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Diurnal amplitude variations were found in the man-made ELF radio wave recorded at the Antarctic station "Bellinshausen". The propagation path was 16.25 Mm long. Average level of the field corresponds to the literature data. Diurnal amplitude variations in the record arise from the day-night non-uniformity of the waveguide. The ambient day level is by 5 - 6 dB lower than that at the ambient night conditions in the vertical electric field component, it is by 4 - 5 dB lower in the horizontal magnetic field. Computational results are presented for the fields in the spherical Earth-ionosphere cavity with the 'whole day' and 'whole night' ionosphere models taken from literature. Model data contradict to observations indicating on inexact literature value of the phase velocity of radio waves. Model data match the experiment after the phase velocity is corrected by 10%. Since the 'whole day' and the 'whole night' ionosphere models do not account for the wave diffraction at the day-night interface, we conclude that more exact modeling is desirable.