Apex (radio band)
They employed amplitude modulation (AM) transmissions, although in most cases using a wider bandwidth than standard broadcast band AM stations, in order to provide high fidelity sound with less static and distortion.
In 1937 the Federal Communications Commission (FCC) formally allocated an Apex station band, consisting of 75 transmitting frequencies running from 41.02 to 43.98 MHz.
However, during the nighttime, changes in the ionosphere resulted in additional long distance skywave signals, that were commonly reflected for up to hundreds of kilometers.
Investigations of increasingly higher frequencies found that, above around 20 MHz, signal propagation by both groundwave and skywave generally became minimal, which meant that station coverage now began to be limited to just line-of-sight distances from the transmitting antenna.
This was considered to be a valuable characteristic by the FCC, because it would allow the establishment of broadcasting stations with limited but consistent day and night coverage, that could only be received by their local communities.
[4] Because the standard AM broadcast band was considered to be too full to allow any meaningful increase in the number of stations, the FCC began to issue licenses to parties interested in testing the suitability of higher frequencies.
[6] While monitoring the first group of stations, it was soon realized that, due to the strengthening of the ionosphere during periods of high solar activity, at times the lower end of the VHF frequencies would produce strong, and undesirable, skywave signals.
In May 1940, the FCC decided to authorize a commercial FM band effective January 1, 1941, operating on 40 channels spanning 42–50 MHz.
Beginning in May 1946, consulting radio engineer Sarkes Tarzian operated a 200-watt experimental AM station, W9XHZ, on 87.75 MHz in Bloomington, Indiana.
Tarzian promoted this as a low-cost alternative to expensive FM transmitters and receivers, saying that a $5.95 converter could be added to existing AM radios that would allow them to pick up the HIFAM stations.
[26] Moreover, after his station's final license expired on June 1, 1950, the FCC denied Tarzian any further renewals,[27] concluding it would not reverse its earlier determination that there was no need for a second AM broadcast band.
