Geophysical data obtained from data bases of the Internet are collated with radio propagation parameters on the high--latitude HF path Heiss Island--St. Petersburg (D=2500 km) for the intensive magnetic storm occurred on 14-17 May 1997. Main attention was given to variations of the operating frequency range of the maximum and the lowest observed frequencies (MOF--LOF). This range was shown to bring some valuable information about ionospheric dynamic. Acquisition of diagnostic data such as the MOF and the LOF values is actually the method of the oblique ionospheric sounding that works properly during magnetic storm periods as opposed to the method of vertical sounding. Specific regularities are revealed for frequency range variations that are substantiated from the physical point of view. Some recommendations are given for radio communications, for example, on operating frequencies selection during a disturbed period in particular