Physics of wireless signalling
Several different electrical, magnetic, or electromagnetic physical phenomena can be used to transmit signals over a distance without intervening wires. The various methods for wireless signal transmissions include:
Electrical Conduction through the ground, or through water.
Magnetic induction
Capacitive coupling
Electromagnetic waves
All these physical phenomena, as well as more speculative concepts such as conduction through air, have been tested for purposes of communication. Early researchers may not have understood or disclosed which physical effects were responsible for transmitting signals. Early experiments used the existing theories of the movement of charged particles through an electrical conductor. There was no theory of electromagnetic wave propagation to guide experiments before Maxwell's treatise and its verification by Hertz and others.
Capacitive and inductive coupling systems today are used only for short-range special purpose systems. The physical phenomenon used generally today for long-distance wireless communications involves the use of modulation of electromagnetic waves, which is radio.
Radio antennas radiate electromagnetic waves that can reach the receiver either by ground-wave propagation, by refraction from the ionosphere, known as sky-wave propagation, and occasionally by refraction in lower layers of the atmosphere (tropospheric ducting). The ground-wave component is the portion of the radiated electromagnetic wave that propagates close to the earth's surface. It has both direct-wave and ground-reflected components. The direct-wave is limited only by the distance from the transmitter to the horizon plus a distance added by diffraction around the curvature of the earth. The ground-reflected portion of the radiated wave reaches the receiving antenna after being reflected from the earth's surface. A portion of the ground-wave energy radiated by the antenna may also be guided by the earth's surface as a ground-hugging surface wave.
Early theories and experiments
Several scientists speculated that light might be some kind of wave connected with electricity or magnetism. Around 1830 Francesco Zantedeschi suggested a connection between light, electricity, and magnetism . In 1832 Joseph Henry performed experiments detecting electromagnetic effects over a distance of 200 feet and postulated the existence of electromagnetic waves. In 1846 Michael Faraday speculated that light was a wave disturbance in a force field".
Complete theory of electromagnetism
Based on the experimental work of Faraday and other physicists, James Clerk Maxwell developed the theory of electromagnetism that predicted the existence of electromagnetic waves. He did not transmit or receive radio waves.
Innovations and laboratory experiments
Hughes
In 1879, during experiments with his induction balance, David E. Hughes transmitted signals which he attributed to electromagnetic waves. Hughes' contemporaries claimed that the detected effects were due to electromagnetic induction. Hughes used his apparatus to transmit Morse code using a transmitter controlled by clockwork.
Hertz
Heinrich Rudolf Hertz was the experimental physicist who confirmed Maxwell's work in the laboratory. Hertz, though, did not devise a system for actual general use nor describe the application of the technology. From 1886 to 1888 inclusive, in his UHF experiments, he showed that the properties of radio waves were consistent with Maxwell’s electromagnetic theory. He demonstrated that radio radiation had all the properties of waves (now called electromagnetic radiation), and discovered that the electromagnetic equations could be reformulated into a partial differential equation called the wave equation.
Hertz’s setup for a source and detector of radio waves (then called Hertzian waves or Hertz waves in his honor), comprised a primitive radio system capable of transmitting and receiving radio waves through free space.
Hertz used the damped oscillating currents in a dipole antenna, triggered by a high-voltage electrical capacitive spark discharge, as his source of radio waves. His detector in some experiments was another dipole antenna connected to a narrow spark gap. A small spark in this gap signified detection of the radio waves. When he added cylindrical reflectors behind his dipole antennas, Hertz could detect radio waves about 20 metres from the transmitter in his laboratory. He did not try to transmit further because he wanted to prove electromagnetic theory, not to develop wireless communications.
Hertz seemed uninterested in the practical importance of his experiments. He stated that "It's of no use whatsoever ... this is just an experiment that proves Maestro Maxwell was right - we just have these mysterious electromagnetic waves that we cannot see with the naked eye. But they are there."
Asked about the ramifications of his discoveries, Hertz replied, "Nothing, I guess." Hertz also stated, "I do not think that the wireless waves I have discovered will have any practical application." Hertz died in 1894, so the art of radio was left to others to implement into a practical form. His discoveries would later be taken up by entrepreneurs looking to make their fortunes.
Tesla
Around July 1891, Nikola Tesla constructed various apparatus that produced between 15,000 to 18,000 cycles per second. Transmission and radiation of radio frequency energy was a feature exhibited in the experiments by Tesla which he proposed might be used for the telecommunication of information.
After 1892, Tesla delivered a widely reported presentation before the Institution of Electrical Engineers of London in which he suggested that messages could be transmitted without wires. Later, a variety of Tesla's radio frequency systems were demonstrated during another widely known lecture, presented to meetings of the National Electric Light Association in St. Louis, Missouri and the Franklin Institute in Philadelphia. According to the IEEE, "the apparatus that he employed contained all the elements of spark and continuous wave that were incorporated into radio transmitters before the advent of the vacuum tube". However, "he almost perversely rejected the notion of transmission by Hertzian waves, which he considered to be wasteful of energy."
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