Design and analysis of axial mode helical antenna for underwater communication systems at 433 mhz in freshwater environments

The underwater environment includes both seawater and freshwater areas, where reliable communication systems are essential for practical applications such as remote sensing, environmental monitoring, aquaculture, and surveillance operations in lakes and rivers. These applications demand dependable short-range communication systems, with antenna performance playing a critical role. However, previous studies have been limited to basic experimental data on propagation attenuation and low-gain antenna configurations. Practical implementations involving high-gain antennas and validated radio link design equations for underwater use, particularly in freshwater, remain unclear. Motivated by the need to enhance short-range underwater communication, this research investigates the development of high-gain antennas for freshwater applications. The study systematically examines the effects of signal reflection and refraction due to varying water conditions to understand propagation behaviour and signal attenuation. A radio link design equation, specifically developed and validated for freshwater environments, is proposed to serve as a reliable model for analysing and improving underwater signal transmission. In this research, freshwater is characterized by a conductivity of 0.06 S/m and a relative permittivity of 76, and these values were used during the antenna design calculations as they significantly influence electromagnetic wave propagation and antenna performance. The research focuses on improving communication in freshwater environments over distances ranging from 0.5 to 3 meters. A frequency of 433 MHz from the ISM band is selected. For a high gain antenna, an axial mode helical antenna of seven turns is used. It was chosen for its ability to provide directional radiation and circular polarization in underwater conditions. The use of directional antennas provides higher gain and circular polarization, improving link stability even when slight misalignments occur. The antenna is enclosed in a polyethylene terephthalate (PET) capsule to prevent direct contact with water and ensure mechanical stability. To minimize signal loss and detuning effects, the capsule is filled with distilled water, which has near-zero conductivity and provides a suitable low-loss medium. This design approach enables effective antenna gain, and results show a gain of 9 dBi with an end-fire radiation pattern, indicating strong performance for short-range underwater communication. The study compares the degradation in power density over distance between electromagnetic (EM) simulations and theoretical predictions. The radio wave propagation method, incorporating the Friis equation, is used to analyse signal attenuation and validate the link design equation under both deep and shallow freshwater conditions. Experimental measurements conducted in a 1.2-meter-deep swimming pool support the simulation findings, with water surface reflections identified as a contributing factor to signal variation. In conclusion, this study advances underwater communication by presenting a high-gain antenna design and a validated link design model adapted for freshwater environments, offering improved system performance, accuracy, and reliability.