Observations of the reflected radio wave from a high-power transmitter show at times of not too high ionospheric absorption a drop of signal strength of typically almost 10db in the first few milliseconds after the arrival of the ionospheric echo following switching on a transmitter with 130 MW equivalent power. This decrease in signal strength is attributed to the excitation of the parametric decay instability. During about the next 5 s a further drop in signal strength by typically almost 10 db is observed. This decrease is attributed to the excitation of short-scale field-aligned irregularities which scatter the incident radio wave into Langmuir waves. These preliminary observations tend to confirm the theoretical threshold of the parametric decay instability. There are also indications that the formation of short-scale field-aligned irregularities inhibits the excitation of the parametric decay instability, as was previously inferred from the 'overshoot' effect in the Arecibo radar observations.