AM, FM & Armstrong

Andre Kesteloot andre.kesteloot at
Mon Oct 31 19:52:27 CST 2005

On Oct 30, 2005, at 9:13 AM, BobD1300 at wrote:

> I've been doing some reading about FM and Edwin Howard Armstrong. I  
> found the
> following articles quite interesting. He contributed, in so many  
> ways to the
> communications we enjoy every day and he really didn't get enough  
> credit during
> his lifetime. De Forest and Sarnoff made life very difficult for  
> him. There
> are four different articles here with some details are repeated...

And quoting from one of the articles:

> He enrolled in electrical engineering at Columbia, and in 1913,  
> while still
> an undergrad, made his first great discovery, regeneration. At that  
> time Lee De
> Forest's audion tube, the first triode vacuum tube, had been around  
> for several years.
> No one had done much with it, though, because De Forest himself
> didn't understand how it worked and because it was a really poor  
> amplifier.
> Armstrong discovered that the gain of a triode amplifier could be  
> enormously
> increased by feeding some of the amplifier output back into the  
> input, i.e. by using
> positive feedback.

Yes, indeed!  Here's an excerpt from my draft history of radio "from  
Joseph Henry (1831) to E. Howard Armstrong (1921)," which I never  
completed.  This is on Armstrong's regenerative circuit.  I wrote it  
after reading this:

  E Howard Armstrong, “The Regenerative Circuit”, a monograph  
published in The Legacies of Howard Armstrong,  Radio Club of  
America, 1990.

The regenerative, or feedback, circuit was “the result of a streak of  
luck that comes once in a lifetime.”  Its discovery began when Howard  
decided to find out for himself how DeForest’s audion worked as a  
signal detector.  When he started, the audion was “a device whose  
action was clouded in the mystery of DeForest’s gas ionization theory.”

Howard found that placing a capacitor across the headset terminals  
sometimes gave an increase in signal strength, suggesting that high  
frequency currents were flowing in the plate circuit.  Following the  
axiom that “whenever there are high frequency signals, try to tune  
the circuit,” he put a tunable inductance in the plate circuit and  
started playing with it, whereupon “various things began to happen.    
As the plate inductance was increased, the signals were boosted in  
strength to an intensity unbelievable… until suddenly the  
characteristic tone of [the Marconi press station he was monitoring]  
disappeared, and in its place, was a loud hissing tone… recognizable  
only by the characteristic swing of the messages transmitted.”

“A number of things contributed to the suspicion that the hissing was  
due to the production of local oscillations by the system…  Once  
[that] was apparent…, the explanation of another phenomenon became  
plain.  I had observed… a whistling note would frequently appear in  
the telephones, which could be varied by adjustment of the receiving  
apparatus… particularly when listening to a wireless telephone  
station.  [It] became apparent the system was acting as a heterodyne  

“Measurements of the signal energy in the telephone receivers show  
that an amplification of from 100 to 1,000 results from the  
regenerative action, the value depending on the strength of the  
incoming signals, the greater amplification being obtained on the  
weaker signals.”  Also, “the selectivity of the system is greatly  
increased, the gain in selectivity becoming more pronounced the lower  
the damping of the incoming wave.”

André Kesteloot, N4ICK, noted in another reference that “This  
positive feedback increases the gain at the frequency of interest and  
narrows the bandwidth of the circuit, leaving gain/bandwidth product  

Armstrong concludes: “Three operations are therefore carried on  
simultaneously in the non-oscillating state:
  •  the high frequency currents are regeneratively amplified,
  •  the selectivity of the system is increased, and
  •  the amplified high-frequency currents are rectified and
         converted into currents of telephonic frequency.”

You may recall that photo I took of an early De Forest Triode laid on  
top of Con Concannon's amateur radio license from 1920.  You may not  
know that Con, originally licensed as 6RQ, died last February just  
short of his 99th birthday.  You can see it again here:

Last week his son, Chuck, dropped by to say hello and to present me  
with that little triode as a remembrance of Con.  It's sitting on my  
bookshelf awaiting attention.  I'm thinking about mounting it in a  
deep frame with, say, a picture of Con (or the license if Chuck is  
willing to part with it) in the the background.  It really is a  
treasure, with its little pig-tails sticking out each end.  One pair  
is for the filament, which looks like it belongs in an Edison light  
bulb, and the other pair of leads go to the plate and grid inside the  
straight cylindrical tube.

Thanks for the articles on Armstrong.


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