Transatlantic Radio Communication

Spark gap

• In 1901, Guglielmo Marconi built a spark gap transmitter in Cornwall, England, and a receiver in Newfoundland, Canada. In December of that year, he reported reception of a signal consisting of the letter "S" in Morse code; this was the first-ever claimed transatlantic radio communication.

  The spark gap transmitter is simply a coil of wire with a gap in it, to which a momentary electrical pulse of sufficient voltage to jump the gap is applied. The length of wire in the coil determines the frequency of the transmitted signal.

  For reception of the signal, Marconi used a device called a "coherer," in which a pair of electrodes in a glass tube are separated by metal filings. A battery is connected to the electrodes, as are the terminals of a large receiving antenna, and a set of headphones is placed in the circuit from the battery. Received signals change the electrical resistance of the metal filings, and the resultant changes in current may be heard as clicks in the headphones.

Improvements in receiver technology

• During the early 1900s, several improved radio receivers were developed, including regenerative and superheterodyne receivers. These receivers had greater sensitivity and selectivity than earlier designs, which improved the ability to receive weak signals.

  In a regenerative receiver, a vacuum tube oscillator is adjusted to the frequency of the received signal, so that it converts to audio and amplifies the changes to the signal that occur when the signal is modulated or keyed on and off. The superheterodyne differs by mixing the outputs of two or more oscillators that add or subtract to arrive at the frequency of the received signal, then amplifying the modulated changes in the signal.

High-frequency radio

• Early amateur radio operators were restricted to frequencies above 1.5 MHz, which were considered to be worthless. Through experimentation, these operators soon discovered high-frequency skywave propagation, in which signals were reflected by the ionosphere. The ionosphere is a region of the upper atmosphere that contains ionized particles, which reflect radio waves below a frequency that varies based on the season, time of day and radiation from the sun. This allowed more reliable transatlantic communications than the low-frequency groundwave propagation that had been used earlier.