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We consider the features of propagation of decametric radio waves along the transpolar ionospheric paths. The results of recording round-the-world trip echo signals, which was performed during the many-hour sessions in November and December 1971, are analyzed. A facility emitting waves at the maximum usable frequency in the zero-azimuth direction and recording the signal arriving from the opposite direction, which was located in the south of Ukraine, was used. The data obtained on November 23, 1971 for the radiation frequencies f o =16-17 MHz are considered a standard example. We also present the results of calculating the diurnal variation in the maximum usable frequency of the F 2 layer for a radio-path hop of 4000 km in length in the forward and backward paths Novorossiysk-USA north-west coast in May 1980, which were obtained by the standard methods and correspond to the experimental data. It is shown that the obtained data can be interpreted on the basis of radiophysical phenomena, namely, (i) the formation of a grazing radio wave near the top of the first hop with a length of 3000-4000 km for the radiation at the maximum usable frequency and (ii) the night-time retaining of the daytime properties of the F 2 region in the case of oblique heating by a high-power radio wave. The performed study allowed us to estimate the possibility of the Alaska HAARP facility to emit to the territory of the Russian Federation.

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