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The transmission line approximation (TLA) method has been widely used for the design of electromagnetic absorbers, because it does not need the tremendous calculations required in numerical analysis methods. However, the method has intrinsic application limits as a low-frequency approximation. In this paper, we described full-wave analysis on the periodic structure with a finite element method (FEM) and investigated the accuracy and limitations of TLA by comparing TLA with FEM results for the reflection loss of a dielectric pyramid. It was found that TLA is valid when λ⩾2p, where λ is the wavelength of the frequency of interest and p is the distance between periodic cone arrays