Electromagnetic waves in this part of the spectrum are very well reflected in the electrically charged layers of ions that exist in the upper layers of the atmosphere (ionosphere). Since radio signals have wavelengths from tens to thousands of meters, they can be diffracted around small obstacles, such as trees or buildings, which do not prevent their transmission, but hills of some size and mountain ranges generate "shadow areas" where the signal does not arrive. Either radio waves that can travel long distances, directly or using repeaters, are very useful for carrying information to all types of radio.
How Radio Works
Used to using Wi-Fi internet and wireless devices every day, the radio can seem like a very simple invention. However, radio was a revolution: it was the first wireless information transmission system in history. The knowledge and discoveries made by several generations of scientists and inventors allowed information to be transmitted by waves from one side of the Atlantic Ocean to the other in 1901, a milestone that ushered in a new era.
One of the works that had a major impact on the development of radio was that of the Scottish scientist James Clerk Maxwell. In 1865, Maxwell first defined the electromagnetic field and how it works: it propagates through space in the form of waves that travel at the speed of light.
Two decades later, Heinrich Rudolf Hertz put Maxwell's theory into practice and built the first apparatus to demonstrate the existence of electromagnetic waves.
Hertz designed an electrical circuit that produced electromagnetic waves and an isolated receiver that captured them from several meters away. This device could be considered the first precursor of radio.
By the way, thinking of all the different and all types of radio you must have gone through a dozen examples but have overlooked that a modern mobile phone is still a cellular radio transceiver since it receives and sends radio signals, hence a radio.
The discovery of radio waves and their use in communications was something that had to happen sooner or later after the Hertz experiments and, therefore, in the last decade of the XIX and early years of the XX century the discovery was produced independently in various places. Thus, Aleksandr Stepanovich Popov made his first demonstrations in Saint Petersburg (Russia); Nikola Tesla in St. Louis (Missouri, USA); Guglielmo Marconi in the United Kingdom or Major Julio Cervera in Spain.
However, its commercial developer was Marconi, who in 1909 would receive the Nobel Prize in physics, shared with Karl Ferdinand Braun, from whom Marconi admitted to having "borrowed" the patent for the crystal rectifier in his "wireless telegraphy" experiments. He also borrowed several patents from Tesla, who reportedly declined the award for not wanting to share it with Marconi, whom he considered unworthy of the honor.
The point is that Marconi, the most industrious of all those involved, managed to send a radio signal from Nova Scotia (Canada) to Ireland in December 1902. In January 1903, the President of the United States sent a message by radio from a station built by Marconi in Massachusetts to King Edward VII of England. By 1904 ships crossing the Atlantic could subscribe to a nightly news service.
It was thus demonstrated that long-distance communication by radio waves was possible because the waves were reflected in, at the time only hypothetical, ionosphere, thus saving the curvature of the Earth. The hypothesis of the existence of the ionosphere was formulated in 1839 by Carl Friedrich Gauss to explain the observed variations in the Earth's magnetism.
Radio communication is based on changing the signal according to the pre-agreed code between the transmitter and the receiver. The first radio communication was achieved simply by turning the transmitter on and off according to a pattern. The most popular pattern was that developed by Samuel Morse and Alfred Vail for use on the telegraph; today we would say that Morse code is digital. Subsequently, sounds were coded by continuous variations in the amplitude (intensity) of the wave that was emitted. This is known as amplitude modulation or amplitude modulation (AM); domestic receivers can pick up the “medium wave” (OM) in AM, that is, between 500 and 1700 kilohertz. More recently, the information could be encoded as variations in the frequency of the emitted wave, which is known as modulated frequency (FM).
In broadcasting, decoding is carried out in the receiver by taking the signal to a loudspeaker, which performs the inverse process to the microphone in the transmitter, the message that comes out of the receiver in the form of sound having the same form as it had in the transmitter. Radio stations regularly announce their broadcast frequency, in megahertz (MHz) for FM and in kilohertz (kHz) for AM.
To prevent the stations from interfering with each other or with other communications, all types of radio there are local, national and international organizations that regulate both the frequencies and the broadcast powers, general and in certain directions, and the hours of the broadcast of the different radio stations.