As other countries, notably Mexico and Cuba, have developed their own broadcasting services, there has been a need to standardize technical practices, reduce disruptions and distribute clear channel allocations more equitably. In addition, the development of better frequency regulation, and in particular indicative antennas, has enabled additional stations to operate on identical or close frequencies, without significant displacement. An important objective for the United States was for Mexico, in exchange for a clear distribution of channels, to eliminate the powerful English-language “grenzblaster” stations, which had directed their programming to the United States, causing serious disruption to American and Canadian channels.  However, a first international meeting held in Mexico City in the summer of 1933 failed, mainly due to the lack of agreement on the number of clear frequencies to be allocated to Mexico.  Due to improved radio communications design, the agreement also reduced the minimum separation of the “same market” from 50 to 40 kHz (Mexico decided to continue to introduce a 30 kHz “same market” spacing, unless it was contrary to the “limit zones” of a neighbouring country.) This narrower distance was particularly important for the two highest local frequencies: 1420 and 1500 kHz, since the stations were moved on these frequencies at 1450 and 1490 kHz, a separation of 40 kHz.  The North American Regional Broadcasting Agreement (NARBA, Spanish: Convenio Regional Norteamericano de Radiodifusi`n) refers to a series of international treaties defining technical standards for am-band (medium wave) radio stations. These agreements also focused on the distribution of frequency allocations among signatories, with particular emphasis on the allocation of efficient and clear channels. The 1950 NARBA provisions are still in effect for the Bahamas, the Dominican Republic and the United States as these countries have not officially repealed NARBA.  The United States also has active bilateral agreements with Canada (“Agreement between the Government of the United States of America and the Government of Canada through the am Broadcasting Service in the Medium Frequency Band” (1984)” ] and Mexico (1984) “Agreement between the Government of the United States of America and the Mexican Government on Broadcasting Service in the Median Frequency Band (1986)  Under the terms of the agreement , its implementation should take place within one year of its adoption. the four main signatories to the Pact, the United States, Canada, Cuba and Mexico. On December 22, 1937, Cuba was the first country to be ratified and followed by the United States on June 15, 1938 and Canada on November 29, 1938. Meanwhile Mexico, the United States and Canada entered into a frequency agreement in 1939 on the basis of contractual standards.
Mexico finally approved the treaty on December 29, 1939, and began work on its broad provisions. The am radio program began in the early 1920s and the United States quickly dominated the North American radio airwaves, with more than 500 stations until the end of 1922. Due to a change in the ionosphere after sunset, the nocturnal signals of am bands are reflected for distances for hundreds of kilometres. This is important for radio programming for low-density areas with very powerful transmitters.