When broadcasting licenses were first granted in 1921, all stations were assigned a common wavelength of 360 meters (equivalent to a frequency of 833 kHz). This of course would mean that no two stations in the same area could be on at the same time. In December 1921, a second wavelength, 485 m (corresponding to a frequency of 619 khz) was assigned for some special types of broadcasts like weather reports and farm information. But rather than assigning some stations to one frequency and some to the other, the division was made by content, so a station would switch frequencies when it was time to give a weather report.
In September 1922, a new entertainment frequency of 750 khz was added. In theory, 750 kHz was assigned to better quality, higher powered stations. These stations were designated "Class B" outlets, while those on 833 kHz became known as "Class A" stations. About thirty stations nationwide would eventually qualify to use 750 kHz.
By early 1923, it became obvious that two entertainment frequencies were not going to be enough, and a conference was called by the Secretary of Commerce (later to be President), Herbert Hoover. The conference was held from March 20 to 24 of that year, and issued a report recommending an expansion of the band. These recommendations were used as a basis for the new frequency reassignments.
The report contemplated setting up two new classes of broadcasting station, assigning each a frequency range, while allowing a third class of stations that could continue to use the 833 kHz frequency:
- Class A stations (restricted to power not exceeding 500 watts) would be allotted frequencies between 1000 and 1350 kHz (described in the report in terms of wavelengths between 300 and 222 meters).
- Class B stations (allowed power up to 1000 watts) would be allotted frequencies between 550 and 800 kHz (described in terms of wavelengths between 545 and 375 m) and between 870 and 1000 kHz (described in terms of wavelengths between 345 and 300 m).
- Class C stations would be allowed to continue at 833 kHz (360 m), but no new class C licenses would be issued. It was envisioned that over the passage of time, these would disappear by attrition, which is exactly what occurred.
On May 15, 1923, the government started assigning different frequencies to different stations, at 10 kHz intervals. (This was also the beginning of the use of frequency rather than wavelength in assignments in the licenses.) Initially, these went only from 550 to 1360 khz. The frequencies from 550 to 1040 kHz were set aside for Class B stations, while the remaining frequencies were designated for Class A. (Note that this is slightly, but not greatly, different from the recommendations in the report.) Not all stations left the 833 kHz frequency at this time, however, and these stations were designated as Class C. These gradually cut over to frequencies in the 10 kHz-interval setup over the next few years. The most important stations, however, changed frequencies at this time.
Stations changing frequencies at that time
The United States Department of Commerce, which oversaw radio licensing at that time, issued monthly Radio Service Bulletins with news on new and altered licenses. The June 1, 1923 issue gave a list which showed stations whose licenses were altered, meaning, in general in this case, those whose frequencies were changed from 833 kHz to something else. Most of these must have changed on May 15, though it is not possible to distinguish between those stations which changed frequencies on the 15th and those whose frequencies were changed between the 16th and 31st. It can be assumed, however, that nearly all, if not all, stations on this list changed their frequencies on the 15th:
|Call sign||Frequency on June 1, 1923|
Several stations which had been authorized to operate on both 750 and 833 kHz were allowed to keep operating on 833, but eliminated the added 750 kHz frequency. According to the June 1 Radio Service bulletin they were: