All television and radio stations in the United States will go off the air for half an hour this morning, signaling the start of the annual national civil defense drill. The office of Civil and Defense Mobilization said the unusual silence will begin at 11:30 a.m. All this wll be part of Operation Alert 1959, designed to test the nation's readiness to cope with a nuclear attack by planes and missiles. Operation Alert will continue through the summer in various stages.