English: Drawing of experimental 450 MHz spark radio transmitter and parabolic antenna built by German physicist Heinrich Hertz in 1888 and used in his historic first researches on radio waves (Hertzian waves). This is the earliest example of a parabolic antenna. The antenna is described in Heinrich Hertz, Daniel Evan Jones (1893) Electric Waves, 2nd Ed., McGraw-Hill, New York, p. 175 as a 2 m x 2 m sheet of zinc attached to a wooden frame to make a cylindrical parabolic reflector with aperture 2 m high by 1.2 m wide, with focal length of 12.5 cm. A section of the reflector is shown cutaway to reveal the antenna. Along the focal line is suspended a Hertzian 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. The dipole elements are 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 wavelength of the waves produced was measured by Hertz at 66 cm, making the corresponding frequency 454 MHz.