An Investigation into the Effects of Multi-Path and NLOS Propagation on Antenna-Based Soil Moisture Sensors in the RFID Band

Antenna-based soil moisture sensors are a growing technology, there is limited research into the impact of the reader antenna's placement and polarization. Studies assume a clear Line-of-Sight (LoS) link. This paper presents the first investigation into the effects of multi-path propagation on antenna-based sensors using an omnidirectional loop antenna designed for the 868 MHz ISM band. The performance of the sensing antenna is evaluated using two polarizations for the reader in both LoS and Non-Line-of-Sight (NLoS). It is shown that the loop antenna achieves a gain sensitivity of around -7dB to the added water content, multi-path effects can significantly vary this when the orientation of the sensor is changed. It is demonstrated that polarization mismatches between the reader antenna and the sensor are not as significant as the multi-path interference. It is concluded that the placement of the reader is more significant than the polarization alignment in a multi-path environment.