Image: Hertz radio wave experiments - parabolic antennas

Description: Diagram of the transmitter and receiver Heinrich Hertz used for some of his historic 1888 experiments determining the properties of radio waves. The transmitter (center) is a dipole antenna 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 induction coil 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 parabolic reflector 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 standing waves, diffraction, refraction, and polarization of radio waves, proving that they are electromagnetic waves like light waves, just with longer wavelength.
Title: Hertz radio wave experiments - parabolic antennas
Credit: Retrieved December 17, 2015 from George Washington Pierce (1910) Principles of Wireless Telegraphy, McGraw-Hill Book Co., New York, p. 51, fig. 28 & 29 on Google Books
Author: George Washington Pierce
Usage Terms: Public domain
License: Public domain
Attribution Required?: No