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Marconi was a great inventor, but he was not the one who invented wireless communications. Several other people designed wireless telegraphy equipment before Marconi. There were people working on wireless communications even before Hertz performed his 1888 experiment that proved the existence of the electromagnetic waves predicted by Maxwell. The earlier wireless devices used induction, and had very little range.
What Marconi did was find an important new use for existing wireless inventions. It was “known” by the leading scientists in 1901 that light was made up of electromagnetic waves, and that radio waves traveled in a straight line, just like light waves. It was therefore considered to be impossible to transmit beyond the horizon. It had been demonstrated that wireless signals can go over hills, but this had been accounted for by the diffraction of the radio waves. It was certainly considered to be impossible to send a wireless signal across the Atlantic. Over a distance of 1800 miles, there is a “hill” about 100 miles high due to the curvature of the earth. In his 1901 experiment, Marconi demonstrated that, by using vertical polarization of the electric field, the signal travels along the surface of the earth, provided that the carrier frequency is low enough. Today, this is known as groundwave propagation. Some people did not believe that Marconi’s 1901 experiment actually worked, because that would have meant that the laws of physics were violated. In modern times, conventional scientific opinion has usually been correct, but this is an example of when it was not.
Marconi’s antennas were electrically short vertical antennas over ground, similar in principle to what is used for Part 15 AM today. He had to use electrically short antennas because the wavelengths he used were very long.
For the first two decades of the Twentieth Century, the principal mode of radio communications was telegraphy at low frequencies. Later, most low-frequency, long distance, communications was replaced by HF telegraphy and telephony. Today, of course, satellites are used.