Default value for maximum measurement distance is 4m, since HC-SR04 sensor can't measure reliably beyond that. However, if you are using another sensor or if you you don't care about distances larger ...

Abstract: This paper presents a compact sensor system for estimating the dielectric properties of materials based on commercial, off-the-shelf (COTS) modules. The dielectric constant and conductivity ...

Abstract: Two compact single-chip 94-GHz frequency-modulated continuous-wave (FMCW) radar modules have been developed for high-resolution sensing under adverse conditions and environments. The first ...