Radio communication was initially unregulated in the United States, and at first Herrold used a variety of self-assigned identifiers for his station, including FN and SJN, plus, for audio transmissions, "San Jose calling". The Radio Act of 1912 established the licensing of stations through the U.S. Commerce Department, and in late 1915 Herrold was issued an Experimental Station License with the call sign 6XF. Although Herrold reported success in developing his system, his arc-transmitters were low-powered and would only work at wavelengths above 600 meters (frequencies below 500 kHz). The concurrent development of vacuum-tube technology, which did not have the same limitations, started making arc technology obsolete.