A Microwave System for Measuring Relative Humidity of Hollow Concrete Blocks: A New Proposal and a Sensitivity Analysis

Abstract In this article, the authors propose changes to a microwave system for measuring the relative humidity of hollow concrete blocks presented in the literature, aiming to improve the system’s sensitivity. The operating frequency range was changed from 2.5 GHz to 5.8 GHz. This reduced the dimensions of the antenna arrays used in the system. Five block samples were used in this work. The frequency selective surface (FSS) used as a sensor has been modified, from a double circular ring to a square loop, which presents a single-band frequency response. The results obtained in this work showed a high correlation between the frequency variation and the relative humidity level. First, second, and third-order functions were compared to see which would better relate resonance frequency to relative humidity. The second-order function presented the best results, considering the Mean Absolute Percentage Error (MAPE), the Mean Square Error (RMS), and the coefficient of determination. We performed a sensor sensitivity analysis showing that the new proposed system is more sensitive than others presented in the literature.