Design and validation of an automated and remote free space measurement system for nondestructive testing of fiber composites

Assessing electromagnetic constitutive parameters is crucial to prescribe the macroscopic properties of composites and their prospective applications. Free space methods are widely used for this purpose, due to their nondestructive/noncontact nature and their applicability on composites incorporating large inclusions or over-frequency bands where waveguide measurements are impractical. However, there still exists issues associated with automation, accurate calibration, remote controlling, and multifunctional characterization. Here, we designed and implemented a microwave-integrated laboratory including a test bench for permittivity/permeability and impedance measurements of individual inclusions and a free space setup for transmission/reflection measurements of fiber-based composites. Easy switching between the bench and antenna measurements was enabled by a homemade RF multiplexer. A three-stage calibration was applied: 2-port error correction (12-term model) of the vector network analyzer and the cables connecting it to the multiplexer, de-embedding of the cables connecting the multiplexer to the switches within the antenna pillars, and thru, reflect, and line (TRL) error correction for the antennas and free space. Exploiting robotics for precise antenna movement and TRL calibration enabled adjustment of the antenna distance to the test frame to a maximum of 2.5 m with a 100 μm accuracy. A multifunctional frame for external stimuli application was also designed. Apart from automation, remote control was realized through user-friendly graphical interfaces and remote access software allowing to swiftly respond to challenges faced during the global pandemic. The free space setup effectiveness was then validated by measuring the transmission/reflection of microwire-based composites from 0.5 to 20 GHz under various magnetic fields.