FM RADIO DEVELOPMENT

by R. J. Reiman, Historian

Edwin Howard Armstrong had been known for the invention of the superheterdyne circuit for radio receivers and went on to development and advocacy of wideband frequency modulation (FM) for radio broadcasting. Did Armstrong invent FM ? This was bitterly disputed by RCA and only after Armstrong's tragic death did RCA settle with his estate on his patents, tacidly admitting his primary claim. Had David Sarnoff conceided this during Armstrong's life, it may have averted his suicide. Armstrong was a very proud man and wanted the recognition to an intense degree.

Should FM broadcasting be permitted ? This was a very easy answer, for the FCC, the industry and the public were clamoring for more licensable broadcast channels and the technical advances in fidelity that FM promised. Only RCA held back, for they were dominant in AM and didn't want a competitor for their vested interests.

What portion of the radio spectrum should be allocated to FM ? In the period before World War II, the FCC allocated forty 200-kHz channels in 8 MHz of spectrum space from 42 to 50 MHz for educational and commercial FM broadcasting, 5 and 35 respectively. The FCC raised the question

"The effect of sky-wave interference until additional stations are placed in operation and --- later developments are known." The development of radar and FM broadcasting research during the war were to change the allocation. Meanwhile, following the allocation, 58 stations had construction licenses and 400,000 receiver sets were sold, many by Zenith. The War Production Board halted further development of FM on December 7, 1941.

At the beginning of the war, VHF technology was in its infancy, transistors did not exist, knowledge of the properties of VHF radio waves was limited and few engineers and technicians had training or experience in the design and maintenance of VHF equipment. All this would change with war-time experience. Another result was the war-time field measurements by the War Department and Bureau of Standards. An apparent problem existed at about 30MHz, the lower limit of the VHF range, in that certain abnormal ionospheric conditions, sky-wave transmission sometimes occurred, and this might cause interferences between stations at the end of their broadcast range, particularly in rural areas. Sky waves result from the reflection of radio signals by layers of ions in the upper atmosphere. Ionization is caused by the sun's rays with the degree varying from day to night, the latitude, the season, and the sun-spot cycle. The sky-wave propogation of short-waves (3 to 36 MHz)

provides an effective long distance transmission medium during the daytime. Short-wave propogation is to weak to be of practical use for communications at frequencies above 30 MHz, but under abnormal conditions can cause objectional interference. The FCC had a case to reallocate FM frequencies to a higher range, and this would obsolete the 400,000 receivers already sold and Zenith's technical edge. RCA, however, would benefit since they had little investment in FM.