Electromagnetic Analysis of Radio Propagation in Fresh Water and Measurement by Axial Mode Helical Antenna at 433 MHz

In the next generation of 6G mobile system, communication network will be extending to the underwater area. Underwater comprises of seawater and freshwater areas. Recently, the need of communication tools in freshwater are rising particularly in remote sensing, monitoring, aquaculture and surveillance operations in lakes and rivers. Previous studies were limited to some experiment data in propagation attenuation and low-gain antenna configurations. Previously, because antenna performances in the water condition was not clearly analysed, radio link design could not well discuss. In this paper, antenna design and electrical performance in underwater use is clarified. Then, analysis of electric field distributions underwater and radio link design equation are clarified using electromagnetic simulations. A frequency of 433 MHz is selected from the ISM band. For a high gain antenna, an axial mode helical antenna is selected because of structural simplicity and adaptability of gain change by changing number of turns. The antenna is placed in a capsule to prevent direct contact with surrounding water. In order to achieve effective antenna gain, it is shown to fill the capsule with distilled water of zero conductivity. In the analysis of radio propagation, the increment of power density degradation at distance is compared between simulation and theoretical results. From the good agreement of simulation and theoretical results, effectiveness of antenna gain in the water condition is ensured. To evaluate the link design equation, simulation results for both the transmitting and receiving conditions of the antenna were obtained. It is clarified that the Friis transmission formula is useful for the link design equation. Finally, propagation measurement results at a swimming pool of 1.2-meter depth are compared with the simulation results. It is noted that water surface reflections disturb propagation attenuation.