North Korea targets between 10 and 15 frequencies used by international short-wave broadcasters, such as U.S.-funded Radio Free Asia and stations operated by South Korea's government, for up to 18 hours a day, and on major occasions like the April centennial, it jams radio signals around the clock, Williams said.
The North appears to have recently installed more sophisticated transmitters acquired from a Chinese company, although jamming operations have been up and down this year, likely because of technical problems or power shortages, he said.
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