Radio waves were first identified in German physicist Heinrich Hertz's 1887 series of experiments to prove James Clerk Maxwell's electromagnetic theory. Hertz used spark-excited dipole antennas to generate the waves and micrometer spark gaps attached to dipole and loop antennas to detect them. These precursor radio receivers were primitive devices, more accurately described as radio wave "sensors" or "detectors", as they could only receive radio waves within about 100 feet of the transmitter, and were not used for communication but instead as laboratory instruments in scientific experiments and engineering demonstrations.
Spark era
The first radio transmitters, used during the initial three decades of radio from 1887 to 1917, a period called the spark era, were spark gap transmitters which generated radio waves by discharging a capacitance through an electric spark. Each spark produced a transient pulse of radio waves which decreased rapidly to zero. These damped waves could not be modulated to carry sound, as in modern AM and FM transmission. So spark transmitters could not transmit sound, and instead transmitted information by radiotelegraphy. The transmitter was switched on and off rapidly by the operator using a telegraph key, creating different length pulses of damped radio waves ("dots" and "dashes") to spell out text messages in Morse code.
Therefore, the first radio receivers did not have to extract an audio signal from the radio wave like modern receivers, but just detected the presence of the radio signal, and produced a sound during the "dots" and "dashes". The device which did this was called a "detector". Since there were no amplifying devices at this time, the sensitivity of the receiver mostly depended on the detector. Many different detector devices were tried. Radio receivers during the spark era consisted of these parts: Chapter 1
An antenna, to intercept the radio waves and convert them to tiny radio frequency electric currents. A tuned circuit, consisting of a capacitor connected to a coil of wire, which acted as a bandpass filter to select the desired signal out of all the signals picked up by the antenna. Either the capacitor or coil was adjustable to tune the receiver to the frequency of different transmitters. The earliest receivers, before 1897, did not have tuned circuits, they responded to all radio signals picked up by their antennas, so they had little frequency-discriminating ability and received any transmitter in their vicinity. Most receivers used a pair of tuned circuits with their coils magnetically coupled, called a resonant transformer (oscillation transformer) or "loose coupler". A detector, which produced a pulse of DC current for each damped wave received. An indicating device such as an earphone, which converted the pulses of current into sound waves. The first receivers used an electric bell instead. Later receivers in commercial wireless systems used a Morse siphon recorder, which consisted of an ink pen mounted on a needle swung by an electromagnet (a galvanometer) which drew a line on a moving paper tape. Each string of damped waves constituting a Morse "dot" or "dash" caused the needle to swing over, creating a displacement of the line, which could be read off the tape. With such an automated receiver a radio operator did not have to continuously monitor the receiver. The signal from the spark gap transmitter consisted of damped waves repeated at an audio frequency rate, from 120 to perhaps 4000 per second, so in the earphone the signal sounded like a musical tone or buzz, and the Morse code "dots" and "dashes" sounded like beeps.
The first radio wave based communication system was a wireless telegraphy system developed by Guglielmo Marconi, beginning in 1894–5. Sarkar et al. (2006) History of Wireless, p. 349-358, archive In 1894 English physicist Oliver Lodge had laid out the basic techniques of radio detection in his demonstrations on Hertz's discovery. Within a year Marconi and Russian physicist Alexander Popov were building receivers based on Lodge's techniques. Lodge's focus was a scientific demonstration of the quasi optical nature of "Hertzian waves" and Popov was constructing a radio based early warning lightning detector for the forest service so Marconi is usually given credit for building the first radio receiver used for communication.
Coherer receiver
The first radio receivers used by Lodge and then by Marconi and Popov in 1894–5 used a primitive radio wave detector called a coherer, invented in 1890 by Edouard Branly and improved by Lodge and Marconi.Phillips, Vivian 1980 Early Radio Wave Detectors, p. 18-21 The coherer was a glass tube with metal electrodes at each end, with loose metal powder between the electrodes.McNicol, Donald (1946) Radio's Conquest of Space, p. 107-113 It initially had a high resistance. When a radio frequency voltage was applied to the electrodes, its resistance dropped and it conducted electricity. In the receiver the coherer was connected directly between the antenna and ground. In addition to the antenna, the coherer was connected in a DC circuit with a battery and relay. When the incoming radio wave reduced the resistance of the coherer, the current from the battery flowed through it, turning on the relay to ring a bell or make a mark on a paper tape in a siphon recorder. In order to restore the coherer to its previous nonconducting state to receive the next pulse of radio waves, it had to be tapped mechanically to disturb the metal particles.Phillips, Vivian 1980 Early Radio Wave Detectors, p. 38-42 This was done by a "decoherer", a clapper which struck the tube, operated by an electromagnet powered by the relay.
The coherer is an obscure antique device, and even today there is some uncertainty about the exact physical mechanism by which the various types worked.Nahin, Paul J. (2001) The Science of Radio, p. 53-56Phillips, Vivian 1980 Early Radio Wave Detectors, p. 57-60 However it can be seen that it was essentially a bistable device, a radio-wave-operated switch, and so it did not have the ability to rectify the radio wave to demodulate the later amplitude modulated (AM) radio transmissions that carried sound.
In a long series of experiments Marconi found that by using an elevated wire monopole antenna instead of Hertz's dipole antennas he could transmit longer distances, beyond the curve of the Earth, demonstrating that radio was not just a laboratory curiosity but a commercially viable communication method. This culminated in his historic transatlantic wireless transmission on December 12, 1901, from Poldhu, Cornwall to St. John's, Newfoundland, a distance of 3500 km (2200 miles), which was received by a coherer. However the usual range of coherer receivers even with the powerful transmitters of this era was limited to a few hundred miles.
The coherer remained the dominant detector used in early radio receivers for about 10 years, until replaced by the crystal detector and electrolytic detector around 1907. In spite of much development work, it was a very crude unsatisfactory device. It was not very sensitive, and also responded to impulsive radio noise (RFI), such as nearby lights being switched on or off, as well as to the intended signal. Due to the cumbersome mechanical "tapping back" mechanism it was limited to a data rate of about 12-15 words per minute of Morse code, while a spark-gap transmitter could transmit Morse at up to 100 WPM with a paper tape machine.
Other early detectors
The coherer's poor performance motivated a great deal of research to find better radio wave detectors, and many were invented. Some strange devices were tried; researchers experimented with using frog legs and even a human brain from a cadaver as detectors.Phillips, Vivian 1980 Early Radio Wave Detectors, p. 198-203
By the first years of the 20th century, experiments in using amplitude modulation (AM) to transmit sound by radio (radiotelephony) were being made. So a second goal of detector research was to find detectors that could demodulate an AM signal, extracting the audio (sound) signal from the radio carrier wave. It was found by trial and error that this could be done by a detector that exhibited "asymmetrical conduction"; a device that conducted current in one direction but not in the other.Phillips, Vivian 1980 Early Radio Wave Detectors, p. 205-209 This rectified the alternating current radio signal, removing one side of the carrier cycles, leaving a pulsing DC current whose amplitude varied with the audio modulation signal. When applied to an earphone this would reproduce the transmitted sound.
Below are the detectors that saw wide use before vacuum tubes took over around 1920. All except the magnetic detector could rectify and therefore receive AM signals:
Magnetic detector - Developed by Guglielmo Marconi in 1902 from a method invented by Ernest Rutherford and used by the Marconi Co. until it adopted the Audion vacuum tube around 1912, this was a mechanical device consisting of an endless band of iron wires which passed between two pulleys turned by a windup mechanism. Stone, Ellery (1919) Elements of Radiotelegraphy, p. 209-221Fleming, John Ambrose (1910) The Principles of Electric Wave Telegraphy and Telephony, p. 446-455Phillips, Vivian 1980 Early Radio Wave Detectors, p. 85-108 The iron wires passed through a coil of fine wire attached to the antenna, in a magnetic field created by two magnets. The hysteresis of the iron induced a pulse of current in a sensor coil each time a radio signal passed through the exciting coil. The magnetic detector was used on shipboard receivers due to its insensitivity to vibration. One was part of the wireless station of the RMS Titanic which was used to summon help during its famous 15 April 1912 sinking. copied on Stephenson's marconigraph.com personal website
Electrolytic detector ("liquid barretter") - Invented in 1903 by Reginald Fessenden, this consisted of a thin silver-plated platinum wire enclosed in a glass rod, with the tip making contact with the surface of a cup of nitric acid.McNicol, Donald (1946) Radio's Conquest of Space, p. 115-119Fleming, John Ambrose (1910) The Principles of Electric Wave Telegraphy and Telephony, p. 460-464Phillips, Vivian 1980 Early Radio Wave Detectors, p. 65-81 The electrolytic action caused current to be conducted in only one direction. The detector was used until about 1910. Electrolytic detectors that Fessenden had installed on US Navy ships received the first AM radio broadcast on Christmas Eve, 1906, an evening of Christmas music transmitted by Fessenden using his new alternator transmitter.
Thermionic diode (Fleming valve) - The first vacuum tube, invented in 1904 by John Ambrose Fleming, consisted of an evacuated glass bulb containing two electrodes: a cathode consisting of a hot wire filament similar to that in an incandescent light bulb, and a metal plate anode.Lee, Thomas H. (2004) The Design of CMOS Radio Frequency Integrated Circuits, 2nd Ed., p. 9-11McNicol, Donald (1946) Radio's Conquest of Space, p. 157-162Fleming, John Ambrose (1910) The Principles of Electric Wave Telegraphy and Telephony, p. 476-483 Fleming, a consultant to Marconi, invented the valve as a more sensitive detector for transatlantic wireless reception. The filament was heated by a separate current through it and emitted electrons into the tube by thermionic emission, an effect which had been discovered by Thomas Edison. The radio signal was applied between the cathode and anode. When the anode was positive, a current of electrons flowed from the cathode to the anode, but when the anode was negative the electrons were repelled and no current flowed. The Fleming valve was used to a limited extent but was not popular because it was expensive, had limited filament life, and was not as sensitive as electrolytic or crystal detectors.