  This manuscript presents a novel method for characterizing the permittivities
of low-loss dielectric slabs in sub-terahertz (sub-THz) frequencies,
specifically above 100 GHz using a quasi-optical system. The algorithm is
introduced with detailed derivations, and the measurement sensitivity is
analyzed through simulations. Subsequently, the method's validity is
established via simulations, demonstrating high accuracy (error 0.1% for the
loss tangent) for a 30 mm thick plate material and relatively lower accuracy
(error <5% for the loss tangent) for a 6 mm thick plate material. Notably, this
accuracy surpasses that of the approach presented in [1] when the same window
width is used to extract signals. Furthermore, a comparison between the
permittivities of plexiglass with a 30 mm thickness characterized by the
proposed method and the approach in [1] reveals a maximum difference in the
dielectric constant of 0.011 and in loss tangent of 0.00071 from 140 to 220
GHz. Finally, the relative complex permittivities of plexiglass at 142.86 GHz
obtained by both methods are compared with the reference values provided in
[2], exhibiting differences of 0.06 in the dielectric constant.
