The Havana Treaty of 1937
AM broadcasting in North America had grown in al1 countries to such an extent that many channels caused interference with other assignments - especially at night-time. There may have been some cases where channel re-arrangements were made with an adjacent country, but these proved to be ineffective because the interferences were continent-wide - not just country to country. Thus a meeting of all North American countries was held in Havana Cuba in 1937 which classified AM channels and set the maximum interference levels permitted from other stations. It then took a year to have each country confirm that the Treaty would apply in their domain.
A key development that was undertaken before the Havana meeting could be called was to have research done and charts developed to show how far signals would go. This had taken years of measurement and analysis, especially for sky-wave signals which are quite variable. To acknowledge this problem the Treaty basically said that for night-time signa1s protected station would have its protected contour at a line where the signal was a specific strength for 50% of the time, and the interfering signal (and that protected contour) would be there only 10% of the time. In addition, the interfering signals were specified as being several times lower than the protected signal. These "protection" end "interference" levels were set differently for each class of station.
There were four major classes stations created including Class 1 ("clear" channels), Classes 2, 3, and 4, - plus some sub-classes such as 1-A, 1-B mid 1-C. Most 1-As did not have another station on their channel at night anywhere on the continent while I -Bs usually had two stations over 1,000 miles apart and mutually protecting each other. Class 2s could be on a I-A channel, but perhaps daytime only. Class 4 dealt with the local channels and Class 3 with Regional channels. The Classes 2, 3 and 4s had progressively poorer protections in order to provide enough stations to serve the many communities.
The Havana Treaty took effect March 29, 1941
Vice-President Engineering, (ret. 1986)
CFRB Limited, Toronto Ontario
Clive Eastwood - September, 2001