Two events demonstrating periodic narrowband millisecond pulsations of radio emission both in intensity and polarization degree are analyzed. Large time delays between L-and R-polarization components are found. The radio emission is shown to be generated as unpolarized by a plasma mechanism at the second harmonic of the upper-hybrid frequency. Observed oscillations of the radiation polarization degree arise due to group delay between extraordinary and ordinary modes along the line of sight. The

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... theoretically predicted dependence of the group delay on frequency (∼f −3 ) agrees excellently with the observed delay frequency dependence. Physical parameters of the emission source and "delay site" are determined within the proposed model.