Hertz wireless transmitter (Rankin Kennedy, Electrical Installations, Vol V, 1903)

One of the first spark-gap radio transmitter (Hertzian oscillator) invented by German physicist Heinrich Hertz during his historic 1893 researches into radio waves (Hertzian waves). It consists of a half-wave dipole radiator made of a spark gap between two metal balls attached to two metal plate "capacity areas" (A,B), powered by high voltage pulses from an induction coil (bottom). The spark between the balls excites damped sinusoidal oscillating radio frequency currents in the dipole, at its resonant frequency, which are radiated into space as electromagnetic waves. The direction of maximum radiation, shown by the dotted line, is perpendicular to the axis of the dipole. Hertzian oscillators were the first type of radio transmitter. The wavelength of the waves produced was roughly twice the width of the resonator, and for Hertz's devices was in the UHF range, around 500 MHz.