The design of wireless sensor devices for data transmission via wireless networks often faces the challenge of Free Space Path Loss (FSPL), which is a major factor in the degradation of communication efficiency, especially in open outdoor areas without obstacles. This directly affects signal quality. This study aims to analyze the impact of Free Space Path Loss on data transmission efficiency using an ESP32 board connected to a temperature sensor via a wireless network. The Received Signal Strength Indicator (RSSI), packet loss, and throughput were evaluated at various distances. The experimental results when placing the device in a vertical orientation using an omni-directional antenna show that temperature and humidity vary over time. In the morning, when the temperature is between 23–25 °C and the humidity is between 80–90%, the successful transmission rate is high and packet loss is low. During the daytime, when the temperature rises to 31–35 °C and the humidity drops to 50–60%, using the onboard (internal) antenna led to an average signal strength reduction of approximately 25.0% between 30–60 meters, 17.5% between 60–90 meters, and 11.9% between 90–120 meters. In comparison, using the external antenna resulted in greater reductions of 29.2%, 16.6%, and 8.6%, respectively. Two-way ANOVA analysis revealed that antenna type and distance had a statistically significant effect on signal strength (Sig. < 0.05), while their interaction was not significant (Sig. = 0.065). Shorter distances consistently yielded stronger signals for both antenna types. These results indicate that selecting the appropriate antenna and managing transmission distance are key factors for optimizing wireless signal performance, particularly in outdoor environments where high temperatures and low humidity increase wave energy loss in the air. This study aligns with the research objectives and provides guidance for designing more efficient and stable wireless sensor devices by evaluating the Received Signal Strength Indicator (RSSI) and calculating the Signal-to-Noise Ratio (SNR) in open outdoor environments.