For seven years, Kim Seongmin has been facing a cancer that has spread to his lungs, brain and liver. Doctors recently gave him only months to live. He can’t sleep at night without painkillers.

       Still, Mr. Kim broadcasts into North Korea twice a day, bringing its people news and information they are cut off from because of Pyongyang’s strict censorship laws.

       “North Korea is keeping its people like frogs trapped in a deep well,” ？said Mr. Kim？, 62, during an interview at his rural home on this island west of Seoul, where he records and edits shows for Free North Korea Radio. “We broadcast to help them realize that there is something wrong with their political system.”

       For two decades, North Korean defectors living in South Korea have been infiltrating the North with outside news and entertainment, through balloons floated across the border or broadcasts such as those from Mr. Kim’s radio station.

       But Kim Jong-un, the North Korean leader, has grown increasingly sensitive to “anti-socialist and nonsocialist” influences that could threaten his totalitarian grip on power, and he is cracking down on such efforts like never before.

       The authorities are searching homes and pedestrians, meting out harsh punishments, including public executions, to people who consume news and TV dramas ？from South Korea, or even if they sing, speak？, dress ？and text-message like South Koreans, according to North Korean documents and a South Korean government report.

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