 Due to the high power of radio frequency signals that pass through the room, it is completely shielded in copper and is off-limits during broadcasts.On January 30, 1940, the Communications Research Laboratory (the predecessor of NICT), began operations of JJY as a shortwave station, broadcasting at 4, 7, 9, and 13 MHz.Over the years, these were dropped and by the late 1950s, JJY was transmitting its time signal on standard frequencies of 2.5, 5, 8, 10, and 15 MHz.