In the Radio Act of 1927, amateur radio lost 38% of its overall frequencies, despite efforts of the ARRL & the IARU. However, it kept 160, 80, 40, 20, & 5 meters, and also gained 10 meters.
Amateur radio grew, & during the Depression grew the fastest, from nearly 17,000 to nearly 47,000. The Communications Act of 1934 divided amateurs up into Class A, Class B, & Class C. 17 years later they would becomes the Advanced Class, the General Class & the Conditional Class.
The Cairo Conference in 1938 took away 40 meters from the amateurs, but the FCC added 2 new "UHF" bands (as frequencies above 25 MHz were then called) -- 2.5 and 1.25 meters (112 MHz & 224 MHz). It had become known every since Marconi had found it unworkable to transmit those high frequencies across the Atlantic that that these high frequencies were not as useful for distance communication. However, means of making them more useful had already been invented.
Radio pioneer Engineer Edwin A. Armstrong received a patent on frequency modulation (FM) in 1933 that he had submitted 3 years before. Armstrong is generally considered the inventor of the superheterodyne receiver, although his patent for it awarded to him in 1918 was disputed and awarded instead to Levy of France in 1928. Armstrong explored FM receivers. and finally convinced other engineers of the advantages of FM broadcasting on the very high frequency (now called VHF) band. FM radio was demonstrated to the FCC in 1936, & they granted a permit for the 1st station (W1XOJ) in 1937 -- an experimental station created by Armstrong. In 1940 by the FCC set apart 35 channels on the VHF band for commercial FM broadcasting and 5 for non-commercial educational use, all in 42 to 50 MHz band. The 1st modern commercial station W47NV (becoming WSM-FM) began in Nashville, TN in 1941.
These led to FM transmissions in the US in the 1940s. The advantages of FM over AM are: (1) It makes room for many more stations in the VHF band. (2) It gives improved signal-to-noise ratios in many areas, allowing for a much wider dynamic range of sound. (3) FM receivers are relatively insensitive to impulsive electromagnetic interference. (4) Only the loudest signals are normally heard, so there is much less interference. There are also disadvantages, such as the increased complexity of circuits needed for AM/FM reception. However, the advantages well outweighed them in the development of the VHF band.
When the US entered WWII all amateur activity was suspended. When WWII ended in 1945, a major battle ensued between ARRL (amateur radio), Armstrong (FM Broadcasting) & Sarnoff (RCA & NBC TV) over frequency allocations. The FCC moved FM commercial stations to the 88-108 MHz frequency band because of David Sarnoff, who desired disruption of FM development to further AM development, & moved the ham band to 144-148 MHz, allowing hams to come back on the air at 10m & 2m. Armstrong lost much of the investment he had made into FM stations in the previous band.
By late 1947 there were only 3,000 FM receivers in use for this new band, whereas there were 21,000 obsolete ones for the old band. FM did not really recover from this 1945 setback till the upsurge in hi-fidelity equipment in the late 1950's.
In addition, RCA claimed invention of FM radio & won its own patent on it, claiming to have developed it independently. Armstrong sued them for interference in 1948. After 6 years of costly patent fights with RCA & because of losses from Sarnoff's inluence in altering the use of FM, he was destitute & distraught. Edwin Armstrong committed suicide in 1954. Philo Farnsworth, who had patented a television system, also went broke battling RCA, and the millions he had dreamed of from his invention went to RCA. Armstrong's wife continued the battle over FM with RCA, & in 1967 the Supreme Court ruled Armstrong (not RCA or any of the other companies accused of infringing his patent) indeed had invented FM.
The superhet uses both amplifiers & oscillators, & it is worth noting a historical contribution of local significance. Popular oscillators had been developed in the 1920's, but George W. Pierce of Elgin, TX came up with a important innovation in them in the 1930's.
Pierce found that if an inductor in the Colpitt's oscillator is replaced by a quartz crystal, it becomes very stable because the crystal is piezoelectric (it deforms mechanically when a voltage is applied to it.). The resonance is precise, & one does not have to build an LC tank circuit to achieve the resonance frequency. A patent for this low-cost oscillator was issued to Pierce on October 18, 1938.
Manual repeaters were built as early as 1932, & required a control operator to be present before they could be used. Seversl manual AM repeaters appeared in the 1950's. However, in the 1960's fully automatic repeaters for FM were developed by hams from revamping VHF commercial equipment. That equipment became available because the FCC narrowed the allowed commercial transceiver bandwidth, so the wideband transceivers were no longer usable commercially. FM broadcasts & automatic repeaters were used extensively by hams in 1969 when Hurricane Camille devastated the Gulf Coast & Virginia. The growth of FM repeaters took off in the 1970's.
By 1970 the FCC proposed the 1st repeater rules, specifying the input & output frequencies allowed for repeaters on the 6m band, the 2m band, the 1.25m band, and the 70cm band. They did not allow automatic repeaters, but required a licensee in attendance to monitor all transmissions. However, rules like this were relaxed in the next few years, and in 1978 the FCC eliminated regulations against automatic repeaters.
Austin has 9 repeaters, with 4 on the 2m band, 1 on the 1,25m band, 2 on the 70 cm band, and 2 on the 23cm band. One of the 2m repeaters has an autopatch on it for telephone access.
Radio was considerably changed as the personal computer developed and became available as an increasingly useful tool in it use. This was predicted in 1964 by David Sarnoff, the man who found RCA in 1919.
From Sarnoff's biography, he said: "The computer will become the hub of a vast network of remote data stations and information banks feeding into the machine at a transmission rate of a billion or more bits of information a second. Laser channels will vastly increase both data capacity and the speeds with which it will be transmitted. Eventually, a global communications network handling voice, data and facsimile will instantly link man to machine--or machine to machine--by land, air, underwater, and space circuits. [The computer] will affect man's ways of thinking, his means of education, his relationship to his physical and social environment, and it will alter his ways of living... [Before the end of this century, these forces] will coalesce into what unquestionably will become the greatest adventure of the human mind."
Most of this prediction has happened, as the World-Wide Web has become the most used internet resource, with Email the 2nd most used of those resources. This has transformed the world & had its impact on the development of digital radio, including wave packets, the Automatic Position Reporting System (APRS) & the new HF digital mode PSK31.
2 computer-ham servces are pimarily 21st century developments. One is Hamsphere, which is a worldwide HF service using connections of all hams over its internet software. The other is Broadband-Hamnet services which are being developed at the upper UHF & even higher frequencies. The latter started with the labelling of High-Speed MultiMedia (HSMM)-MESH. It is a connection that is expanding across Texas & will soon hit nearby states.
"ARRL at 100: A Century of Ham Radio" at http://www.youtube.com/watch?v=jerSTUDql7s&feature=youtu.be
Continelli, William (W2XOY), History of Amateur Radio, 1996 & 2001, online at http://www.rollanet.org/~n0klu/Ham_Radio/History%20of%20%20Ham%20Radio.pdf
Douglas, Alan, "Who Invented the Superheterodyne", & material in issue on "The Legend of Edwin Howard Armstrong" Radio Club of America, vol. 64, #3 (Nov., 1990); the article on the superhet is reprinted online at http://antiqueradios.com/superhet .
"Edwin Howard Armstrong, " online at http://en.wikipedia.org/wiki/Edwin_Howard_Armstrong
"FM Broadcasting in the United States" at http://en.wikipedia.org/wiki/FM_broadcasting_in_the_United_States
Hawker, J P, Radio and Television (Hart, 1966).
Lyons, Eugene, David Sarnoff (1966).
Maxwell, Jim W6CF, "Amateur Radio: 100 Years of Discovery", QST (Jan., 2000) online at http://www.arrl.org/files/file/About%20ARRL/Ham_Radio_100_Years.pdf
"More About Sarnoff" in "Big Dream, Small Screen" on American Experience, online at http://www.pbs.org/wgbh/amex/technology/bigdream/masarnoff2.html
"Pierce Oscillator", online at http://en.wikipedia.org/wiki/Pierce_oscillator