[ARC5] How Edwin Armstrong invented the superhet

[Al Klase]

OK, let me take a whack at this.
We have to go back to the WWI era.  Most radio operations were taking 
place well below 1 MHz. Communication receivers we generally crystal 
sets connected to large wire antennas.  There was a need for 
direction-finding on enemy transmitters.  That required a loop antenna, 
but the output was too low to work with a crystal set or even a one-tube 
regen.


The solution was multi-tube amplifiers.  The French had a leg up here 
having produced the first mass produced tube, the "TM" valve. See also 
R. J. Round and the Brit DF "B-Stations."




French amp among the Armstrong artifacts at the former Ft. Monmouth museum.

Armstrong was working at the Division of Research and Inspection in 
Paris.  This was in effect an AT&T laboratory.

To get amplification at HF frequencies, where the existing tubes didn't 
have much gain, he leveraged his knowledge of heterodyne frequency 
conversion gained from his work with the regenerative receivers.  He 
built a tunable frequency converter to feed a French amplifier operating 
at about 100 KHz.   The circuit was originally called the supersonic 
(now we say ultrasonic) heterodyne circuit, where the incoming signal in 
not converted to audio, but to an intermediate RF frequency.

Hope the pix come through.

Al

ARK Sig Block Al Klase - N3FRQ
Jersey City, NJ
http://www.skywaves.ar88.net/
On 1/7/2023 7:37 AM, releazer at earthlink.net wrote:
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