Electromagnetic characterization of anisotropic or heterogeneous materials

In recent years, exploiting the physical properties of certain materials has led to a number of technological advances in the microwave field. These include, for example, the miniaturization of circuits thanks to the use of high-permittivity dielectric substrates, the realization of signal processing functions such as attenuation, isolation and phase shifting, based on the properties of ferrimagnetic media, and the reduction in manufacturing costs made possible by the development of composite materials. In this context, the electromagnetic (EM) characterization of materials is essential for controlling manufacturing procedures and the performance of materials used in microwave devices, as well as for developing new materials.

Two main classes of media - dielectrics and ferrites (ferrimagnetic ceramics) - are commonly used in high-frequency applications. However, to meet the specifications set by certain applications, new materials have been developed or are currently under study. These include: metamaterials used for high-resolution lenses, compact and directional antennas, and EM wave absorption; ordered photonic bandgap materials (BIPS), which prevent the penetration of EM radiation at certain frequencies, and whose main application is in antenna reflectors; and superconductors used to reduce ohmic losses in circuit conductors. This list is by no means exhaustive, but it does illustrate the complexity of the work involved in implementing characterization techniques, given the diversity of properties of the materials studied (absorbent, heterogeneous, anisotropic, etc.) and their packaging (liquids, thick films, thin films, paints, inks, etc.).

The measurement of microwave permittivity and permeability of isotropic and homogeneous dielectric and bulk magnetic materials is currently well mastered, thanks to a wide range of techniques based on the combined use of the latest generation of network analyzers and resonant structures (closed, semi-open cavities, etc.) or propagation structures (coaxial line, triplate, waveguide, etc.). On the other hand, the conditioning of certain materials, e.g. in the form of thin films, or the study of the influence of certain external constraints, e.g. mechanical, electrical (control voltage), magnetic (control current) or thermal, pose as yet unsolved EM characterization problems.

In this context, engineers need to be given a basic introduction to the measurement of media with conventional properties, e.g. linear, homogeneous and isotropic (LHI) dielectric or magnetic materials. dielectric or magnetic materials that are linear, homogeneous and isotropic (LHI), and help in choosing the measurement technique best suited to materials with strong singularities due to their properties (anisotropic, heterogeneous),...