Macroscopic characterization of materials using microwave measurement methods — A survey

Characterization of materials using microwave waveguide techniques is reviewed. The microwave measurement systems for the characterization of materials are increasingly important and widespread due to the rapid development of synthetic materials in this era, such as graphene, metamaterials, carbon nanotube, conductive polymer, high-temperature superconductor, and aerogel. Most of these materials are used in electrical and electronic engineering applications, such as RF electronic components, passive microwave circuits, printed circuit board (PCB) substrates, microwave components/cable shielding, EMI absorbers, microwave energy harvesting, thin-layer conductive coating, and nano/dielectric antennas. Normally, three macroscopic constitutive parameters of the materials are desired to be observed, namely permittivity, ε_r, permeability, μ_r and conductivity, σ, since these parameters are used to measure the performance and determine the suitability of the material used in specific applications. Microwave material characterization techniques can be categorized into either resonant technique (based on shifting resonant frequency) or non-resonant technique (based on S-parameters measurements). Typically, the S-parameters or shifting resonant frequency of the material under test is initially measured using an impedance analyzer (below GHz) or vector network analyzer (up to GHz). Based on the raw measurements, the constitutive parameters (ε_r, μ_r and σ) of the material can be predicted via electromagnetic models. With the aid of a recent VNA technology, the constitutive parameters measurements are capable of achieving 1.1 THz. In this paper, an international standard setup, accuracy, limitations, formulations and sub-principle of measurements for each technique were compared and briefly summarized as a reference for researchers and engineers in order to choose appropriate measurement techniques based on their material dimensions, type of materials, accuracy requirement, particular environmental condition, range of operating frequency and cost budget.