their own radio broadcasting services, the need arose to standardize engineering practices, reduce interference, and more fairly distribute clear channel assignments. Moreover, the development of better frequency control, and especially directional antennas, made it possible for additional stations to operate on the same or close by frequencies without significantly increasing interference. A key objective for the United States was that, in exchange for receiving clear channel assignments, Mexico would eliminate the high-powered English-language "border blaster" stations that had been directing their programming toward the U.S. and causing significant interference to U.S. and Canadian stations. However, an initial international meeting held in Mexico City in the summer of 1933 failed, primarily due to a lack of agreement over how many clear channel frequencies would be assigned to Mexico. 1937 "Havana Treaty" In 1937, a series of radio conferences, this time successful, was held in Havana, Cuba, and the initial NARBA agreement was signed on December 13, 1937 by representatives from the United States, Canada, Cuba, Mexico, the Dominican Republic and Haiti. The most significant change was the formal addition of ten broadcasting frequencies, from 1510 to 1600 kHz, with the 106 available frequencies divided into Clear Channel (59 frequencies), Regional (41) and Local (6) designations. The official lower limit remained at 550 kHz, as it was not possible to add stations at the bottom of the broadcast band due to the need to protect 500 kHz — a maritime international distress frequency — from interference. (Although operation on 540 kHz was not covered by the Agreement, unofficially it became an additional Canadian clear channel frequency.) Under the Agreement, most existing stations operating on 740 kHz or higher would have to change frequencies. Open frequencies were created throughout the band by "stretching out" the existing assignments, achieved by following a table which in most cases moved all the stations on a common frequency to a new, higher, dial position. This provided gaps of unassigned frequencies, most of which became clear channels allocated to Mexico and Canada. A majority of the frequency shifts were limited to between 10 and 30 kHz, which conserved the electrical height of a station's existing vertical radiator towers, an important factor for readjusting directional antenna parameters to accommodate the new frequency. Individual stations were specified to be Class I, II III or IV, with the class determining the maximum power a station could use and its interference protection