In this work the effect of a high frequency radio wave incident upon the ionosphere is studied. The changes in the ionospheric parameters are derived, A particular case is examined, namely, the values found for conditions over the Arecibo Observatory, The radio source is a transmitter, with 160 NH, tunable from 5 to 25 MHz, using the dish antenna (305 meters of diameter) of the Arecibo Observatory, The uses of the ordinary or the extraordinary waves are discussed, The different conditions in the non deviative and deviative absorption regions are considered, The probing instruments are: the 430 MHz radar (at the Arecibo Observatory), photometers, and an ionosonde,