Characteristic Mode-Assisted Permittivity Measurement of TWT Dielectric Rods

The resonant method is very useful for permittivity measurements of traveling wave tube (TWT) dielectric rods, but it suffers from limitations, such as typical material shapes and coaxial placement. How to establish a model without these limitations is a critical issue for enhancing measurement capability. To solve it, this letter proposes a new approach for permittivity measurement by the resonant method using the characteristic mode (CM). The surface integral equation and the volume-surface integral equation are adopted to solve the resonant fields before and after loading the sample, respectively. In addition, the feature of the resonant mode under permittivity variations is extracted to assist in identifying and calculating the permittivity. Measurement results demonstrate that the CM-assisted method has good accuracy and stability when applied to multitype TWT dielectric rods.