This paper reports for the first time a long-range interrogation (> 50 meters) of wireless and batteryless humidity sensors combining a Van-Atta retrodirective array and a 3D beam scanning using a 24GHz Frequency-Modulated Continuous-Wave radar. Van-Atta cross-polarization properties, as well as the use of dedicated statistical estimators and Synthetic Aperture Radar technique allow the long-range measurement of the relative humidity at a distance of 58 meters. A measurement sensitivity of 0.2dB to 0.4dB per %RH was measured as a linear variation of the proposed estimator with a standard error of ±0.005dB.