Harris RF-7800W Student Manual page 7

1917 - The U.S. Army started using radios as they entered WWI.
1924 - Edward Appleton proved that the blanket of electrically charged (Ionized)
particles in the earth's atmosphere (Ionosphere) was capable of reflecting radio
waves. In the late 1920's, Scientists applied this theory and developed ways to
measure and predict the refractive properties of the ionosphere.
1942 - Hedy Lamarr and George Antheil received a patent for Frequency Hop.
After it was patented, the government used frequency hop as top-secret
communications until the 1980's. Today, this secure communications method is
used in SINCGARS radio systems
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