A STUDY OF RADIO WAVE PROPAGATION IN THE CAVE FOR DEVELOPING THE THROUGH-THE-EARTH APPLICATION

Abstract

Radio waves are significant for operating in an underground tunnel or a cave, such as two-way radio systems or emergency communication during rescue operations. Most applications used in the tunnels require wired connections because of the high attenuation of the wireless propagation. In particular, the attenuation in caves is higher than in tunnels because of the non-uniform nature of the walls. Therefore, this paper presents a study of the radio wave propagation within a cave. The experiment covers the low-frequency (LF) to ultra-high frequency (UHF) bands that aim to analyze the attenuation and behavior of waves in caves. Study results show that the low frequency and medium frequency (MF) bands can penetrate cave walls due to the deeper skin depth. At the same time, higher frequencies perform well in line-of-sight (LOS) propagation over short distances, albeit encountering significant attenuation in non-uniform cave environments. The skin depth condition obtained in the propagation result led to the Through-the-Earth (TTE) experiment by making two hand-made transceivers in the beginning band of MF at 350 kHz for transmitting and receiving waves between the mountain surface and the cave passage directly through the rock layer. The results of the TTE experiment show that the transceivers can transmit to 571 meters with a received power of -85.0 dBm before encountering an obstacle that makes it impossible to continue. Finally, the results are significant for developing the application of radio frequency within caves or tunnels and improving the application of the TTE technique with more versatility and efficiency.