It provides at least grade B coverage as far southeast as Chattanooga, as far northwest as Evansville, Indiana, as far west as Jackson, Tennessee and as far south as Huntsville, Alabama. At night, it can be heard in much of the eastern and central United States. The station traditionally played country music in the nighttime hours, when listeners from around the United States would tune in. During daytime hours, the station broadcast long-form radio, including both local and NBC network programs, in addition to music. After television became popular (thus largely eliminating the audience for full-length radio programs), WSM adopted a middle of the road (MOR) music format during the daytime hours, and continued to play country music at night. It was not until 1980 that WSM adopted the 24-hour country music format of today. WSM is credited with helping shape Nashville into a recording industry capital. Because of WSM's wide reach, musical acts from all across the eastern United States came to Nashville in the early decades of the station's existence, in hopes of getting to perform on WSM. Over time, as more acts and recording companies came to Nashville, the city became known as the center of the country music industry. Disc jockey David Cobb is credited with first referring to Nashville as "Music City USA", a designation that has since been adopted as the city's official nickname by the local tourism board. In 1996, the station was named Radio Station of the Year at the International Bluegrass Music Awards. Tower WSM's unusual diamond-shaped antenna (manufactured by Blaw-Knox) is visible from Interstate 65 just south of Nashville (in Brentwood) and is one of the area's landmarks. It is located near the I-65 exit 71 interchange with Concord Road (State Highway 253). When the 878-foot tower was built in 1932, it was the tallest antenna in North America. Its height was reduced to 808 feet (246 m) in 1939 when it was discovered that the taller tower was causing self-cancellation in the "fringe" areas of reception of the station (it is now known that 195 electrical degrees, about 810 feet, is the optimum height for a Class A station on that frequency). For a period during World War II it was designated to provide transmissions to submarines in the event that ship-to-shore communications were lost. It is now one of the oldest operating broadcast towers in the United States.