Patihan Cave (sandstone). The measurement will focus on analyzing and comparing factors, propagation mechanisms, and behavior, which cover the frequencies at 300, 1000, 1650, 2325, and 3000 kHz, to develop the low-frequency attenuation model within the cave proposed in this paper. The 3D cave models obtained from a LiDAR scanner were used to calculate the physical factors of the cave walls, and then the path losses were calculated using our proposed model and compared to the experimental results. In addition, the path losses calculated using 1) CST simulation software and 2) the waveguide model were also compared with the results from the proposed model. The results obtained from the proposed model provide attenuation values similar to those of the experimental results. Finally, the concept of the low-frequency attenuation model can be used for the prediction and analysis of the propagation performance of low-frequency waves in caves, tunnels, or underground mines and lead to the development of cave communication technology and electromagnetic knowledge in the future.