Who Invented the Superheterodyne?

By Alan Douglas

Originally published in the Proceedings of the Radio Club of America, Nov. 1990, Vol.64 no.3
"The Legacies of Edwin Howard Armstrong."

Of E.H. Armstrong's four principal inventions—regeneration, superregeneration, the superheterodyne, and frequency modulation—the superheterodyne has always seemed one of the least controversial.  "Everyone" knows that Armstrong invented it. He devised it during World War I, patented it shortly afterward, sold his patent to Westinghouse who cross-licensed RCA and the radio industry, and that was that. Some Frenchman named Lévy claimed he was first, but whoever heard of him?

All of Armstrong's inventions were involved in controversies. Lee de Forest got legal credit for regeneration (and others might have, with better counsel, notably Robert Goddard [1]). John Bolitho had discovered much of the superregeneration principle before Armstrong, who prudently bought Bolitho's patent before negotiating with RCA. FM had been gathering dust on theoreticians' shelves for decades before Armstrong took it up, but as soon as he had made it worth fighting over, he was beset from all sides. So, if the superheterodyne was his most valuable invention—and it is fundamental to essentially every radio and television made since 1930—it would be surprising if Armstrong had not had his priority disputed.

The dispute ended in defeat. In 1928 Armstrong lost his superheterodyne patent in an interference proceeding within the Patent Office, when most of its claims were transferred to a Lévy patent owned by AT&T. Since AT&T was in the same patent pool as Westinghouse and RCA, this transfer had no effect on the industry and attracted little notice [2]. Lévy did not publicly press his claims outside of France, and even there, Armstrong was often credited with the invention.

In view of this apparent misappropriation of credit, it is worthwhile to take a careful chronological look at the superheterodyne, to see precisely how it was invented and how it was introduced into practice.

The Heterodyne

First came the heterodyne. The principle of "beats" or difference tones between simultaneous audio pitches was well known since antiquity, but Reginald Fessenden in 1901 was the first to apply the principle to radio transmissions [3]. Originally both radio frequencies were to be transmitted, received with two antennas, and combined in a detector. Later a local oscillator was substituted for one of the transmitter-receiver combinations and the heterodyne as we know it was born. Fessenden himself coined the term, from the Greek heteros (other) and dynamis (force).

For years Fessenden was the lone proponent of continuous waves, and possessed the only such transmitter, the radio-frequency alternator later perfected by E.F.W. Alexanderson of General Electric. Cyril Elwell followed with his development of the arc generator, the basis of the Federal Telegraph Co. For its detector Federal interrupted the incoming signal at a radio-frequency rate with a rotating commutator. The heterodyne worked better but had to await the development of suitable low-power RF sources: small alternators, arc generators, or vacuum-tube oscillators.

Heterodyne detection provided an apparent amplification of the received signal, an important effect since at first no other method of radio-frequency amplification was known (the Audion was used only as a detector for several years after its 1906 invention, not as an audio- or radio-freqency amplifier).

From 1912 to 1915 radio engineers Hogan, Cohen, Latour and Liebowitz attempted theoretical explanations of heterodyne amplification, variously obtaining results of 1.27, 2, or 4 times the ratio of local oscillator strength to received signal strength. Not only did the numbers differ, but there was also disagreement on whether it was true amplification or a result of increased detector efficiency. The discussions in the I.R.E. Proceedings became more and more heated, as the mathematical expressions lengthened. So Armstrong, ever distrustful of mathematics, set out to discover the truth for himself.

With a permanent teaching position at Columbia University as Michael Pupin's assistant, Armstrong had the full use of a well-equipped engineering laboratory. He presented his experimental findings to the Institute of Radio Engineers in October 1916 [4], more or less corroborating Liebowitz's mathematics. Heterodyne amplifications of 100 or more were measurable, which in turn could be increased fifty times in a regenerative circuit connection. But most importantly, by the time Armstrong had finished his work, he was intimately familiar with the practical handling of heterodyne circuitry.

The War

When the United States entered World War I, Armstrong joined the Army Signal Corps and was posted to France. The Division of Research and Inspection had just been created, to evaluate existing apparatus and propose changes, and to inspect equipment being manufactured in Europe for the American Expeditionary Force. Captain Armstrong was placed in charge of the Radio Group of the Research Section.

On his way to France, stranded for three days when fog closed the Channel, Armstrong had taken the opportunity to visit London. Stopping at the Marconi Co. offices, he met Captain H.J. Round, for the war's duration in charge of a chain ofwireless direction-finding stations for the Admiralty. Here Armstrong came close to some of the war's best-kept secrets. For, using information supplied by these listening stations, the Admiralty could not only keep continuous track of many German ships and submarines, but had also broken the German ciphers and could read nearly all the messages [5].

What most interested Armstrong, however, was Round's short-wave equipment. The Germans used low-powered "buzzer" sets for shipboard intercommunication while at anchor in their home ports, confident that they could not be heard more than a few miles on their 200-meter (1.5MHz) wavelength. Round's multi-stage amplifiers however could pick them up, and fix their positions. A small change in position could mean that a ship had moved downriver, getting ready to put to sea (the largest naval battle of the war, the Battle of Jutland, was brought about because of a 1½-degree change in bearing of the German flagship). With advance warning of German sorties, the British could not only ready their defenses, but ideally hoped to bring the German fleet to action against their own superior forces.

Round had been able to make such short-wave amplifiers operate by designing his own vacuum tube, the V24, with very low interelectrode capacitances. One of his standard amplifiers used eleven V24s in cascade, for a total gain of 2000, and where more amplification was needed, two amplifiers could be connected in tandem. Some direction-finding stations ran as many as 130 tubes, and used prodigious numbers of spares, not to mention battery power, but to the Admiralty the results were well worth the expense [6].

Round's direction-finding amplifier. Coils A1 and A2 were
connected to large stationary single-turn loop antennas, 90 degrees apart.

Such quantities of V24 tubes would never be available to the army in France, and no American tube was remotely suitable for this RF-amplifier service, but Armstrong sent the information back to the Signal Corps laboratories for future development. For the moment, the AEF settled on the latest French design by Marius Latour: a four-tube, six-stage model L-3. Armstrong's problem was immediate— the German army was rumored to be using very short waves for front-line communication, waves too short to be picked up on the French receivers.

Since one regenerative detector tube would have performed nearly as well as Round's multi-stage creations, one may wonder why this was not done. After all, American hams had been operating on short waves for years (although, to be truthful, very few were anywhere near the legal boundary of 200 meters). Paul Godley's "Paragon" receiver (grid and plate circuits tuned with self-resonant variometers, for regeneration) was well known. H.J. Round gave two explanations in 1920: the multi-stage amplifiers were less microphonic than a single tube, and an oscillating detector directly coupled to an antenna would have wiped out other direction-finding stations trying to pinpoint the same signal [7].

Armstrong's Paris Laboratory (U.S. Army Signal Corps photos)

One more bit of knowledge is needed to set the stage for Armstrong's discovery: the heterodyne was not considered suitable for spark reception. Spark signals were somewhat like present-day AM, in that they were modulated at an audio rate and occupied a large bandwidth. Tuning a heterodyne detector to a spark signal's center frequency was out of the question. Neither the signal nor the local heterodyne oscillator had anything like the necessary stability, and in addition there was no obvious way to tell when zero-beat was achieved. A mis-tuned heterodyned spark signal had a raspy hissing sound, much more difficult to read than an audio tone, and not easily distinguished from interfering signals or atmospherics.

The Invention

As Armstrong later explained it, his conception of the superheterodyne was the result of three chance occurrences. First, he knew all about heterodyne circuitry. Second, his London meeting with H.J. Round had set him to thinking about reception of weak high-frequency spark signals. As he related in 1943,

"The third link came months later as I happened to be watching a night bombing raid and wondering at the ineffectiveness of the antiaircraft fire. I may say that night bombing was not very dangerous in those days, either for the man on the ground or the man in the airplane. Thinking of some way of improving the methods of locating the position of the airplane, I conceived the idea that perhaps the very short waves sent out from them by the motor ignition system might be used. The unique nature of the problem, involving the amplification of waves shorter than any ever even contemplated and quite insoluble by any conventional means of reception, demanded a radical solution. All three links of the chain suddenly joined up and the superheterodyne method of amplification was practically forced into existence. Not one link in the chain could have been dispensed with. This, I think, is the only completely synthetic invention I have ever made." [8]

This happened in Paris in March 1918, as he was walking back to his apartment after watching the air raid. Years later he swore he could find in the dark the particular street where the thought had come to him, if set down in the city blindfolded.

The signals were too weak to be detected directly, and had to be amplified. The heterodyne would amplify them, but would lose the natural spark tones. Armstrong already possessed a tuned amplifier and detector, for long waves. His flash of insight was to use the heterodyne to bring the short-wave signals down to the range of his long-wave amplifier. This heterodyning, it turned out, did not alter the modulation content of the original spark signals, but preserved all the original sidebands and therefore the characteristic tone that allowed each spark transmitter to be distinguished aurally from others. The final detection could now be done by rectification, in the normal manner [9], because there was a large amplified signal available.

>

One half of Armstrong's first model, as it was displayed in the Army Communications Museum
at Ft. Monmouth, New Jersey in 1980. The right-hand box, the headphone, and the VT2 tubes with
the caps resting on them, are all incorrect, but the set has been displayed this way since at least 1954.
This model, with its four-tube amplifier box, is shown in its original condition in Radio News, Feb. 1920 (ref.15).
U.S. Army photo, courtesy of H.L. Chadbourne.

That was the invention, but a great deal of experimentation was needed to prove its workability. Armstrong proposed the method to his superior Major Buckley in June 1918. Over the next few months, up to the time of Armstrong's French patent application in December, the sequence of events was as follows:

"Preliminary experiments which showed the practicability of the method were made at this time, but on account of the large amount of more pressing work they were discontinued until about August 1. At this time Sergt. Pressley was assigned to work on the reception of undamped waves by this method. In the course of a few days apparatus was set up, and exceedingly good results were obtained. More pressing work, however, in tank radio, for which Sergt. Pressley was required, prevented continuation of this problem.

"The development of the method for receiving damped and modulated continuous waves was the next step. On account of the fact that no men capable of handling the work were available this development was turned over to Sergt. MacDonald, who was regularly stationed at Orly Field, but who volunteered to work on the problem in his own time. The lack of help greatly delayed the development. Toward the middle of August Sergt. Lewis was available and was also assigned on the development. About the middle of September the experimental and developmental work was completed and the problem of putting the apparatus into practical form was taken up. It was decided to use six tubes. Two of these were used in transforming the incoming high frequency to the lower frequency, three for aplifying this frequency, and one for detecting it. This work was placed in charge of Sergt. Lewis, assisted by Sergt. Houck. On account of many unforeseen difficulties and the great amount of work required to compleete the detail design of the various parts, the first model was not turned out until about the 1st of November. In preliminary tests the model gave several thousand times the amplification of the L-3, and the advantage could be increased by the addition of a two-stage audible frequency amplifier. Tests were completed, and it was ready for trial at the front at the time of the signing of the armistice." [10]

Armstrong's second model, now in the Smithsonian.
(photos by Donald Patterson)

Publicity

Armstrong returned to the United States in time to present a paper to the Institute of Radio Engineers on December 3, 1919, outlining his new system. He concluded the talk,

"The new practice of this method involves the use of many known inventions, but in connection with the production of a superaudible frequency by heterodyning I wish to make due acknowledgment to the work of Meissner, Round, and Lévy, which is now of record. The application of the principle to the reception of short waves is, I believe, new and it is for this reason that this paper is presented." [11]

During all of 1920 Armstrong was preoccupied with his regeneration patent, and particularly his legal troubles with Lee de Forest. Having little income to pay his mounting bills, he needed an ally. He is said to have approached the large independent manufacturer Amrad, backed by J.P. Morgan Jr., with an offer of a half interest in his regeneration patent for $500 [12]. But then his attorneys hit upon the idea of licensing all the makers of regenerative ham receivers, and by September had signed up 18 of them, assessing a royalty of 5% of sales price. At the time, the amateur market was negligibly small, and some licensees were no more than high-school boys working in their attics. The fact that they might grow up to become such firms as Crosley and Zenith was unforeseen.

Armstrong found his ally in Westinghouse. Having become involved in radio during the war, and wishing to set up a world-wide communications business like the British-controlled American Marconi company, Westinghouse invested heavily in Fessenden's old company and its valuable patents, only to be checkmated by its rival General Electric. GE, with the Navy's blessing, had formed RCA from the old American Marconi company. RCA in June 1920 had concluded cross-licensing agreements with GE and AT&T and signed exclusive traffic agreements with nearly every important country in the world, before Westinghouse could blink.

Westinghouse RA tuner and DA detector-amplifier, designed in 1920 by Frank Conrad and Donald Little

Westinghouse executives however were not myopic. Frozen out of the commercial field, they began radio broadcasting to create a market for their manufactured radio sets, and moved quickly to strengthen their bargaining position with RCA and its allies by purchasing Armstrong's regeneration and superheterodyne patents in October 1920 [13]. It is tempting to assume that Westinghouse appreciated the advanced technical features of the superheterodyne and was therefore willing to spend so much money on the patent, but it is more likely that regeneration was the real prize, and that Armstrong insisted on a package deal. The company's broadcast models, already designed and in production, could not be marketed without either a patent license or ownership. Westinghouse made no use of the superheterodyne patent, and for a time neither did anyone else.

Paul Godley with an experimental superheterodyne.
This volume of Wireless Age, one of a complete set, was discarded by the
Columbia University Library and might very well have been used by Armstrong.

In the Fenruary 1920 issue of Wireless Age (affiliated with American Marconi/RCA and generally considered authoritative) Paul Godley described Armstrong's system in some detail [14]. Godley had been with American Marconi during the war as its receiver expert. He was a partner in the Adams-Morgan Co., the country's foremost maker of ham receivers, and had been the first to make Armstrong's regenerative circuit work on short (200 meter) waves. Simultaneously the February and March issues of Radio News (Published by Hugo Gernsback and aimed more at young hams and tinkerers) carried lengthy articles by Harry Houck, who had been Armstrong's assistant in France [15].

But other than establishing Armstrong as the originator of yet another advance in radio technology, these published articles seemed to have little effect. In those days when the average amateur counted himself lucky to afford even one vacuum tube, the idea of running six to eight of them must have seemed quite far-fetched. On October 29, 1920 the Wireless Society of London discussed the superheterodyne, Wireless World in London published a report on November 13 [16], and finally in February 1921 the I.R.E. published Armstrong's 1919 paper. Still no hint that the circuit had any practical civilian uses.

The 1921 A.R.R.L. Transatlantic tests woke people up. The idea that a bunch of rowdy kids with limited equipment, wavelengths, and power, could accomplish what the commercial interests supposedly could not— "get across"—gave these amateurs swelled heads for decades afterward [17]. (Actually, much experimentation with short waves had already been done, and Marconi himself probably knew as much about them as the hams did). QST magazine was filled with the exploits of Paul Godley, who had been sent to Scotland especially for the tests, and who had used — a superheterodyne! [18] Wireless World for February 4, 1922 likewise ran a long story on the equipment and the results [19]. Superheterodynes began to acquire some mystique. However they were still very expensive. Vacuum tubes for instance cost $5.00 to $7.50, and with tube filaments drawing 1 ampere each, upkeep of batteries alone would break most piggybanks.

Popularity

In early 1922 the radio boom hit America. Radio broadcasting, which earlier had interested mainly kids, now began to appeal to a far wider and more affluent audience. Armstrong himself termed his new circuit "the Rolls Royce method of reception" and, as with the automobile, the superheterodyne attracted many patrons precisely because of its expense and complexity.

RCA, which could have sold superheterodynes, refused because Elmer Bucher, its sales manager, insisted that his models must have no more than two tuning controls, to be simple enough for the public to operate [20]. RCA did commission GE to build a commercial model, designed by A.F. Van Dyck in 1921 and installed with appropriate fanfare on the passenger liners Leviathan and America in early 1922. But of course this was far from a domestic radio.

Elmo N. Pickerill in the radio room of the Leviathan, 1923.
(Steamship Historical Society photo)

First to cater to the public taste, in RCA's absence, was Charles Leutz, formerly Godley's assistant at American Marconi. Leutz introduced his model in September 1922, updating it every few months with the latest improvements.

Leutz model L, 1922 (Photo by H.L. Chadbourne)

He dared not sell complete radios, for fear of a patent-infringement suit, but did a thriving business in blueprints, components, and kits, publicizing his wares in full-page ads and in a popular series of books titled Modern Radio Reception. Modern it may have been, but his first model was certainly not for novices; it had six tuning dials and seventeen other controls. Elmer Bucher singled it out as a prime example of what the public did not need. Leutz's 1923 model was vastly simpler, and more successful [21].

Meanwhile, in the early 1920s, AT&T was stirring. Its engineers had been using superheterodynes in one form or another for several years, largely for point-to-point experimental reception [22]. AT&T had bought Lucien Lévy's American patent application, in the hope it might be judged fundamental (as noted, and as will be explained later in more detail, it was so judged in 1928). After joining the "radio group" in July 1920 with RCA and GE (Wireless Specialty was admitted in March 1921, and Westinghouse in June), AT&T was cross-licensed under all their radio patents, including the superheterodyne. AT&T moved aggressively into radio broadcasting, supplying most American radio stations with transmitters and studio equipment, and operated WEAF in New York City, unquestionably the country's finest station both technically and in programming. AT&T's executives were seriously considering claiming all radio broadcasting, or at least all sponsored broadcasting, as their exclusive prerogative [23].

Manufacturing radio receivers for public sale would have been a natural next step; after all, AT&T's foreign affiliates were doing it. And Western Electric was already building receivers to be supplied to broadcast stations as part of their studio equipment. All stations had to monitor the 500 kHz marine distress frequency, and shut down in the event of an SOS; they also used the receivers to check their own transmission quality. Basing their design on a portable field-strength-measuring receiver built by the Engineering Dept. in New York, Western Electric's engineers created a seven-tube superheterodyne model 4A by October 1922. One was sent in January 1923 to Dr. Alfred Goldsmith, RCA's Director of Research, and rumors circulated that another was about to be installed in the White House.

Western Electric model 4A (photos courtesy Hall of History, Schenectady)

Western Electric model 4D

Northern Electric (Canada) model R4

For additional photos of GE assembly lines, and two Radiola 26 prototypes by Arthur F. Van Dyck (who moved from GE to RCA in 1922), click here.

The cross-licensing agreements among the "radio group" members had been drawn up before radio broadcasting was thought important, and while some categories such as radio transmitters were carefully defined, the companies' respective rights to build and sell radio receivers to the public were not so clear. AT&T wanted to get its nose into the tent. Its superheterodyne was said to have given RCA's sales manager Elmer Bucher "the jitters" which, considering RCA's archaic model lineup at the time, was probably true. RCA's affiliates GE and Westinghouse, which did all the actual design and manufacturing, had planned more of the same for next year's model line.

RCA's Gamble

But in February 1923, just a month after Goldsmith had seen Western Electric's 4A, Howard Armstrong walked into David Sarnoff's office at RCA with his own simplified model. By using WD11 tubes and combining functions, he had whittled his model down to a (just barely) portable [24]. It would need further work to adapt it for commercial production—much more, in fact, than anyone thought—but it looked feasible and Sarnoff convinced his associates to take the gamble. He cancelled millions of dollars' worth of just-placed orders with GE and Westinghouse, hoping to scoop the industry with a model that no one else could match [25].

Time was very short, to be designing an entirely new radio model for the 1923-24 season. Most manufacturers tried to have their engineering done by June, to take orders during the summer, and to run their factories from September through December. Neither GE nor Westinghouse was especially fast on its feet, yet RCA expected them to scrap their existing designs and put an untried circuit into commercial form in three or four months!

Westinghouse declined [26] and for a time GE wished it had done the same. At one point during the development, the GE engineers were ready to give up, a sentiment echoed to Sarnoff by the usually-optimistic Goldsmith. A blank look and the question "What'll I do now?" by Sarnoff to his secretary Marion MacInnis brought the response, "Why don't you call Armstrong?" [27] He did, and along with Hull and Langmuir of the GE Research Laboratory, Armstrong helped solve the problem of hiss in the mixer tube [28] while his associate Harry Houck solved the oscillator-pulling with his "second harmonic" invention [29]. For this bail-out work, the two received an additional 18,900 RCA shares, making Armstrong the company's single largest stockholder. And Howard did even better: he married Marion MacInnis.

As the 1923 Christmas selling season came and went, with nothing available but last year's leftover turkeys, Sarnoff must have been besieged by RCA's panic-stricken dealers. But in February 1924 the new lineup finally appeared. It was a tremendous success, with eventual production of 148,300 superheterodynes, and made more money for RCA than anything up to the AC-powered sets of 1927-1928. To remove competition, Leutz was now hit with lawsuits and injunctions [30], and AT&T was convinced not to upset the ongoing arbitration with RCA by publicizing its new 4B model [31].

Armstrong wrote a lengthy paper for the I.R.E. detailing the many development steps he had gone through, and this paper also appeared in the widely-circulated magazine Radio Broadcast [32]. RCA's considerable ballyhoo even reached Japan; a radio magazine there printed photos of Howard and Marion on the Florida beach, listening to her wedding present, a portable superheterodyne. Armstrong's name was by now closely linked to his creation, and he was recognized universally as its inventor. In all of the universe, that is, except for France.

France

In France an entirely different line of development was going on, dating from 1916. In that year Lucien Lévy, an officer with the Télégraphie Militaire, was working on the 1½ kW radiotelephone transmitter at the Eiffel Tower, under the direction of Col. Gustave Ferrié [33]. Lévy had the idea of obtaining secrecy by modulating the RF carrier with a supersonic wave which would itself be modulated by an audio signal. This scheme, neither practical (at that time) nor original, suggested however to Lévy that if the supersonic wave were instead produced in the receiver, by heterodyning the received signal against a local oscillator, this wave could be selected by a tuned circuit before being finally converted to audio. In other words the signal could be doubly tuned: once at the incoming frequency, and again at the "intermediate" (to use the modern term) frequency. Lévy applied for a French patent on this arrangement on August 4, 1917 (issued August 19, 1919, no. 493,660) [34]. On October 1, 1918 Lévy's second French application disclosed an even more elaborate multi-stage amplifier and filter at the intermediate frequency (issued May 27, 1920, no. 506,297).

Lucien Lévy presenting Lee de Forest with one of his superheterodyne models (Hemardinquer, La Superhétérodyne et la Superréaction, 1926, p.166. Reference 48. Copy courtesy of the John Crerar Library, Chicago).

Information on Lévy's original circuit had been publicized among his military colleagues as one page of a report by C. Gutton [35] in 1917, and his final scheme in a hectographed paper distributed to the AEF radio-research branch in Paris on October 20, 1918.

Lévy in 1919 tried to sell his American patent application to entrepreneur Emil Simon for $5000, telling the skeptical Simon that Armstrong had stolen his idea [36]. Later that same year he offered the rights to Le Materiel Téléphonique, the French arm of Western Electric, and in this way Lévy's work came to the attention of AT&T's engineers. They of course had been working along the same lines for years, but had evolved the superheterodyne principle so gradually that they essentially didn't know what they had [37]. Lévy's patent seemed to cover the most practical form, so AT&T bought his American application for $20,000 [36].

Lévy Wins

Lévy eventually formed his own company, Les Établissements Radio L.L., which he headed for some years [38]. His superheterodyne patents were publicized in the magazine Radioélectricité in April and May 1921, but it was April of 1923 before he could advertise a superheterodyne broadcast receiver. As he explained in 1924,

"The superheterodyne could not reach its ultimate capabilities in France, on account of the government's slowness in expropriating the (German) Meissner patents covering the heterodyne and high-frequency amplifier coupling. Nonetheless, a model was built in 1919 which at Paris, with a 1-meter loop antenna, easily picked up boats in the Mediterranean."[39].

At left: L'Onde Électrique, Feb. 1923. Below: April 1924.

Radio L.L. produced three home models in 1923 and, as the superheterodyne became more and more popular, other companies joined in too. By the end of 1926 Lévy had 65 French licensees.

In the United States—but not necessarily in France, at that time—two valid patents could not cover identical subject matter. Lévy had filed first, but because his patent had a different purpose from Armstrong's, and the claims were quite different, the patent examiner had apparently not noticed the conflict and had allowed Armstrong's patent to issue on June 8, 1920 (no. 1,342,885). But Lévy­­or AT&T­­noticed. Lévy broadened his claims to purposely create an interference, by copying Armstrong's claims exactly. The Patent Office would then have to choose between the two inventors.

Now despite the indignant rantings of Armstrong's biographer Lawrence Lessing [40] there was nothing sneaky or underhanded about Lévy's procedure. Copying a rival's claims was in fact required by Patent Office rules, to remove any ambiguity over whether or not an interference existed. The only question was whether the invention that Lévy disclosed, in 1917, would cover the new claims. The disclosure was not altered. After several years of legal wrangling, the Court of Appeals of the District of Columbia ruled that Lévy's original disclosure would indeed support the new claims; in other words, all the features of Armstrong's superheterodyne were spelled out in Lévy's description [41]. Therefore, since Lévy's filing date was seven months earlier than Armstrong's first date of conception, Lévy was entitled to a patent and accordingly one was issued on November 5, 1929 (1,734,038) with a priority date of August 4, 1917. It incorporated seven of Armstrong's nine claims; the two remaining went to Alexanderson of GE and Kendall of AT&T in similar fashion (Aitken, reference 36).

While French patent procedure was fairly lax, the Germans were even more thorough than the Americans, and a similar interference proceeding there resulted in a patent to Lévy on October 1, 1931 (no. 536,049) again with a priority date of August 4, 1917. [42].

There were in fact a number of quasi-superheterodyne systems invented earlier than either Armstrong's or Lévy's. Walter Schottky, who was active in this field himself, listed three in 1926: [43]

The idea of employing the advantages of heterodyne reception for radio telephony also, by selecting an inaudibly high beat frequency, was probably published originally in 1913 by Mr. Hogan in the course of a discussion {8}. The idea of producing a beat frequency by means of a local source of oscillation, which was not intended to make the signals audible, but expressly to provide for another tuning and thereby increased selectivity, has been patented by Graf Arco and A. Meissner {9}, and by H.J. Round {10}. Round's application also lays stress on providing inaudible beat frequencies, but actually offers no good selectivity against interference owing to the inherent necessary detuning of the aerial.

{8} Hogan, Proceedings of the Institute of Radio Engineers, 1, 97 (1913)
{9} English Pat. 252, 1914, filed January 5, 1914 and D.R.P. 300896, January 15, 1917.
{10} English Pat. 27,480, 1913, filed November 11, 1913.

A.M. Morse in the Electrician of July 31, 1925 also cited the equivalent British patents of the various contestants, with much the same comments [44].

Lévy Loses

Even in France, the very birthplace of chauvinism [45], the Frenchman Lévy found it tough sledding to obtain public credit for his invention. RCA's 1924 publicity reached his country when Radio-Revue published a translation of Armstrong's 1924 I.R.E. paper in which, unlike the 1921 article, Lévy's name did not even appear. This oversight prompted a lengthy rebuttal by Lévy in the same issue (reference 38).

Radioélectricité, Dec. 25, 1924.

But Lévy's struggles were not solely with Armstrong. In the popular weekly L'Antenne, a discussion began in late 1925 on the relative merits of the frequency-changing circuits used by the Lévy and Ducretet companies. Lévy used a separate oscillator tube, and called his mixer tube a "detector," while Ducretet's engineers used a "bigrille" or double-grid tube for both functions and called theirs a "modulator." By late 1925 Lévy was beginning to sign up his competitors for royalty licenses to use his invention, and it is more than likely that Ducretet had commercial reasons for not admitting its circuit to be a superheterodyne.It is also more than likely that many others in the French radio industry felt similarly hostile toward Lévy, since L'Antenne quickly became a forum for vituperative personal attacks on him, chiefly by the magazine's own editor, Henri Étienne. When Étienne learned that another engineer attached to Ferrié's group during the war, Paul Laüt, had proposed most of what Lévy had patented, in a memo written six months earlier, he reprinted the original memo and demanded that Lévy explain himself. Lévy could only offer some weak excuses and "arguments specieux" and there the controversy rested, with his opponents having the last word. [46]. Lévy had his patent and, as Étienne put it, "filled his pockets" but as late as 1955 had to write a bristling full-page reply to L'Onde Électrique, France's foremost electronics magazine, to correct a published story crediting Armstrong with the superheterodyne (reference 42).

Lévy always felt that Armstrong had stolen his invention, but there is no direct evidence for this [47]. Lévy's ideas had indeed been published in military reports distributed to the American radio personnel; however the first such report had arrived before Armstrong was in Paris, and the second came after he had already done a good deal of experimental work and was preparing his patent application.

It is true that Armstrong, in his capacity as head of the radio research laboratory, was in close contact with French manufacturers, since inspection of incoming French equipment was being done at the same Paris location. And it was his job to keep abreast of French technical developments and to coordinate his group's research with them. Given Lévy's emphasis on secrecy systems and selectivity, Armstrong probably felt that he had contributed little of novelty to the prior art, and only discovered the superheterodyne's potential after Armstrong pointed the way. Lévy, conversely, knew that Armstrong did not deserve an all-encompassing patent, and he was stung by Armstrong's unwillingness to credit prior researchers in his 1924 paper ("It is unfortunate that Mr. Armstrong, who in his 1920 I.R.E. paper had recognized our priority, has forgotten, in the midst of his glory, the source from which he drew)." [48].

Summary

Walter Schottky summed it up accurately in 1926:

"Finally, the aforementioned patent of Lucien Lévy is of fundamental importance to the whole field; he must be considered, at least from the point of view of patent law, as the true originator of the super-heterodyne method, since the super-imposition of an adjacent frequency, an intermediate circuit tuned to inaudible frequencies, and a further rectification in order to convert into the desired signal, are described explicitly in his application (as one of several constructions). In regard to earlier existent publication, there may be a doubt as to whether the information would have brought about the desired technical progress we owe to the super-heterodyne method, as conceived by Mr. Armstrong and also described in the German application. After all, the actual aim of the high-frequency transformation or super-heterodyning principle consists in providing a suitable and relatively convenient radio-frequency amplifier for short waves, whereas the selectivity effects that Lévy solely had in view are less important, according to the above considerations, and might be obtained as well by the use of a slightly attenuated or reaction-coupled radio frequency syntonizing circuit. The drawings of this application also leave it doubtful whether the elimination of the square-law rectifying action, which is so essential for the commercial use of the apparatus, would have been obtained by means of experimental sets constructed on the principle indicated in the application.

"The "word" seems, at any rate, to have been far less important than the "deed," and there appears to be no doubt that it is Mr. Armstrong and his collaborators to whom we owe the deed, which has made the super-heterodyne method such an invaluable instrumentality in radio engineering." (reference 43)

Footnotes

(Click on a footnote number to return to the main text or here to get to the top.)

[1]  A.E. Anderson, "Robert H. Goddard: Original Inventor-Patentee of the High Frequency Vacuum Tube Oscillator" (unpublished manuscript, 1981)

[2]  It would not have affected most of the industry anyway, as RCA did not license other manufacturers under its superheterodyne patents until 1930. But it surely would have changed RCA's fortunes, if RCA and AT&T had not reached an amicable settlement of their broadcasting dispute, and if AT&T had therefore gone into the radio business, selling superheterodynes to the public.

Archer, Big Business and Radio (New York: The American Historical Co., 1939).

[3]  U.S. Patent 706,740 filed Sept. 28, 1901, issued Aug. 12, 1902.

[4]  Edwin H. Armstrong, "A Study of Heterodyne Amplification By the Electron Relay," Proc. I.R.E. 5 (April 1917), pp.145-168.

[5]  Sir Arthur Hezlet, Electronics and Sea Power (New York: Stein and Day, 1975, pp.83-155.

[6]  H.J. Round, "Direction and Position Finding," Journal I.E.E. 58 (March 1920), pp.224-257.

[7]  H.J. Round, "Direction and Position Finding," Journal I.E.E. 58 (March 1920), p.240.

[8]  Armstrong, "Vagaries and Elusiveness of Invention," Electrical Engineering 62 (April 1943), p.150.

[9]  Normally a grid-leak detector would have been used, which amplified as well as detected, but Armstrong had used crystal rectifiers in his heterodyne researches. Any device, worked over a nonlinear portion of its characteristic, would partially rectify an applied signal, and serve as a detector.

[10]  Report of the Chief Signal Officer, 1919 (Washington, Government Printing Office, 1919. Reprint by Arno Press, New York, 1974), pp.288-289.

[11]  Armstrong, "A New System of Short Wave Amplification," Proc. I.R.E. 9 (Feb. 1921), pp. 3-27.  QST 3 (Feb. 1920), pp.5-9, 13.

This paper uses the term superaudible heterodyne, from which superheterodyne is derived. The British tended to use supersonic. Incidentally, the first use of the word superheterodyne that I have seen, is in QST for March 1921 (p.41) but evidently from the context it was in common use by then.

[12]  Amrad's boy-wonder president H.J. Power declined Armstrong's offer.

Douglas, Radio Manufacturers of the 1920s, Vol.1 (Vestal, NY: Vestal Press, 1988) p.39.

[13]  Option purchased on October 5, exercised November 4, 1920, for $335,000 plus $200,000 if Armstrong should win his interference with de Forest over the regeneration patent. The purchase included 4 issued patents and 16 applications, by Armstrong, Pupin, or the two jointly.

[14]  Wireless Age 7 (Feb. 1920), pp.11-14.

[15]  Radio News 1 (Feb. 1920), pp.403-405, 439; (March 1920), pp.469-471, 508-510.

[16]  Wireless World 8 (Nov. 13, 1920), pp.581-583.

[17]  DeSoto, Two Hundred Meters and Down (West Hartford, CT: The American Radio Relay League, 1936).

[18]  QST 5 (Feb. 1922), pp.7-40.

[19]  Wireless World 9 (Feb. 4, 1922), pp.689-694.

[20]  Archer, Big Business and Radio, p.92.

RCA nonetheless marketed its share of complex apparatus. The Radiola VI from this period indeed had only one tuning dial and one bandswitch, but also sported an amplification control and six filament rheostats. It sold for $162.50 without antenna, tubes, batteries, or speaker.

[21]  For a detailed story of Leutz's career and radio models, see Douglas, Radio Manufacturers of the 1920s, Vol.2 (Vestal, NY: Vestal Press, 1989), pp.122-131.

[22]  Espenschied, Proc. I.R.E. 47 (July 1959), pp.1257-1258.

A History of Engineering & Science in the Bell System. The Early Years (1875-1925) (Bell Telephone Laboratories, 1975) pp.349-465.

[23]  Archer, Big Business and Radio, pp.55, 75-78, 89.

[24]  Figs. 9 and 10 in Armstrong's 1924 I.R.E. paper (reference 32). The Westinghouse WD11 was electrically equivalent to Western Electric's 215-A "peanut" tube, drawing 0.25 Ampere at 1.1 Volts from dry cells. GE's belated answer, used in all its portable sets including the superheterodynes, was the UV199 which consumed .06 Ampere at 3.3 Volts. The standard radio tube of the day, the UV201A, drew 0.25 Ampere at 5 Volts from a storage battery.

[25]  Archer, History of Radio to 1926 (New York: The American Historical Society, 1938), p.297.

[26]  Author's correspondence with W.L. Carlson, superheterodyne design engineer at GE 1924-1930.

[27]  Lessing, Man of High Fidelity: Edwin Howard Armstrong (Philadelphia: Lippincott, 1956. New York: Bantam Books, 1969), p.148/119.

[28]  Hull's screen-grid tube grew out of this work. See Physical Review 27 (April 1926), pp.432-438, 439-454. Also Proc. I.R.E. 16 (April 1928), pp.424-446; 16 (June 1928), pp.840-843.

[29]  To allow use of silicon-steel transformer cores, and to get the proper bandpass, the IF was set at 42 kHz. To economize on tubes and battery power, one triode served as oscillator and mixer, and the RF tube was also the first IF amplifier. But it proved impossible to avoid interaction between the RF and oscillator tuned circuits, only 42 kHz apart. Houck's solution was to run the oscillator at half the usual frequency, so that its second harmonic was 42 kHz from the RF signal. One disadvantage of this arrangement was that a station could be tuned in at several points on the dials, but at that time there were fewer stations on the air, than now. These models in fact work quite well, even today.

[30]  RCA had already incurred Congressional wrath with its monopolistic practices, prompting a full-scale Federal Trade Commission investigation in 1923. If it had sued Leutz in 1923, before its own superheterodynes were on sale, RCA would surely have been denounced as a dog in the manger.

[31]  This time AT&T actually delivered a 4B to the White House, to RCA's consternation since it beat the top-of-the-line Radiola Super-VIII in competitive tests (but it had three more tubes, and a better loudspeaker). RCA and AT&T finally settled their differences in 1926 with the sale of WEAF to RCA. WEAF became the flagship station of the new National Broadcasting Company (later WNBC). See Archer, Big Business and Radio.

[32]  Edwin H. Armstrong, "The Superheterodyne­­Its Origin, Development, and Some Recent Improvements," Proc. I.R.E. 12 (Oct. 1924), pp.539-552. Also (with a different fig.1) Radio Broadcast 5 (July 1924), pp.198-207.

[33]  Col. (later General) Gustave Ferrié (1868-1932) was an influential proponent of military radio, and his Eiffel Tower laboratory was at the center of new developments.

L'Onde Électrique 11 (Feb. 1932), pp.45-52.

[34] Corresponding foreign patents:

U.S.     1,734,038  applied Aug.12, 1918,  issued Nov.5,1929
Britain    143,583  accepted June 3, 1920
Germany    536,049  issued Oct.1, 1931

In accordance with international convention, these all had priority dates of Aug. 4, 1917.

[35]  Gutton collaborated with Gen. Ferrié on short-wave studies in the 1920s, and later was director of the Laboratoire National de Radioélectricité.

[36]  Aitken, The Continuous Wave (Princeton, NJ: Princeton University Press, 1985), p.467.

[37]  Espenschied, reference 22.

[38]  15 years, according to Champeix. Lévy also ran his own broadcast station.

Champeix, "Qui a Inventé le Superhétérodyne?" La Liaison des Transmissions 116 (March-April 1979), 117 (April-May 1979).

[39] 

"Le superhétérodyne Lévy ne put atteindre tout le développement dont il était susceptible en France, à cause de la lenteur avec laquelle les services de l'Etat procédaient à l'expropriation des brevets Meissner, dont l'emploi était nécessaire pour la r&#233alisation des hétérodynes du super-hétérodyne et pour le réglage facile de l'accrochage des amplificateurs à haute fréquence.

"Pourtant, malgré ces difficultés, un modèle fut créé en 1919, lequel permettait facilement à Paris sur cadre de 1m. la réception des côtiers et bateaux de la Méditerranée."

Lévy, "L'Histoire du Super-Hétérodyne," Radio-Revue 3 (Oct. 1924), pp.186-188.

[40]  Lessing, Man of High Fidelity, p.118 (original ed.) or p.93 (paperback ed.)

While Lessing is usually trustworthy, occasionally hero-worship gets the better of him. His statement here that the French government never allowed Lévy's claims is absolutely false. And his description of Lévy's patent and AT&T's conduct is, to say the least, misleading. Lessing also forgets to mention that Armstrong's superheterodyne patent was void after 1928. Champeix (reference 38) after paraphrasing Lessing's account in his very thorough 1979 paper on the superheterodyne's invention, follows with a single sentence, "Voilá comment on &#233crit l'histoire." (Loosely, "See what passes for history.").

[41]  29F(2d)953. Armstrong v. Lévy, decided Dec. 3, 1928.

[42]  Lévy, "Au Sujet du Superhétérodyne," L'Onde Électrique 35 (May 1955), p.548.

[43]  Schottky, "On the Origin of the Super-Heterodyne Method," Proc. I.R.E. 14 (Oct. 1926), pp.695-698.

Hogan's comment, by the way, was an answer to "How do you receive radiotelephone signals with a heterodyne detector?" His reply was to keep the beat frequency inaudibly high. The "correct" answer of course is to zero-beat them with the local oscillator, which makes you wonder about the state of the art in those days! Hogan, it should be noted, was an extremely competent experimenter and engineer.

[44]  Morse, "The Superheterodyne," Electrician 95 (July 31, 1925), p.121.

[45]  Chauvinism: vainglorious or exaggerated patriotism, from Nicolas Chauvin, whose demonstrative patriotism and attachment to Napoleon came to be ridiculed by his comrades. (Webster).

[46]  Champeix, "Qui a Inventé le Superhétérodyne?" (reference 38).

Champeix met Paul Laüt by accident in 1968 and heard the story from him, later reconstructing the affair from the published letters in L'Antenne. In the end, however, Champeix awards the laurel to Lévy and Armstrong.

Laüt contracted tuberculosis and was sent away to the countryside to recuperate, for a year. He used this time to grapple with theoretical problems assigned by Ferrié, reporting his conclusions by letter. His superheterodyne proposal involved frequency-changing by the heterodyne, amplification at the intermediate frequency, and detection. But it did not include any IF tuning. Lévy claimed in 1926, "Il me semble bien qu'à ce moment, le "remarquable' petite note de M. Laüt a'avait pas attiré outre mesure l'attention" ("It seems to me that at that time, Mr. Laüt's 'remarkable' short note did not attract much attention"), an opinion corroborated by his superior in a subsequent letter to L'Antenne;. Laüt stated in 1968 that on his return to Paris in 1917, he was chagrined to learn that Lévy had patented some of his ideas, but was told by Ferrié not to let personal considerations interfere with the war effort. Of course, it is a matter of record that Laüt did not contest Lévy's patent and, whatever he stated later (hindsight is always 20-20) he must have felt at the time that the matter was not worth pursuing. And in truth, Laüt seems not to have gone much beyond what Round or Arco and Meissner had devised.

[47]  As Champeix points out, Laüt had good reason to feel the same way about Lévy!

[48]  "On pourra enfin regretter que M. Armstrong, qui avait, dans sa première communication à la Société des Radio Engineers de New-York, reconnu notre antériorité, ait oublié au sein de sa gloire, la source à laquelle il était venu puiser." (Radio-Revue, reference 39). When the same material was reprinted, with additions, in a 1926 book on the superheterodyne, cooler heads prevailed and the phrase "au sein de sa gloire" was omitted.

Hemardinquer, La Superhétérodyne et la Superréaction (Paris: Étienne Chiron, 1926).

[49] 

Superheterodyne assembly and testing at GE. (photos courtesy Hall of History, Schenectady).

A.F. Van Dyck moved from GE to RCA in 1922, heading the Technical & Test Dept. One of his pet projects was a portable superheterodyne; the two models shown had evolved by 1925 into the Radiola 26. Model #2 is now in the Ford Museum.

I am grateful to Dr. Anders Widell (Lund, Sweden) for introducing me to Champeix's paper on Lévy, to John M. Anderson of General Electric for searching that company's photographic archives, and to Joseph de Veer of the Marine Biological Laboratory Library (Woods Hole, Mass.) for tracking down elusive references. Other contributors are credited under particular illustrations. Copyright © 2004 by Alan S. Douglas.

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