1865 - James Maxwell Theorizes Radio Waves
  • Humans can hear sound waves with their ears, and light waves with their eyes.

  • In 1865, English scientist Maxwell first theorized that magnetic fields and electric fields could combine to form to form self-sustaining radio waves that can travel over distances.

  • This was just a theory, as we cannot see or hear a radio wave with just our ears or eyes.

1888 - Heinrich Hertz Proves Radio Waves
  • On a laboratory table, he created a small spark using a simple spark generating apparatus.

  • On the same table, he had a separate circular wire loop with a tiny gap in it.

  • When he adjusted the gap to just the right distance, a spark (signal) would jump this gap when he generated a spark on the first apparatus.

  • He showed that the signal possessed all of the qualities of the electromagnetic waves Maxwell had theorized.

  • He also proved that the radio waves travel at the speed of light (186,00 miles per second).

1897 Guglielmo Marconi Starts Making Radio Waves Useful
  • Italian Marconi becomes interested in developing a "wireless telegraph. 

  • He succeeds in making a spark transmitter with a telegraph key to tap out Morse code (short sparks and long sparks that represent letters and numbers).

  • His receiver uses a radio wave detector, called a "coherer".  When a radio wave is received, tiny metal particles in a glass tube cling (cohere) together, acting like a swtich to ring a bell.

  • The English Navy becomes very intersted in the invention for possible use in communicating with ships at sea.

  • Marconi continues experimenting.  In the early 1900's, he succeeds in sending a signal across the Atlantic Ocean.

1906 - Lee de Forest Invents the Revolutionary Triode Vacuum Tube
  • The first vacuum tube had two elements inside: a "filament" that generated electrons, and a "plate" that received them as a strong current.

  • American engineer de Forest conceived the idea of putting a third element (the "grid") between the filament and plate.  He called his new tube an Audion.

  • The weak radio or audio signal is connected to the grid, which would vary the stronger current between the filament and plate to exactly match.  In other words, the vacuum tube would act as a powerful amplifier, enabling the use of loudspeakers so that more than one person with a headset could listen.

1914 and later - Edwin Armstrong, Inventor Extraordinaire
  • Armstrong  advanced the understanding of how deForest's Audion tube worked.  This led him to try feeding back some of the amplified signal back to the grid, over and over again, causing it to be amplified many thousands of times.  Around 1914, he patented this idea, which he called "regeneration."  It made possible the development of inexpensive radios for home use that could replace the crystal radios that could only use a headset.

  • He also discovered that, when too much of the plate signal was fed back to the grid, it could cause the tube to oscillate at a chosen radio frequency - becoming a transmitter!  This enabled development of practical, reliable radio transmitters.

  • During World War I, Armstrong developed another new radio technology which he called "super heterodyne."  It was more sensitive (able to receive weak stations) and more selective (able to separate stations with close frequencies) than previous radios.  It is a very good design and was used for many decades after its development.

  • The static-free FM radios we use today?  FM is another invention of Edwin Armstrong, who also pioneered its use when others continued to promote static-prone AM.

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1920s - Radio Goes Mainstream and Many Advancements Occur
  • Numerous inventors and companies create new circuit designs and improved components.  Click here for a summary.