"Radio Sweeping Country: 1,000,000 Sets in Use"-Variety headline, March 10, 1922
The history of commercial radio in the United States began when the Department of Commerce licensed KDKA Pittsburgh on Nov. 2, 1920, just in time for the Harding-Cox election results. Within a few years hundreds of transmitters were broadcasting to millions of receivers, as virtually every household in America had either bought or made a simple crystal set. Those early sets needed no power source other than radio waves picked up by wire antennas (the longer and higher the better), allowing farmers to hear the latest stock prices and Los Angeles sports fans to catch the first Rose Bowl on New Year's Day 1923.
I thought of this last Saturday listening to the Shopping Show on KINS-980 AM. We live in Old Town two miles from the KINS transmitter (next to Eureka slough). Magically, enough of the station's 5,000 watt output reached my 100 feet-long wire antenna strung outside our apartment to - just barely - power a crystal set: no batteries, no source of power other than the radio waves themselves.
Operation of a basic crystal set starts with the-
Antenna, which picks up amplitude-modulated (AM) radio waves and converts them to electric current for the-
Tuned circuit, which selects one signal out of all the radio frequencies received by the antenna by creating resonance between two energy-storing components: a variable capacitor tuner, which stores electrons within its dielectric insulator; and a coil inductor, which stores electrical energy as a magnetic field. When the resonance period matches that of the radio carrier wave, a signal is sent to the-
Rectifier (or detector), which strips out half of the radio wave, leaving a rectified radio-frequency (RF) signal. The original crystal sets were so named because their rectifiers were "cat's whiskers" (thin wires) touching crystals of galena (lead sulfide). Karl Braun discovered the rectifying property of certain crystals in 1874, allowing reception of "spark-gap" signals from transmitters hundreds of miles away. Nowadays, hobbyist crystal sets use semiconducting germanium diodes, which are far more reliable than galena crystals. The rectified RF signal then goes to the-
Earpiece. Moving-iron earphones were the most expensive component of a crystal set. Earphones converted the rectified signal into sound waves, filtering out the high-frequency "carrier" wave in the process. Hobbyists now use smaller and lighter piezoelectric earpieces. (Piezoelectric crystals bend when an electric field is applied-and conversely, they generate a current when bent, as in gas-grill spark igniters.)
The demise of crystal sets began with the invention of "Audion" vacuum tubes (precursors of transistors) which amplified the weak signals sufficiently to power loudspeakers. They enjoyed a brief revival during WWII after allied soldiers at Anzio were forbidden from using personal radio receivers, because the Germans could pick up signals from the sets' superheterodyne oscillators. Some inventive GIs realized that a pencil lead sitting on parallel razor blades made for a pretty good rectifier, and "foxhole radios" were born.
The signal generated by a crystal set is tiny, taking full advantage of the incredible sensitivity (10-16 watts/cm2) of the human ear. Devoted "xtal" aficionados on the West Coast can even pick up shortwave broadcasts from the BBC in London-a low-tech triumph in our high-tech era.
Barry Evans' (firstname.lastname@example.org) communications expertise also extends to the "two-cans-on-a-piece-of-string" system. Field Notes (the book) is available at Northtown Books and Eureka Books.
CAPTION: Crystal set from a Ramsey Electronics "CS-1" kit: coil at top, variable condenser at bottom, piezoelectric earpiece on right. Note that either the diode (above the crystal) or the crystal can be used for rectification. Inset: "Cat's whisker" in contact with a sweet spot on the galena crystal.