I will see if I can find another source on the internet, but one of the most fascinating stories of the 'APEX" stations is the story of W41MM, licensed to WInston-Salem NC, but transmitting from Clingman's Dome over 100 miles away. ERP was 50kW with an antenna 6000+ ft in the air.
Gordon Gray claimed that Nazi U boats used the W41MM signal to "home in" on the coast of NC. That would have been 300+ miles or so from the mountaintop site.
The station was operated by Gordon Gray, who is a fascinating "broadcaster" of the era. Gray was also Secretary of War, and was high up in Reynolds Tobacco. He owned WSJS-AM, in Winston-Salem, and paired W41MM with WSJS.
W41MM eventually became either WKNC-FM 97.3 in North Wilkesboro NC, or WMIT 106.9, Black Mountain NC (depending on which fork you follow as Gray sold the station, because he could not make any money with it.
Several decades later, WSJS was again paired with an FM at 104.1, and a TV station on Channel 12.
Having worked at WSJS, it was a very historical place for NC broadcasting.