Harnessing The Airwaves: From Hertz To Armstrong

In the late 1880s Heinrich Rudolf Hertz (1857-94), a German physicist, proved an earlier theory of the English physicist James Clerk Maxwell that electromagnetic waves could move through space. To do this, Hertz attached copper plates to two separate metal spheres. When he generated an alternating current between them, sparks jumped from one sphere to the other. As crude as it was, this device served as the first transmitter. For a receiver Hertz held a loop of copper wire a few feet away. When a spark jumped between the original two spheres, another, smaller spark jumped through this receiver. Before his death, at thirty-seven, Hertz was also able to measure the length and velocity of electromagnetic waves and determine that they had similar properties to waves of heat and light. His machinery became the basis of early spark-gap transmitters.

In the summer of 1896 a young Italian inventor named Guglielmo Marconi demonstrated an improved Hertzian transmitter linked to an elevated antenna system, which made it possible to send radio waves several hundred yards. For a receiver, Marconi employed a coherer—a tube of silver and nickel filings that exhibited lowered electrical resistance and “cohered” when subjected to currents from radio waves. The receiver was connected to an apparatus that could print the dots and dashes of Morse code. In December 1901, Marconi sent a signal from Poldhu, Cornwall, England, to a receiving site in Newfoundland. In 1902, Marconi developed a magnetic detector that used a band of iron wires inside a coil to pick up signals. The “Maggie,” as it came to be called, was considerably more reliable and faster than the coherer. It was widely adopted by the Marconi Wireless Telegraph Company.

In 1906, in Massachusetts, Reginald Fessenden successfully impressed voice and music information on a radio wave by using a high-frequency alternator as a transmitter. The alternator’s great advantage was that it generated truly continuous waves, unlike the earlier transmitters. This device was improved further by Ernst Alexanderson of the General Electric Company. By 1918 the alternator was the state of the art in radio transmitting appliances.

Also in 1906, two Americans—Henry C. Dun woody and Greenleaf W. Pickard—independently developed the first crystal detectors. Consisting of a wire impressed against a piece of crystal, the device removed the audio-frequency component from a radio signal and allowed it to be picked up by a set of headphones. In the early broadcasting era this “cat’s-whisker” galena detector was fairly popular.

In 1904 John Ambrose Fleming of England had developed a diode, a two-electrode vacuum tube capable of detecting wireless signals, based on an effect observed two decades earlier by Thomas Edison. While perfecting the incandescent bulb, Edison had noticed that a weak current would pass across a space between the bulb’s filament and a metal plate placed above it. Fleming discovered that the bulb-plus-plate would pick up a radio signal and transform it into pulsed direct current, which then could be used to indicate telegraph signals.

Two years later Lee De Forest, a Yale-educated entrepreneur and inventor, placed a zigzag grid between the filament and plate of a two-element tube and discovered that a small voltage change on the grid would create a much greater one in the plate circuit. This made possible the amplification of weak radio signals.

In the regenerative detector developed by Edwin Armstrong between 1912 and 1914, a portion of the amplified signal in the plate circuit was fed back to the grid in phase with the incoming signal, thereby reinforcing it.

Armstrong’s superheterodyne circuit (from the Greek heteros , meaning “other,” and dyne, “beat”), the basic component of all radios today, amplified weak signals more than ever before. Each block in the first diagram below represents a stage in the process. The incoming signal is mixed with a second frequency, which remains constant. The resultant signal is then amplified and sent to a detector, where it is converted to an audio frequency and sent to an audio amplifier.

The receiver in Armstrong’s frequency modulation system consisted of the same circuits found in the superheterodyne circuit, plus additional limiter and discriminator circuits. The signal passes through a mixer and intermediate amplifier (as is the case with the superheterodyne) to a limiter circuit that cuts out any amplitude variations caused by static. The FM wave is converted into an AM wave in the discriminator and then (again as in the superheterodyne) passes through the detector to the audio amplifier.