Communications Satellite Corp. yesterday announced a slight increase in profits for 1984 of $1.1 million, putting earnings at $51.2 million ($2.83 per share) on revenue of $442.3 million. Earnings in 1983 were $50.1 million ($2.77) on revenue of $440 million.

Profits for the fourth quarter of 1984, ending Dec. 31, were $14.3 million (79 cents) on revenue of $120.3 million, an increase of $5.6 million (32 cents) from the fourth quarter last year. Revenue in the fourth quarter last year was $107.7 million.

Comsat attributed the increase in income to the sale of its one-third stake in Satellite Business Systems, a digital business communications provider, to International Business Machines Corp. and Aetna Life & Casualty Co., the two other partners in the venture. Other factors contributing to the increase were higher earnings in rate-regulated satellite services and higher investment tax credits associated with a new NBC programming distribution system Comsat will provide.

These amounts were partially offset by Comsat's termination of its involvement in direct satellite-to-home television broadcasting through its Satellite Television Corp. subsidiary.

Profits also were offset by increased losses from the write-down of inventory in the corporation's telecommunications equipment business and anticipated reductions in revenue for services provided through the Comstar and Marisat satellite systems Comsat operates. The Federal Communications Commission ordered the company to lower its rates slightly during the third quarter.

Income from continuing operations in 1984 was $45.2 million; $28.8 million less than the comparable amount for 1983. Of this decrease, $13.5 million was attributed to write-offs of Satellite Television Corp. and $4 million to telecommunications equipment write-offs.

Some analysts said yesterday they expect strong growth in Comsat's earnings over the next 12 months because of the divestiture of Satellite Business Systems and Satellite Television Corp. Analysts said the core business of providing satellite services through Intelsat, the global satellite consortium in which Comsat holds a 23 percent share, will continue to be moderate.