Wireless communication within caves or tunnels has been researched for a long time. The major problem in cave communication is the limitation of wave propagation within the conductive cave wall, which makes it like the waves traveling in the waveguide. In addition, cave environments with curved and non-uniform paths that cause high attenuation differ from propagation in free space. However, the nature of the low frequencies, with their diffraction abilities and skin effects, allows them to communicate within caves. Therefore, this paper presents a study of low-frequency wave propagation in the cave by investigating the mechanism of low-frequency propagation within the cave. The method used was an experimental study by transmitting low-frequency waves through the cave cavity, which revealed the attenuation that occurred as the wave propagated in the cave. In conclusion, the significant factor is the skin depth generated by the electrical conductivity of the cave wall, which made it possible to explain the low-frequency propagation behavior in the cave and will use to develop a low-frequency propagation model.

Effective communication in caves is crucial for rescuers during accidents or disasters. Previous studies have investigated wave propagation in tunnels and limestone caves. This paper presents an experimental study of wave propagation in a sandstone cave at Wat Tham Patihan Cave in Thailand, covering the MF to UHF bands in both line-of-sight (LOS) and non-line-of-sight (NLOS) scenarios. The study aims to investigate the behavior of waves while traveling in the sandstone cave. The study found that in LOS, the path loss decreases with higher frequencies, contrary to the propagation theory in free space. Since waves traveling in a cave behave like they are in a dielectric-walled waveguide. In NLOS, the path loss in the L-Bend range at low frequencies was lower than that at high frequencies due to the ability to reflect, diffract, and encounter obstructions. In contrast, at 350 kHz, a high attenuation is caused by the skin effect when the wave travels through the arc, penetrating the cave wall. The study results will aid in developing communication systems and antennas for use in caves.

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.

This study investigates Through-The-Earth (TTE) communication using electrodes operating at a frequency of 350 kHz in the Medium Frequency (MF) band. These electrodes, situated at the end of the wire of an earth-grounded antenna, facilitate signal transmission between the Earth's surface and subterranean regions by inducing electric currents. The simulation study evaluates the impact of electrode types—specifically Y-shape, X-shape, and V-shape, commonly encountered—on parameters such as transmission coefficient (S21), surface current, and impedance. CST software is employed to analyze these effects and identify the most suitable electrode shape for use with the earth-grounded antenna. Analysis of simulation results shows that the X-shape electrode exhibits superior performance, displaying the highest S21 and surface current while exhibiting the lowest real impedance compared to other shapes. Because the X-shape electrode has a higher contact area than all other electrodes, it results in a low contact impedance value. As a result, the X-shape electrode appears as the optimal choice for integration with the earth-grounded antenna, particularly in Through-The-Earth communication applications.

This paper presents a systematic numerical investigation of a hybrid plasmonic–photonic Panda-ring antenna with an embedded gold grating, designed to enable efficient dual-mode radiation for optical and terahertz communication systems. The proposed structure integrates high-Q whispering-gallery mode (WGM) confinement in a multi-ring dielectric resonator with plasmonic out-coupling at the metal–dielectric interface, allowing controlled conversion of resonantly stored photonic energy into free-space radiation. The electromagnetic behavior is analyzed through a hierarchical structural evolution, progressing from a linear silicon waveguide to single-ring, add–drop, and Panda-ring resonator configurations. Gold is modeled using a dispersive Drude formulation with complex permittivity to accurately capture frequency-dependent plasmonic response at 1.55 µm. Power redistribution within the resonator system is described using coupled-mode theory, with coupling and loss parameters evaluated consistently from full-wave numerical simulations. Full-wave simulations using OptiFDTD and CST Studio Suite demonstrate that purely photonic resonators exhibit strong WGM confinement but negligible radiation, while plasmonic gratings alone suffer from low efficiency due to the absence of coherent photonic excitation. In contrast, the proposed hybrid Panda-ring antenna achieves stable and directive far-field radiation under WGM excitation, with a realized gain of approximately 8.05 dBi at 193.5 THz. The performance enhancement originates from synergistic hybrid SPP–WGM coupling, establishing a WGM-driven radiation mechanism suitable for Li-Fi and terahertz wireless applications.