In this letter, we discuss the measurement error due to the exclusion of the higher modes when a waveguide with a flange is inserted with lossy dielectric material. The reflection coefficient is calculated by a spectrum-domain approach (SDA), which uses only the dominant (TE10) mode to realize a simplified nondestructive measurement of complex permittivity of lossy dielectric sheets. The analysis shows that the error due to the exclusion of the higher modes decreases as complex permittivity increases. Consequently, we have confirmed that a simplified measurement of complex permittivity is possible by a coverage limitation with the SDA, which excludes the higher modes.

A method for estimating complex permittivity of a material using a rectangular waveguide with a flange is presented by the finite difference time domain (FDTD) method. An advantage of the present method is that it is not necessary to vary the material structure in order to insert it into the waveguide. Therefore estimation errors related to the dimensions of the material are almost negligible. In this case, fluoridated rubber is chosen as the low-loss material. The comparison of the complex permittivity of the material determined by the present method with FDTD and the conventional waveguide method at 10 GHz is performed. It was confirmed that the present method is effective for estimating the complex permittivity under the condition that the length of the flange is about 50 mm (1.7λ) square.