JapanJA0: Shin'etsu
Callsigns beginning with JA0 were issued in Niigata, Nagano, in Japan. The same area appears in the series JE0, JF0, JG0, JH0, JI0, JJ0, JK0, JL0, JM0, JN0, JO0, JP0, JQ0, JR0, JS0, 7J0.
Call area
Every JA0 station counts for CQ zone 25 and ITU zone 45.
How call areas work in Japan
Japan is divided into ten call areas, numbered 1 to 9 and 0, each a group of prefectures: area 1 is Kanto around Tokyo, area 3 Kinki around Osaka, area 8 Hokkaido, area 6 Kyushu and Okinawa. The JA to JS series (JD apart) and 7J share the digits; 7K to 7N are issued only in area 1.
JD1 is used for Ogasawara and for Minami-Torishima, which are separate DXCC entities. All of mainland Japan is in CQ zone 25 and ITU zone 45, so the area matters for domestic awards and contests such as the JIDX, where JA stations send their prefecture number.
Portable and moved stations
Operators away from home often add a suffix such as /P or the number of the area they are in, and many keep a JA0 call after moving. For awards the place of operation counts, so the prefix is a strong hint, not proof.