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 who used Armstrong’s inventions to fulfill his improbable vision of a broadcast empire with himself at the helm.
Table of Contents
David Sarnoff’s Early Life
Sarnoff and Armstrong
The Superheterodyne Method and How it Works
The Superheterodyne Receiver
David Sarnoff’s Early Life
Let’s start with David Sarnoff. Sarnoff was born in a shtetl in Minsk, Russia in 1891 to a poor Jewish painter (think: “Fiddler on the Roof” but with a painter). His father left Russia to improve their fortunes but when Sarnoff, his mother, and younger brothers joined him in NY, Sarnoff found that his father was unable to care for anyone.
At the tender age of 9, Sarnoff became the breadwinner of the family. Years later he recalled, “It was like being tossed into a whirlpool – a slum whirlpool – and left to sink or swim.” Sarnoff’s interest in wireless began when started delivering telegraphs for the Commercial Cable Company (he was actually looking for a job at the New York Herald, but he accidentally opened the wrong door).
Sarnoff then spent a precious $2 (out of his $5-a-week paycheck) to purchase a telegraph key and teach himself how to be a telegraph operator. Soon, he switched to working for the Marconi Company where he was occasionally allowed to send wireless messages when the main operators were sick.
One day in December of 1906, Guillermo Marconi himself visited his New York office and the bold fifteen-year-old introduced himself to the big boss. They had an instant report and at the end of the evening, Sarnoff became Marconi’s personal message boy, which mostly involved deliveries to Marconi’s many mistresses.
With Marconi’s patronage, Sarnoff became a junior telegraph operator. He was a local manager by 1912 when the Titanic sank and he then created a story where he was the first to hear of it in the US (he wasn’t), that he was the only operator (he wasn’t) that he stayed up for 3 nights straight to hear the news (maybe).
Due to his “heroics”, he was promoted to chief inspector and right-hand man to Marconi’s general manager: Edward J. Nally.
Sarnoff and Armstrong
The next year, in 1913, the 21-yr-old Sarnoff went to a demonstration from a wonder kid named Howard Armstrong who had figured out how to make triode vacuum tubes create smooth radio waves and amplify signals up to 5,000 more powerfully than before!
Sarnoff then invited Armstrong to use his devices at a Marconi Companies tower and they spent 13 hours straight in a frigid shack receiving signals from around the world.
Years later Sarnoff said, “Whatever chills the air produced were more than extinguished by the warmth of the thrill which came to me at hearing for the first time signals from across the Atlantic and across the Pacific”.
Sarnoff tried to get the Marconi company to buy the patent but they refused, possibly because the Marconi company was suing the original inventor of the triode vacuum tube for patent infringement and didn’t want proof that the triode tube could be so useful. Sarnoff (and Armstrong) were disappointed but they remained friends.
In May of 1914, Sarnoff witnessed a publicity stunt where Wanamaker’s department store sent music (and advertisements) wirelessly to a boat. The next year Sarnoff wrote his boss a proposition: “I have in mind a plan of development which would make radio a “household utility” in the same sense as the piano”. Nalley was interested but the higher-ups thought it was too radical, so that too came to nothing at the time.
The Superheterodyne Method and How it Works
In April of 1917, America joined WW1 and Sarnoff attempted to join the navy but was denied because his job was vital to the war effort (or, as Sarnoff thought, due to anti-Semitism). Meanwhile, Howard Armstrong volunteered for the army and was stationed in Paris working on radio systems.
In Europe, Armstrong heard that the Germans were using very short-wave (very high frequency) radio communication. However, radio equipment of the day did not work well at a high frequency above around 500 KHz. Months later Armstrong was watching a night bombing raid in Paris and he started thinking about the high-frequency radio waves produced by the motor ignition systems in the airplanes.
He wondered what would happen if he combined them with the high-frequency radio waves he could create in a receiver. Suddenly, he had “a radical solution” to the high-frequency problem! He knew that if two waves were combined with slightly different frequencies, then they produce a beat frequency (or intermittent frequency).
You might have heard this phenomenon if you have ever tried to tune a guitar without a tuner. When you make two notes that are at slightly different frequencies the result is a whomp whomp sound (the beat frequency). Armstrong realized that he could use this method to make lower-frequency waves out of high-frequency waves.
This was actually not a completely new idea, in 1905 a man named Reginald Fessenden had patented his plan to mix two signals so that the beat frequency was audible and you could hear it on headphones. Fessenden called it the heterodyne method (hetero = two different, dyne = force).
Armstrong’s insight was to combine signals to produce radio waves that were manageable but still above hearing, or supersonic. For that reason, Armstrong called his idea the superheterodyne method and if you look into any schematic of it, the basic idea is still the same: you take a radio signal and mix it with another signal at a slightly different frequency producing the beat frequency (or Intermediate Frequency), then you amplify and demodulate it (get the musical signal out).
For example, if you were listening to music at 900 kHz you could mix it with 950 kHz and then get a 50 kHz Intermediate frequency to amplify and interact with. One of the advantages of the superheterodyne method is that you can insure that the intermediate frequency is the same for any radio station so that the bulk of the equipment can be optimized to amplify that specific frequency.
When the war ended in 1919, Sarnoff’s boss formed a brand new company with the goal of having Radio communication in the hands of Americans. For this reason, he called it the Radio Company of America (or RCA). After a few years of wrangling, RCA managed to have the rights to basically everything important in Radio(over 2,000 patents).
Now Sarnoff could implement his scheme to make radio a “household utility”. Meanwhile, between 1920 and 1922, Armstrong received a whopping $735,000 in cash and 60,000 shares of RCA stock (making Armstrong RCA’s biggest stockholder) for his patents (even the ones that didn’t work well!). In addition, through all of these patent deals, Armstrong met Sarnoff’s secretary, Marion MacInnis, and fell in love.
The Superheterodyne Receiver
In February of 1923, Armstrong demonstrated a simplified version of his superheterodyne receiver and Sarnoff convinced his associates to scrap millions of dollars of past orders and focused on creating this new system. Still, the original plan wasn’t workable for mass-produced systems.
Finally, the exacerbated Sarnoff said, “What do I do now?” to which Marion replied, “Why don’t you call Armstrong?” Armstrong swooped in and, with a friend of his named Harry Houke, saved the day.
By February of 1924 RCA finally had its new receivers called Radiola AR-812, the “Rolls Royce of Radio” which only three needed three controls. So how did the Radiola’s controls work? In the Radiola, an adjustable capacitor was used where if you turned a knob more or less metal was put in parallel with each other, which changed how much charge it can hold.
If a capacitor is discharged through a coil it vibrates at a certain frequency, so by adjusting the capacitor you change the frequency of the oscillation. The first knob changed the capacitance of one capacitor, which controlled the frequency of resonance with the antenna; in other words, it determined what frequency the radio was listening to.
The second knob controlled the frequency of the oscillations that you were mixing with the incoming signal (once again, by adjusting a capacitor). This would tune this until the beat frequency was at the note that the rest of the electronics was maximized for: around 45kHz.
Finally, the last knob was for the volume. The Radiola used 6 vacuum triodes and you adjusted the volume by changing the resistance of a resistor next to a battery that was heating the filament of the triodes. The idea is if you lower the resistance of the resistor, then the current through the filament increases which heats the filament more.
As the filament gets hotter more electrons can flow from the filament, which increases the volume in the headphones. Whew! After that, RCA had basically a monopoly on radio.
Sarnoff then formed America’s first broadcasting network (NBC), one of the big 5 studios (RKO pictures), and reined as the president of RCA and boss of the radio, movie, and television world until he retired in 1970 when he was 79 yrs old.
Meanwhile, in 1924, Howard Armstrong was rewarded with another 20,000 shares of RCA stock and Armstrong wrote Harry Houke a personal check for $100,000 (to give you a sense of how much that is, the average income in 1924 was $3,400). Howard Armstrong celebrated by climbing the top of RCAs 400 foot tower and doing insane acrobatics.
Sarnoff crossly wrote, “If you have made up your mind that this mundane universe of ours is not a suitable place for you to be spending your time in, I don’t want to quarrel with your decision, but keep away from the property of the Radio Corporation.” Armstrong didn’t care, he was wealthy, and famous, and Marion had finally agreed to marry him.
He was literally and figuratively on top of the world. Which, of course, meant that he had only one way to go. Unfortunately, Armstrong’s descent would be both figurative and literal and Sarnoff would be significant part of his downfall.How Armstrong invented FM radio and how that was so amazing that it destroyed his relationship with Sarnoff and eventually his life and is next time on the Secret History of Electricity.