Preview referat: Radio receivers
In this way the capacitance of the circuit could be altered, and also its resonance frequency. With every interruption from the battery, a high voltage was produced at the output of the inductor, creating a spark between the narrow placed balls k1 and k2. According to Maxwell's theory, as long as there was a spark, i.e. alternating current in the circuitry, there was an electromagnetic field surrounding the conductors, spreading itself through the surrounding space. A few metres away from this device Hertz placed a bent conductor with metal balls k3, k4 placed on the ends, positioned very close to each other.
This also was an oscillatory circuit, called the resonator. According to Maxwell's theory, voltage induced by the electromagnetic waves should be created in the resonator. Voltage existence would be shown by a spark between the balls k3 and k4. And that's the way it was: Whenever there was a spark in the oscillator between the balls k1 and k2, a spark would also be produced by the resonator, between balls k3 and k4. With various forms of the arrangement in Pic.
1.1, Hertz proved that electromagnetic waves behave as light since they could also be reflected and refracted. It was also shown that light is of electromagnetic nature, as stated by Maxwell. Hertz, however, did not believe in the practical value of his electromagnetic waves experiments. The range of the link was no further than a few meters. The transmitted signal was very weak, therefore the signal in the receiver had a very small amplitude and it wasn't possible to detect it at a greater distance.
The possibility of amplifying the signal in the receiver did not exist at the time. Besides the short range, another shortcoming of the link was noted: If another similar transmitter was working nearby, a receiver detected all the signals at the same time. It did not have the ability of isolation. However crude and simple these experiments were at the time, they represented the birth of a new scientific branch - Radio Engineering.
The pioneers of radio were Popov and Marconi, but the place of honor belongs to Nikola Tesla, who demonstrated wireless broadcasting in 1893, at the Franklin Institute. Pic.1.2 shows the arrangement of this broadcast system. Tesla's idea was to produce electromagnetic waves by means of oscillatory circuits and transmit them over an antenna. « mai multe referate din Fizica