Radio Free Afghanistan, the U.S.-funded station established by Congress in 1985, will begin broadcasting Sunday in Pashto, one of the two principal languages of Afghanistan, sources connected with the station said yesterday.

Six hours of programming in Pashto will effectively double the amount of broadcasting time in which the radio beams uncensored news and information to the Soviet-occupied nation.

The radio has been broadcasting in Dari, the country's other main language, for six hours a week since 1985.

The radio is controlled by the Board for International Broadcasting (BIB), a private corporation whose members are appointed by the president and that is funded by Congress.

The BIB, whose charter grants it independence from government control, operates the Munich-based Radio Free Europe, which broadcasts to East European countries, and Radio Liberty, which broadcasts to the Soviet Union.

These operations are intended to serve as "surrogate stations" providing news, information and cultural programming to countries where the news media are controlled by the government.

Radio Free Afghanistan, which also operates from Munich, is supposed to serve a similar function. Under its original authorizing legislation, sponsored by Sen. Gordon J. Humphrey (R-N.H.), it is to broadcast until the end of the Soviet occupation, which began in December 1979.

In addition to Radio Free Afghanistan, the Voice of America, a part of the U.S. Information Agency, broadcasts to Afghanistan 28 hours a week, half in Dari and half in Pashto.

USIA also runs an Afghan Media Project designed to train Afghan resistance fighters in journalistic techniques so they can return home and produce uncensored news for Western news media.