The Havana Treaty of 1937

The Havana Treaty of 1937

Who doesn't remember being entertained by the phenomenon of overnight AM 'skip'?  Officially and scientifically I am referring to a term, 'skywave' which is the result of the "propagation of radio waves reflected or refracted back toward Earth from the ionosphere, an electrically charged layer of the upper atmosphere."  The position of the ionosphere, some 80 to 1,000 kilometres in altitude, is of great advantage in that radio waves are not limited by the Earth's curvature.  The ability to communicate beyond the horizon and more importantly, at intercontinental distances was and continues to be favoured by ham radio enthusiasts since the 1920's.  Skywave propagation is mostly used in the shortwave frequency bands.

The growing popularity of AM radio broadcasting meant North America became dotted with channels, many causing interference with each other, particularly during night-time hours.  Such times in AM radio were like the old 'wild west'.  Organization and cooperation on an international level was needed to establish control and moderate the strength, signals and patterns radiating from AM transmitter sites.

There was some cooperation between countries but this was not sufficient to resolve the issue as the problem with interference was a continent-wide concern.  An international effort resulted in the Havana Treaty of 1937.  In that year, North American countries came together in Havana, Cuba for the purpose of classifying AM channels by setting maximum interference levels from other stations.  However, prior to the Havana gathering, there was much homework to be done.  A great deal of research, measurement and analysis was conducted.  Charts were developed to illustrate and determine the distance of signals.  The nature of AM radio skip required a night-time solution.  It was deemed that a protected station would have the benefit of a protected contour and to that would maintain a specific signal strength for 50% of the time, while from another source, an interfering signal reaching the protected station's contour would be effective only 10% of the time.  It was also deemed that interfering signals were to be several times lower than the protected signal.

The levels set for 'protection' and 'interference' defined a class to which would be assigned to AM radio stations.  There would be four major classes and some sub-classes.

Following the Havana gathering, it then took a year to receive confirmations from each country indicating that the Treaty would apply in its own domain.  The Havana Treaty took effect March 29, 1941.

As a result of the Havana Treaty, CKSO Radio changed its frequency from 780 to 790 kHz.  Its output power remained at 1,000 watts.