Howard Armstrong, David Sarnoff And the Influential Superheterodyne Receiver

The superheterodyne method is where two signals are mixed to make a lower frequency signal that is easier to deal with, and, according to Wikipedia, “virtually all modern radio receivers use the superheterodyne principle.” But where did it come from and how does it work and why (and how) did it usher radio broadcasting into the mainstream?  This is the story of Howard Armstrong, the brilliant engineer who thought of it in the middle of a bombing raid, and David Sarnoff, the determined immigrant ...