Armstrong Regen Patent

Armstrong’s Regenerative Radio Patent

By Doug Criner

A recent series of U.S. postage stamps commemorating important electrical inventors included a stamp honoring Edwin H. Armstrong. The stamp identified FM radio as his invention. True enough, but Armstrong’s inventions of regenerative radio and of the superheterodyne circuit are arguably at least as significant as his FM work.

His first major invention, patented in 1914, was titled a "wireless receiving system," now know as regenerative radio. That invention improved the amplification of radio signals by feeding the signal from the detector tube’s output back to the input, causing further amplification.

Armstrong applied for his regen patent in 1913, the year before the patent was granted and the same year he graduated in electrical engineering at Columbia University. It seems that much of his work on the regeneration concept was completed as an engineering student at Columbia. Later, after a stint in the U.S. Army Signal Corps during WWI, he returned to Columbia as an instructor.

The Regen Patent

It is interesting to review old patents to possibly gain insight into the original thinking of the inventor and to learn the basic, underlying principles of the invention. Unfortunately, because patents are filtered through the hands of lawyers, they are sometimes disappointing as technical documents.

The 1914 regen patent may be viewed here: http://www.pat2pdf.org/patents/pat1113149.pdf

Armstrong’s explanations in the patent are much more complicated than needed today to explain the principle of regeneration. The patent does not use modern engineering vocabulary such as feedback, mutual inductance, inductive coupling, tickler coils, or even regeneration.

The patent includes six schematic drawings, depicting different circuit arrangements. None of them illustrates what ultimately became the most conventional approach for designing regenerative receivers: essentially a tickler coil in the plate circuit, inductively coupled to the antenna coil in the grid circuit of the detector. The patent’s Figure 1, one of Armstrong’s six schematics, shows the complexity of his concept compared to conventional regen circuits incorporated in many 1920s-era receivers, such as the Crosley Model 50:

One of Six Schematics In Armstrong's Regenerative Patent

Typical "Modern" Regen Receiver Schematic (© Wikipedia)

The patent shows that Armstrong’s essential idea was to couple the output of the detector’s plate circuit to the input, or grid, circuit. However, none of the six schematics shown in the patent appears very practical from the standpoint of commercial radio construction—and are unnecessarily complicated, with a relatively large number of components, including many variable capacitors and inductors. The patent claim does not indicate any mathematical analysis of the circuits, although Armstrong may have performed such calculations.

Each of the patent’s six schematics show a tuned circuit in the grid circuit of the detector. That certainly makes sense; the radio station’s RF must be tuned. But, several of Armstrong’s schematics also show a resonant circuit on the plate circuit (called the "wing" circuit in the patent). When I first noticed this, I could not figure out why the plate circuit would be resonant.

Further study revealed an old circuit, used mostly in the 1920s, called a "tuned-grid, tuned-plate" (TGTP) oscillator. (You can run a search on that term.) In Armstrong’s patent, the idea was to resonate the plate circuit at the same frequency as the tuned RF entering the grid, and feed back the RF signal to the grid circuit, presumably through the Miller grid-to-plate capacitance of the detector triode.

Conclusions

My study of the patent leads me to the following thoughts:

1. Armstrong’s essential concept was positive feedback to increase amplification. Whether he "stumbled" on this or thought of it as a theoretical exercise is difficult to know. Perhaps Armstrong’s biographies, which I have yet to read, shed light on this question.

2. Once he grasped the idea of regeneration, he seems to have experimented with various ways to achieve it. It seems as though his approach was essentially trial-and-error rather than an analytical, rigorous design.

3. In experimenting with regeneration, Armstrong often would have encountered the characteristic audio tone produced when the circuit begins to oscillate and beats against the incoming signal. This is the principle of heterodyning. Perhaps his work on regenerative radios formed the inspiration for his later development of the superhet receiver.

Epilog

The regen patent shows the impact of lawyers, and that impact isn’t very helpful, technically. One wonders about the time, money, and exasperation spent by Armstrong, an electrical engineer, in getting his regen patent issued. In subsequent years, he was to spend untold months and years dealing with the legal system to adjudicate this and other patents and to negotiate contractual and licensing arrangements with Westinghouse, RCA, and others.

Armstrong received seemingly generous payments for his patent rights from Westinghouse and others, but when he died, in 1954, by suicide, he was in dire financial straits.

Those entities that wound up controlling his patents, particularly Westinghouse and RCA, used the patents largely to discourage competition within the growing radio industry. In my opinion, a 3-tube regenerative radio can out-perform many of the 5-tube tuned-radio-frequency (TRF) sets, that dominated the 1920’s consumer market as the only way for competitors, without a patent license, to build radios. A pity—causing untold millions of unnecessary dollars to be spent by radio purchasers in the 1920s and early ’30s.

I wonder, hypothetically, if Armstrong had it to do over, whether he would give away his patents to the public domain and spend his time in the lab at Columbia University. I, personally, think he would have gone back to the lab, inventing what wonderful things we might never know.