File:Small spark gap transmitter and coherer receiver.jpg

English: Photo of a small homemade spark gap transmitter and receiver wireless telegraphy radio set used by amateurs to experiment with Hertzian waves around 1900, from a 1909 book on radio. The transmitter (left) is a "Hertzian oscillator" consisting of an induction coil powered by 4 dry cell batteries which excites oscillations in a dipole antenna consisting of two metal balls with wires sticking out in opposite directions. When the telegraph key is pressed, sparks jump between the balls, causing oscillating electric currents in the antenna wires which are radiated as radio waves.

The radio waves are picked up by the receiver (right). The receiver has a wire dipole antenna connected to an early radio wave detector called a coherer, a glass tube with two electrodes a few millimeters apart, with metal filings in the gap. When the radio waves are applied to the coherer, the filings become conductive. The coherer is also connected to a circuit with a dry cell (right) and an electric bell, so when the filings become conductive the bell rings. The filings remain conductive after the radio wave stops, so the bell arm also serves as a "decoherer", tapping the glass tube to disturb the filings and return them to a nonconductive state to prepare the coherer to receive the next radio signal.

The frequency of the radio waves emitted is determined by the length of the transmitter's antenna. The half-wave dipole is roughly half a meter long, so the wavelength λ of the waves will be about one meter. The frequency will be f = c / λ, the speed of light divided by the wavelength, or about 300 MHz, around the frequency that is used today by cell phones. The range at which the transmitter signal can be received is probably not much more than 100 feet. The source gives detailed instructions on how to construct the set.