Complex sentence: By 1940, the U.S. called upon the Telecommunications Convention to prohibit amateurs from contacting any other HAMS outside of the United States and banned any portable and mobile operation below 56 MHz. Thus, radio amateurs were denied the use of the air.
Split into simpler sentences: By 1940, the U.S. called upon the Telecommunications Convention to prohibit amateurs from contacting any hams outside of the United States. It was at this time that the U.S. banned any portable and mobile operation below 56 MHz. Thus, radio amateurs were denied the use of the air.