Rupert Murdoch, the Australian publisher whose newspaper empire spans three continents, has signed a $75 million agreement with Satellite Business Systems that would enable him to launch a nationwide direct-broadcast satellite-to-home television network by the end of this year.

News Satellite Television, a British-based company controlled by Murdoch, signed an agreement with the McLean, Va., company that would give it the rights to five transponders on the SBS III satellite, which was the first to be launched by the space shuttle last November. These transponders would, in turn, be made available for broadcast to a joint-venture between Inter-American Satellite Television, a California company, and NewsAmerica, Murdoch's U.S. company, which owns the New York Post, the Village Voice, the Boston Herald and several other newspapers and magazines.

A transponder on a direct broadcast satellite acts as a super-transmitter, beaming television programs into homes equipped with special receivers, or satellite dishes. The dishes that the Murdoch joint venture will offer will range from 1.2 meters to 1.8 meters (3 1/2 feet to 5 feet) in diameter. The joint venture seeks to service homes that are not readily accessible by cable television.

"We're going to have five channels of video programming," said William Kommers, the president of Inter-American Satellite Television, "including a 24-hour-a-day premium movie channel, family interest programming, sports and other kinds of programming." However, he would not disclose any details.

Kommers said that "agreements are in place" both to manufacture the dishes and to distribute them to subscribers of the new TV service. "We're excited by this new venture," said Donald Kummerfeld, president of NewsAmerica. "It represents a logical extension for us." However, he says, NewsAmerica will initially provide financial resources to the venture rather than initiate original programming development.

Murdoch, whose tabloid papers are known for bold headlines and spicy gossip, has extensive television interests in Australia and has been involved in British television programming. He was co-executive producer with the Robert Stigwood Organization of the film "Gallipoli." However, he has been conspicuous by his absence in the cable television market in the United States and England.

This move further confuses the fledgling direct broadcast satellite television business in the United States. Originally, the Federal Communications Commission had authorized specially designed high-power satellites to provide direct broadcast services into homes. These satellites were not expected to be launched until 1986.

Communications Satellite Corp., the Washington-based satellite company, had set up a subsidiary, Satellite Television Corp., to provide broadcast services at that time. CBS and RCA had also received FCC permission for direct broadcast satellites.

However, the United Satellite Communications, Inc., a New York-based company, discovered in 1981 that existing low-power satellites could be used to provide a direct broadcast service. With money from both General Instrument Corp. and Prudential Insurance, USCI is expected to launch its satellite service by the end of this year as well. The Murdoch joint venture is very similar in nature to USCI's.