English: Drawing of early 450 MHz spark-gap radio transmitter and parabolic antenna used by German physicist Heinrich Hertz in 1888 during his historic first researches on radio waves (Hertzian waves), from his 1893 book. This is the earliest example of a parabolic antenna. The antenna is described on p. 175 of the source 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. 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 drawing on the left shows 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 wavelength of the waves produced was measured by Hertz at 66 cm, making the corresponding frequency 454 MHz.
Hertz used a similar parabolic antenna with a spark gap for receiving. With these he performed historic experiments demonstrating standing waves, diffraction, refraction and polarization of radio waves, proving that radio waves are electromagnetic waves like light waves, thus confirming the 1867 theory of James Clerk Maxwell.