Marconi's first transmitter incorporating a monopole antenna. It consisted of an elevated copper sheet (top) connected to a Righi spark gap (left) powered by an induction coil (center) with a telegraph key (right) to switch it on and off to spell out text messages in Morse code.
In 1894 the young Italian inventor Guglielmo Marconi began working on the idea of building a commercial wireless telegraphy system based on the use of Hertzian waves (radio waves), a line of inquiry that he noted other inventors did not seem to be pursuing. Marconi read through the literature and used the ideas of others who were experimenting with radio waves but did a great deal to develop devices such as portable transmitters and receiver systems that could work over long distances, turning what was essentially a laboratory experiment into a useful communication system. By August 1895 Marconi was field testing his system but even with improvements he was only able to transmit signals up to one-half mile, a distance Oliver Lodge had predicted in 1894 as the maximum transmission distance for radio waves. Marconi raised the height of his antenna and hit upon the idea of grounding his transmitter and receiver. With these improvements the system was capable of transmitting signals up to 2 miles (3.2 km) and over hills. Marconi's experimental apparatus proved to be the first engineering-complete, commercially successful radio transmission system. Marconi's apparatus is also credited with saving the 700 people who survived the tragic Titanic disaster.
In 1896, Marconi was awarded British patent 12039, Improvements in transmitting electrical impulses and signals and in apparatus there-for, the first patent ever issued for a Hertzian wave (radio wave) base wireless telegraphic system. In 1897, he established a radio station on the Isle of Wight, England. Marconi opened his "wireless" factory in the former silk-works at Hall Street, Chelmsford, England in 1898, employing around 60 people. Shortly after the 1900s, Marconi held the patent rights for radio. Marconi would go on to win the Nobel Prize in Physics in 1909 and be more successful than any other inventor in his ability to commercialize radio and its associated equipment into a global business.
On Christmas Eve 1906, Reginald Fessenden used a synchronous rotary-spark transmitter for the first radio program broadcast, from Ocean Bluff-Brant Rock, Massachusetts. Ships at sea heard a broadcast that included Fessenden playing O Holy Night on the violin and reading a passage from the Bible. This was, for all intents and purposes, the first transmission of what is now known as amplitude modulation or AM radio.
In addition to its vast nighttime coverage area, the station boasts one of the largest daytime coverage areas in the country. It provides at least grade B coverage as far southeast as Chattanooga, as far northwest as Evansville, Indiana, as far west as Jackson, Tennessee and as far south as Huntsville, Alabama. At night, it can be heard in much of the eastern and central United States.
In the early years of aviation the first system of radio navigation was the Radio Direction Finder, or RDF. By tuning in a radio station and then using a directional antenna, one could determine the direction to the broadcasting antenna. A second measurement using another station was then taken. Using triangulation, the two directions can be plotted on a map where their intersection reveals the location of the navigator. Commercial AM radio stations can be used for this task due to their long range and high power, but strings of low-power radio beacons were also set up specifically for this task, especially near airports and harbours.
Amelia Earhart's Lockheed Electra had a prominent RDF loop on the cockpit roof.
Early RDF systems normally used a loop antenna, a small loop of metal wire that is mounted so it can be rotated around a vertical axis. At most angles the loop has a fairly flat reception pattern, but when it is aligned perpendicular to the station the signal received on one side of the loop cancels the signal in the other, producing a sharp drop in reception known as the "null". By rotating the loop and looking for the angle of the null, the relative bearing of the station can be determined. Loop antennas can be seen on most pre-1950s aircraft and ships.
