In this paper, we proposed a high overmoded factor system based on the overmoded waveguide method used in permittivity measurement for lossy materials. It alleviates the air gap effects and extends the rectangular waveguide method to the sub-terahertz band. Compared to the conventional standard waveguide method, errors of \(\epsilon _{r}^{\prime }\) and \(\tan \delta \) are significantly decreased from 24% and 47% to 3% and 4%, respectively, with an air gap of 0.04 mm. To achieve a high mode purity conversion between the standard waveguide and the overmoded waveguide, a multi-channel waveguide transition structure is proposed and designed. It avoids the resonance problem caused by the linear transition structure, thus eliminating the uncertainty introduced by the filtering algorithm. The proposed method was verified by measuring polytetrafluoroethylene (PTFE) samples at W-band. The measured results indicate its high accuracy and high tolerance for air gaps. Then, it is applied to measure the lossy BeO-TiO \(_{2}\) ceramic for gyrotron traveling wave tube design. The proposed method has promising applications in permittivity measurement at sub-terahertz and terahertz frequency bands.
