## 1 Introduction  
Microwave characterization techniques for the measurement of dielectric and conductive properties of materials have been widely described in the literature [1, 2]. In particular, free-space methods have the advantages of being nondestructive and contactless [3, 4]. The general set-up consists of a pair of directive antennas associated to a vector network analyzer (VNA) to measure the free-space \(S\)-parameters of planar materials positioned in the trajectory of the radiated beam assuming far-field conditions [5, 6]. From the calibrated microwave signals in the material reference planes, the complex permittivity can be determined after modelling of reflection at the air--material interface and the propagation in the material. Consequently, the accuracy in the extraction of the complex permittivity depends on the hardware resources but also on the mathematical modeling adopted to express the measured \(S\)-parameters as a function of the geometry and the physical properties of the material under investigation. Most of the works reported in the literature are related to the development of free-space techniques assuming a transverse electromagnetic mode (TEM) model of the waves propagating in the material. Many industrial processes require an instrumentation suitable for contactless methods and real-time on-line monitoring and control. Although microwave free-space techniques present features to address non-ionizing and low-power microwave nondestructive testing and evaluation (MNDT&E) applications in an industrial context, its transposition in an industrial context still remains a challenge [7; 8]. The use of focusing horn-lens antennas are not suitable as the material under test is usually one-sided. Consequently, a monostatic (reflection mode) free-space technique is the most viable solution to be considered. At microwave range, i.e., 300 MHz-30 GHz, the moderate directivity of traditional horn antennas is not in favor for operating in the far-field. Indeed, free-space propagation losses and spatial resolution are function of the stand-off (distance between the antenna aperture and the material plane). As conductive thin-film composite materials are mainly reflective objects with negligible propagation in the bulk materials, by properly calibrating the free-space set-up, the stand-off can be drastically reduced to benefit from both spatial resolution and high signal to noise (SNR). In Section 2, the production line of Adamant Composites FX-ply(tm) prepregs is presented together with the implementation of the microwave free-space nondestructive module. At our test frequency set to 10 GHz, the far-field region is around 4 m. Consequently, running the system in conventional TEM mode is not possible as the distance separation between antenna aperture and material is too big. The electromagnetic beam will not be entirely intercepted by the material and the signal to noise ratio (SNR) will be too low. Consequently, the antenna operates in the near-field region where the propagation of the waves is difficult to model. Consequently, electromagnetic modeling and dedicated free-space calibration procedure are derived for the extraction of the complex permittivity of the material under investigation from the measured microwave signals. In particular, we demonstrate that the conventional calibration procedure can be applied in the near-field region to recover the complex reflection in the reference plane of the material. In Section 3, test materials are prepared to qualify the radiofrequency (RF) instrumentation proposed. Experimental results related to the measurement of the conductivity at 10 GHz of the test materials after calibration are demonstrated. The penetration of microwave techniques in industry is mainly limited by the use of sophisticated equipment commonly found in research laboratory.