Approach to Control Permittivity and Shape of Centimeter-Sized Additive Manufactured Objects: Application to Microwave Scattering Experiments

Controlling the electromagnetic properties of dielectric objects is demanded in microwave applications such as radar cross section (RCS) studies, metamaterial design, and antennas prototyping. Additive manufacturing has made the fabrication of desired shapes easier, but there is still room for improvement in controlling the permittivity. This article proposes a novel approach to control the permittivity of 3-D printed objects, in particular the ones with low permittivity contrasts. The effective permittivity is set by locally varying the material density. The object is first meshed using tetrahedral meshing software. Then its air percentage is controlled by adjusting the diameter of cylinders, which are positioned at each edge of the mesh. By tuning the volume fraction, one can achieve the required effective permittivity. Design, manufacturing, and characterization steps are discussed for the specific case of spheres. With such a canonical shape, the effective permittivity can be directly retrieved by comparing far-field electromagnetic scattering measurements with Mie computations. Bistatic RCS measurements and simulations are provided and discussed. They enable us to assess the validity of the proposed methodology, in particular the good adequacy between the adequately chosen unstructured air/material distribution, the desired relative permittivity, and the good homogenization of the object.