In 1906, Lee de Forest invented the “three-electrode Audion” cathode ray tube. However, by his own admission, saw no use for it in radio.
During his time at Columbia, Armstrong worked with Audion tubes and realized they could recycle a radio signal, amplifying it by sending it repeatedly through the tube. Further, by reversing the process, Armstrong could amplify the reception of a radio signal as well.
de Forest, by his own admission, was focused on using the tube to amplify telephone signals. He had no interest in radio signals.
Companies licensed the tube from both Armstrong and de Forest then fought one another. AT&T initially supported de Forest whereas RCA supported Armstrong. At one point, due to licensing, Armstrong was the largest shareholder in RCA. But RCA eventually decided to side with de Forest, abandoning Armstrong, who continued his lawsuits.
Litigation continued with de Forest losing all the early rounds. Eventually, he appended his initial tube patent to focus not specifically on radio waves but, more generally, on electricity, and won.
The definitive Supreme Court opinion issued in 1932. The third Supreme Court decision in the case, issued two decades after the patent applications were filed, suggests the Court was exhausted by the ongoing litigation. The Court reasoned the patent holder is whomever first makes a thing, not who innovates various uses from it (a position that would change, later, as the law and the Court changed). There is an undertone that Armstrong and de Forest should have settled the case long ago and cross-licensed the technology.
During this time Armstrong extended uses for the tube, inventing FM that had much cleaner sound than AM. Armstrong offered the FM patent to his old friend Sarnoff, who had risen to become CEO of RCA and creator of the National Broadcasting Corporation. Sarnoff saw FM as an enormous breakthrough but, during the Depression, did not think buyers would pay for new receivers and did not want to incur the expense of new transmitters.
RCA had first right of refusal for the technology but failed to strike a deal. Eventually, Armstrong commercialized his radios, selling $5 millions of RCA shares and created a company to compete with RCA and its broadcasters. His company had $2M/year in revenue but was spending as much on expansion.
With never-ending patent battles, and no money left ー and obsessed on the court cases ー his wife of 31 years left him on Jan. 1, 1954. He soon after committed suicide at age 63. The case did not definitively end until Oct. 9, 1967, after a Supreme Court forced the last settlement, by Motorola.
Besides FM radio, Armstrong also invented the Superheterodyne receiver, that used a series of filters to more accurately and cleanly tune into a radio station.