[ARC5] How Edwin Armstrong invented the superhet

[Christopher Bowne]

Surprised that people are forgetting how relatively easy it is to detect CW (and when it came along, SSB) with a regenerative detector. As others has mentioned, regenerative detectors were just about universally used with simple one or
two stages of RF amplification ahead of the regenerative detector, and even with no amplification stage ahead of the detector.  I was puzzled by the comment that one could not DF with a TRF receiver.  Regardless of whether or not an IF stage was used in
a superheterodyne or whether the detection was performed at the signal frequency, DF, at least by use of a steerable loop antenna to determine the null in a signal level of a transmitter of interest can either be done on a CW signal with either a regenerative detector or BFO operating at either the signal frequency or an IF frequency.  Low frequency radio beacons used to, and where still active, send their station ID and sometimes long dashes using MCW tone modulation of a steady carrier so they could be received by receivers that did not have a BFO or regenerative detector.

The 1930s vintage design RAK VLF/LF
receiver is a TRF receiver with two stages of TRF amplification and a regenerative detector.  It has very high selectivity over its entire 15 kHz to 600 kHz, obviously hugest at the lowest frequencies, and comparablen contemporary superhets using the common nominal 455 kHz IF frequencies of the day.

It’s follow on RBA in WW2 was also a TRF was also a TRF design, although I believe it used a tracking RF oscillator at the signal frequency for a BFO.  I believe the injection level of the RBA was adjustable to prevent overwhelming the incoming signal that was to be detected.

Chris AJ1G Stonington CT

Sent from my iPhone

> On Jan 7, 2023, at 11:44, Al Klase <ark at ar88.net> wrote:
> 
>  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
> 
> 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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