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A simple high-power ionosphere sounding equipment for use at low frequencies is described. Its application to the measurement of virtual height at vertical incidence at a carrier frequency of 100 kc is reported. Results of nearly a year of intermittent night-time measurements showed a remarkably large variation in virtual height which ranged from about 84 km to as high as 106 km. Some evidence was found to indicate that frequently at night the reflecting region consists of clouds or patches of ionization, rather than the more nearly uniform ionization characteristic of the regular layers at high frequencies. A rotation of the polarization of the reflected signal with respect to that of the transmitted signal was observed.