Marconi #3

G. Marconi secretly worked on a project he referred to as "the great thing" -- an attempt to signal across the Atlantic Ocean. Theoretical physicists said it couldn't be done because they claimed electromagnetic waves radiated in a straight line into space and would not follow the curvature of the earth. Holders of this view included the great French mathematician and physicist Henri Poincare, who understood the properties of Hertzian waves. But Marconi was convinced that the theoreticians were wrong; he believed electromagnetic waves would bend to follow the curvature of the earth (Marconi, 148).

In December, 1901 Marconi experimented with signals sent from a station in England across the Atlantic Ocean to a station in Newfoundland. At the receiving end he did indeed hear signals "serenely ignoring the curvature of the earth which so many doubters considered would be a fatal obstacle" (174).

Prompted to explain how Marconi had been able to receive a Hertzian wave signal nearly two thousand miles away, two theoretical physicists later hypothesized that there might exist an ionized layer in the upper atmosphere capable of reflecting or refracting radio waves of certain frequencies back to earth (176).

The biography doesn't address the varying range of radio wave lengths/frequencies. However, Wikipedia shows the whole range of radio waves (link1) and includes the following (link2).

"Lower frequency (between 30 and 3,000 kHz) vertically polarized radio waves can travel as surface waves following the contour of the Earth; this is called groundwave propagation."

"In this mode the radio wave propagates by interacting with the conductive surface of the Earth. The wave "clings" to the surface and thus follows the curvature of the Earth, so groundwaves can travel over mountains and beyond the horizon."

"Early long distance radio communication (wireless telegraphy) before the mid-1920s used low frequencies in the longwave bands and relied exclusively on ground-wave propagation. Frequencies above 3 MHz were regarded as useless and were given to hobbyists (radio amateurs). The discovery around 1920 of the ionospheric reflection or skywave mechanism made the medium wave and short wave frequencies useful for long distance communication and they were allocated to commercial and military users."

So it seems both Marconi and Poincare were partly correct and partly incorrect. Marconi's experiment used lower or medium frequency (longer or medium length) radio waves.