FAQ for
Welcome Back, Mr. McDonald (1997) More at IMDbPro »Rajio no jikan (original title)

American radio frequencies on AM, or Medium Wave, have always been 10 kiloHertz apart. The rest of the world used to be the same way. However, in order to get more stations into the same band the rest of the world went from 10kHz wide frequencies to frequencies that were 9 kHz wide. The band for stations in America went from 540-1600 kHz, while the rest of the world had a 531-1620 (or 1629).

Later on, the America band was stretched out to 1700.)

The call signs in Japan start with JO--, therefore JOPP is a valid call sign, but this station is not real.