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. 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.