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.
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