The invention of the regenerative circuit by
Edwin Howard Armstrong in 1912–1913 made it possible to build extremely sensitive radio receivers using relatively few vacuum tubes. By feeding a portion of the output signal of a vacuum tube back to its input circuit, the amplification of incoming signals could be greatly increased, and with stronger feedback the circuit could oscillate and act as a transmitter. The regenerative receiver was immediately recognized as commercially important because it allowed sensitive reception with inexpensive equipment and relatively small antennas, enabling reception of distant signals even in urban locations. Armstrong lacked the financial resources of the large corporations involved in radio patent litigation. To finance legal expenses, his attorneys licensed the regenerative circuit patents to a group of small manufacturers producing receivers for amateurs and experimenters. By November 1920 seventeen companies had accepted such licenses, agreeing to pay royalties of about five percent of net sales. These licenses were non-exclusive and non-transferable and restricted to receivers sold for amateur and experimental purposes. The invention quickly became the subject of competing patent claims. In addition to Armstrong, other inventors asserting related feedback circuits included
Lee de Forest,
Irving Langmuir of
General Electric, and
Alexander Meissner of
Telefunken. The regenerative circuit patent became the subject of extensive litigation that lasted more than two decades. The dispute was ultimately resolved in 1934 when the
Supreme Court of the United States upheld Lee de Forest’s claims relating to the feedback circuit. An editorial in the June 1934 journal
Electronics commented on the long dispute: The regenerative-circuit dispute became one of the most famous patent conflicts in the history of electronics, illustrating both the importance of radio patents and the extensive litigation that accompanied the rapid growth of the industry. The regenerative circuit patent became the subject of extensive litigation that lasted more than two decades. Earlier stages of the dispute reached the
Supreme Court of the United States in the late 1920s, when the Court issued a decision recognizing priority claims associated with Lee de Forest's feedback circuit patents. The controversy continued through additional litigation and appeals before being widely regarded as settled by later rulings in the 1930s.
RCA patent pool The complexity of overlapping radio patents eventually led to large-scale cross-licensing agreements among major corporations. In 1919 the
Radio Corporation of America (RCA) was formed to consolidate the wireless assets of the American
Marconi Wireless Telegraph Company of America and to coordinate radio patent rights among major electrical manufacturers. Subsequent agreements between
General Electric,
Westinghouse Electric Corporation,
American Telephone and Telegraph Company, and RCA effectively created a patent pool covering many key radio technologies. Under these arrangements the participating firms cross-licensed their radio patents while dividing major areas of the business. General Electric and Westinghouse manufactured radio equipment, RCA handled sales and international communication services, and AT&T retained rights relating to radio telephony and long-distance transmission. Important receiver technologies, however, remained outside the initial patent pool. In particular, the
regenerative circuit invented by
Edwin Howard Armstrong was controlled by Westinghouse, while Armstrong's later
superheterodyne receiver patents were held by the
National Electric Signaling Company (NESCO). As a result, several companies were unable to manufacture complete state-of-the-art receivers without risking patent infringement. By the mid-1920s newer receiver technologies began to displace earlier regenerative designs. RCA’s 1924 product line, for example, was eventually abandoned in favor of receivers based on Armstrong’s superheterodyne principle, including designs using harmonic frequency conversion. These technical and legal constraints illustrate the central role played by patent control in shaping the early commercial radio industry. Receiver architecture, product packaging, and even corporate alliances were frequently determined as much by patent ownership and licensing arrangements as by engineering considerations. RCA’s licensing policy became an important source of revenue and a major factor shaping competition within the radio industry. By the late 1920s the company had begun granting licenses more broadly to manufacturers wishing to produce receivers under the RCA patent portfolio. Royalties were initially set at 7½ percent of the net selling price of the receiver package, and by contemporary estimates RCA collected nearly $3 million in royalties in 1927 and more than $6 million in 1928. The licensing system nevertheless generated continuing controversy within the industry. Many manufacturers objected to paying substantial fees to a competitor and criticized RCA as a “patent octopus.” The royalty rate was reduced to 5 percent in 1932 and a licensing bureau was established to assist licensees technically. Despite these changes, disputes continued; one of the most notable involved
Philco, which challenged RCA’s method of calculating royalty payments and ultimately prevailed in court in 1939.
Neutrodyne The patent restrictions surrounding Armstrong’s regenerative circuit also encouraged the development of alternative receiver designs intended to avoid infringement. One of the most important was the
Neutrodyne circuit developed by
Louis Hazeltine in 1922. The Neutrodyne used neutralization of interelectrode capacitance in triode amplifiers to prevent unwanted feedback and oscillation, allowing stable multi-stage radio-frequency amplification without relying on regenerative feedback. Because the design did not employ the positive feedback covered by Armstrong’s patent, it could be manufactured without paying royalties for regenerative receivers. Hazeltine’s patents were administered through the
Hazeltine Corporation, which licensed the Neutrodyne design to numerous manufacturers, making it one of the dominant commercial receiver architectures of the early 1920s before the later adoption of the
superheterodyne receiver. The need for neutralization circuits such as the Neutrodyne largely disappeared with the introduction of the
tetrode vacuum tube incorporating a
screen grid. The screen grid, placed between the control grid and the plate, greatly reduced the interelectrode capacitance that coupled the plate circuit back to the grid. By electrostatically shielding the input circuit from the output circuit, the screen grid suppressed the unintended feedback that had previously caused triode RF amplifiers to oscillate. As a result, stable multi-stage radio-frequency amplification could be achieved without neutralization networks, simplifying receiver design. A widely used example was the Type 224 screen-grid tetrode introduced in the late 1920s, which enabled practical high-gain radio-frequency amplifier stages in commercial broadcast receivers and contributed to the decline of neutrodyne receiver designs.
New vacuum tube (valve) structures The introduction of the
tetrode with a
screen grid solved the feedback instability that had plagued triode radio-frequency amplifiers by electrostatically shielding the control grid from the plate and greatly reducing interelectrode capacitance. However, the tetrode introduced a new problem: secondary electrons emitted from the plate could be attracted to the positively charged screen grid, producing a characteristic “
kink” in the plate-current curve in which plate current decreased with increasing plate voltage. This effect could cause instability and distortion in amplifier circuits. The problem led to the development of the
pentode vacuum tube in the mid-1920s, which added a suppressor grid to repel secondary electrons back to the plate. Because pentode designs were covered by patents held by
Philips, American manufacturers developed alternative approaches, most notably the
beam tetrode, introduced by
RCA in the 1930s, which used aligned grids and beam-forming electrodes to suppress secondary emission while avoiding the pentode patent claims.
Superheterodyne Another major receiver architecture introduced during this period was the
superheterodyne receiver, invented by
Edwin Howard Armstrong during World War I. Because the superheterodyne offered substantially improved selectivity and sensitivity compared with earlier receiver designs, it quickly became one of the most valuable patents in the radio industry and an important element of the RCA patent portfolio. Westinghouse acquired Armstrong’s regeneration and superheterodyne patents in 1920, giving it control over important receiver technologies within the developing radio patent structure. As the broadcast receiver market expanded during the 1920s, licensing policies within the
RCA patent pool played a significant role in shaping receiver design and production. In 1927 the RCA group adopted a policy of licensing other manufacturers to produce receivers under its patent portfolio at a royalty of 7½ percent of the net selling price. These licenses initially covered only tuned radio-frequency (TRF) receivers—including widely used
Neutrodyne designs—and excluded the more efficient superheterodyne receiver, which RCA had recently introduced and reserved for its own products. However, RCA was producing less than half of the radio sets sold in the United States, and many dealers favored TRF receivers supplied by competing manufacturers. As a result, RCA revised its policy in 1928 and began licensing superheterodyne receivers as well, after which tuned radio-frequency sets rapidly disappeared from the market. ==See also==