As a tribute to the station's centrality in country music history, the diamond antenna design was incorporated into the new Country Music Hall of Fame and Museum's design in 2001. The tower is listed as a National Engineering Landmark and was listed on the National Register of Historic Places on March 15, 2011. Former sister stations In 1939, WSM began operating an experimental high-frequency, high-fidelity AM "Apex" station, W4XA, on 26,150 kHz. This was replaced in 1941 by a commercial FM station, initially with the call sign W47NV and operating on 44.7 MHz. This was reported to be first commercial FM to be fully licensed; although a few FM stations had begun broadcasting earlier, they were operating under experimental or "Special Temporary Authorizations" and had not yet been granted operating licenses. In 1943 the call sign was changed to WSM-FM, however the station was shut down in 1951, although its antenna is still mounted atop the Blaw Knox tower at Brentwood. Seventeen years later the current incarnation of WSM-FM was established after a National Life subsidiary purchased WLWM and renamed it WSM-FM in 1968. This WSM-FM (95.5 MHz) was WSM's sister until 2008, when Cumulus Media, the full owner of WSM-FM since 2003, ended its joint sales agreement with the AM (see below). Despite having the same base call sign, the two stations are no longer related; incidentally, both the current WSM-FM on 95.5 MHz and the current occupant of the 103.3 frequency vacated by the original WSM-FM, WKDF, are now sister stations, with each separately broadcasting a country music format. Ownership and facilities For most of its history, WSM, along with WSM-TV and the Grand Ole Opry, was owned by the Nashville-based National Life and Accident Insurance Company. The stations' call letters were derived from the company's motto, "We Shield Millions." The station's broadcast studios were first located in the company's building on Seventh Avenue and Union Street in downtown Nashville; this was the original home of the Opry, until 1934. The studios remained in this location until the mid-1960s, when the company built a new headquarters building downtown and new studios for WSM-TV on Knob Road in west Nashville (the TV station had been located near Belmont College). Upon completion of the new headquarters, National Life and Accident Insurance Company chose to relocate WSM radio to their new TV studios, and WSM radio, joined in 1968 by its