File:Hertz radio wave experiments - parabolic antennas.png

Summary

Diagram of the transmitter and receiver <a href="https://en.wikipedia.org/wiki/Heinrich_Rudolph_Hertz" class="extiw" title="w:Heinrich Rudolph Hertz">Heinrich Hertz</a> used for some of his historic 1888 experiments determining the properties of radio waves. The transmitter (center) is a <a href="https://en.wikipedia.org/wiki/dipole_antenna" class="extiw" title="w:dipole antenna">dipole antenna</a> consisting of two 1 cm dia. brass rods about 13 cm long, with metal balls attached to its adjacent ends to make a spark gap about 3 mm wide. At left is a closeup of the dipole. The dipole elements were attached to an <a href="https://en.wikipedia.org/wiki/induction_coil" class="extiw" title="w:induction coil">induction coil</a> powered by a battery on a table behind the antenna, which applied high voltage pulses which caused sparks in the spark gap, exciting high frequency oscillations in the dipole. The dipole was suspended along the focal line of a cylindrical <a href="https://en.wikipedia.org/wiki/parabolic_reflector" class="extiw" title="w:parabolic reflector">parabolic reflector</a> made of a piece of sheet metal attached to a wooden frame to make a parabolic antenna with an aperture of 1.2 m x 2 m and a focal length of 12.5 cm. The receiver (right) consists of a dipole made of two pieces of wire each 50 cm long, at the focus of a similar reflector, whose ends are brought out to an adjustable micrometer spark gap. The wavelength of the waves produced by the transmitter was measured by Hertz at 66 cm, making the corresponding frequency 454 MHz. These were the first parabolic antennas.

Hertz used the transmitter and receiver in historic experiments to demonstrate <a href="https://en.wikipedia.org/wiki/standing_wave" class="extiw" title="w:standing wave">standing waves</a>, <a href="https://en.wikipedia.org/wiki/diffraction" class="extiw" title="w:diffraction">diffraction</a>, <a href="https://en.wikipedia.org/wiki/refraction" class="extiw" title="w:refraction">refraction</a>, and <a href="https://en.wikipedia.org/wiki/polarization" class="extiw" title="w:polarization">polarization</a> of radio waves, proving that they are electromagnetic waves like light waves, just with longer wavelength.

Licensing

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