The FM story has it's beginning about 1920 with a study by John Remington Carson of Bell Labs that culminated in Notes on the Theory of Modulation in 1922 which described, among other things, the concept of Frequency Modulation. However he considered it to be essentially useless for the intended purpose: to narrow the bandwith required for transmission.
In 1931 the critical breakthrough was realided when it was found that the signal benefited enormously by a dramatic bandwidth increase. By 1933, the new system was ready for demonstration. Although a complete sucess, engineers were skeptical that such improvement was possible. NIH was in full swing.
Armstrong was not deterred and went into development of a true FM station, demonstrating first to the I.R.E. using an amateur station, WQAG on 2.5 meters (120 Mc !), which led to construction of the first commercial station at Alpine, New Jersey in 1937 which was granted the first experimental commercial station W2XMN two years later on July, 18, 1939 followed by W1XPW which began transmission in late 1939. Zenith followed suit with W9XEN which began broadcast in Feb. 1940 almost exactly a year after Zenith had begun television broadcasts with W9XZV as the third FM station and the first in the midwest.
But what was missing was a listener base. Though FM receivers could use traditional audio circuits and superhetrodyne design, new front ends with limiter and discriminator circuits were required to convert FM to comventional IF. Armstrong contracted with GE (showing the rift with RCA) for 25 U$400.00 receivers as early as 1937. However the real start was not until January 5, 1940 when the Yankee Network, a group of allied stations in New England simulcast WQXR though New England from NYC to Boston.
On May 20, 1940 and with over 100 applications for station licenses already received the FCC allocated a frequency band of 42-50 Mc for use by those stations. Commercial FM had arrived.
Zenith was not caught waiting and with the introduction of the 1941 models in the fall of 1940 "New Frequency Modulation Radios" were one of the highlights with console (10H573), chairside (10H551), "spinet" (10H573) & chairside (10H571R) - latter two with phonograph) models starting at U$129.95. All used chassis 10A3(R) and featured the traditional three AM bands of the "S" models plus FM. All had 12 inch speakers and 64 setting "RadioOrgan".
My records show 2,750 10A3 and 2,600 10A3R chassis built that first year (Green "A" serial tags).
This was continued for 1942 with FM moved into the top-of-the-line 22 tube "Arlington" and "Wilshire" models - the first Zenith units with over 16 tubes since the 1934-36 "Stratosphere" - with 50 watt audo amplifier feeding 1 - 6" speaker and 2-12" speakers with a 13 tube radio chassis and 9 tube amplifier chassis using six 6A3s. Lesser models were also available.
These continued with moderate popularity primarily in the New England states until civilian production was curtailed in 1942.
However, even before civilian production resumed following WWII, in the fall of 1944 the FCC held hearings on the allocations of radio frequencies. Broadcasters, notably NBC and CBS suggested that for the common good FM should be moved to higher frequencies since sunpots were less likely to affect reception. To fill the vaccuum, it was suggeted that the previous frequencies be allocated to television.
For some reason the FCC agreed and on January, 15th, 1945 decreed that the 45 Mc band was to be vacated by the end of 1947 and FM stations would henceforth be allocated 20 Mc from 88 to 108 Mc with the first 5 Mc reserved for public broadcast stations.
That this doomed the Yankee Network which was still struggling out from the debt incurred by setting up the network and the fact that Armstrong had donated his FM patents to the war effort while RCA made millions from theirs. This was his reward.
From 1945 through 1947 (by which time the sunspot theory had been shown to be nonsensical) engineers of various companies argued that there was no reason to move the FM band and that it would destroy the investment of hundreds of people and companies. The FCC refused to change its stance and between 1948 and the mid 1960's, FM stations were few and far between. Most automakers did not even bother with FM radios until GM introduced them for the 1963 model line. The Yankee Network was defunct.
There were some interesting radios built in the period from the resumption of civilian production for the 1946 model year and the 1947 termination as many including Zenith produced FM radios with both the 45 Mc and 100 Mc bands including a few for the 1948 model year which was introduced in the fall of 1947, only months before the ban took effect.
After that, while Zenith continued to produce a limited number of FM sets through the fifties, there was really very little difference between the 1948 7H820 and the 1955 T825F except that a handle had been added and the 45 Mc band had been replaced with a "FM-AFC" setting. In fact the first Zenith all-transistor FM Radio, the 1961 Royal 2000 Trans-Symphony uses the same inductive ("permiability tuned" tuned mechamism as the 1948 7H820.
Thus the early 1948 models mark the end of an interesting era in radio in which once again brilliance was no match for politics.