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Asphalt thickness estimation using ground-penetrating radar (GPR) is an important application in transportation infrastructure inspections. Thin asphalt measurement needs to use super-resolution or full-wave methods to process the GPR signals with overlapped echoes. We propose a model for antenna reflection coefficient in the presence of multilayer medium by omitting considerations of evanescent waves and interactions between the antenna and the medium. We formulated it within the spectral domain, enabling the explicit incorporation of the antenna’s radiation pattern. This model is about 100 times faster than the existing full-wave models. In this model, we calculate the plane wave expansion (PWE) of the antenna radiated field. Ignoring the evanescent waves, the PWE of the radiated field is calculated from the far-field pattern of the antenna. Simulation and measurement results confirm the validity and effectiveness of this model in asphalt thickness measurement.