The Communications Satellite Corp. sold yet another division last week, paring the company back more closely to its primary business of providing international satellite service -- a monopoly bestowed on it when it was created by Congress 25 years ago.

Comsat sold its mobile satellite antenna business to Satellite Telesystems International for an undisclosed sum. Among the division's products are portable earth stations -- antennas that weigh less than 100 pounds and can be assembled in less than 20 minutes -- and other devices that are used for communication aboard ships.

The business is one of several that Comsat has divested in recent months as part of a restructuring.

Two other divisions -- an international telephone and data communications subsidiary and a subsidiary that made earth stations for private data networks -- were sold to Contel Corp.

Contel, the country's third-largest independent telephone company, bought the units as part of an agreement to dissolve a $2.3 billion merger between the two companies.

Contel backed out of the deal, citing adverse regulatory decisions against Comsat.

Comsat controls U.S. access to Intelsat, the international consortium of satellite firms, and arranges for companies such as American Telephone & Telegraph Co. to use the system.

The Federal Communications Commission, which sets profits for this part of Comsat's business, recently ordered Comsat to refund millions of dollars to American users of Intelsat satellites.

Comsat last week also closed its sale of Amplica Inc. -- a marginally profitable business that makes equipment for the defense electronics industry -- to Triax Corp., also for an undisclosed sum.

Comsat's restructuring resulted in an $84 million loss in the second quarter that will force the company into the red for the year.

Comsat plans to cut corporate overhead and begin tougher marketing of its existing satellite services.

"I am looking at 1987 and 1988 as a year of focus and performance," said chief financial officer Bob Perry. "I actually believe that Comsat will be able to grow profitably over the long term."

During the next three to six months the number of employes will shrink by up to 50 percent, mainly because of the sales of divisions, said Richard McGraw, a company spokesman.

Beyond that, more corporate staff positions will be cut, but how many has not been decided, he said.

"We will reduce the size of the corporate staff to support a smaller core," McGraw said.

Between Jan. 1, 1986, and Dec. 31, Comsat will have reduced its corporate expenses from $46 million to $33 million a year -- a reduction of 28 percent, he added.

Meanwhile, Comsat has cut the price of leasing satellite circuits and has signed up customers such as AT&T for five- to seven-year contracts.

It also has devised ways to fit more telephone and data information onto the same satellite circuit, which it said will benefit customers.

In addition, Comsat is pressing ahead with signing up customers for private-line international voice and data service over the Intelsat satellite system and increasing sales of its hotel video service, which is expected to beam programs via satellites to 300,000 hotel rooms by the end of the year.

Despite the changes, Comsat's future is not without problems.

With the advent of fiber optic cables -- thin strands of glass used for voice and data communication -- satellites will face tough competition from undersea cables that are superior in quality, said John S. Bain, an analyst with Shearson Lehman Bros.

The first fiber optic undersea cable spanning the Atlantic will go into operation next year. The cable, owned by 29 companies, including AT&T, could give Comsat trouble -- especially if federal regulators remove restrictions on how much overseas communications traffic goes by cable or satellite.

Comsat has estimated that it could lose $311 million in revenue in the next 13 years if the FCC drops its restrictions.

"The first company to come out with an ad that says, 'Use our international telephone service, we have no annoying delay' -- that would be the end for satellite service," said analyst Bain, referring to the bothersome echo effect that is heard on many long-distance calls.

With the advent of fiber optics, "satellite technology as a backbone is over. Its time has come and gone," he said.

Satellites, however, will always be needed for backup service, said Bain, and Comsat is quick to point out that satellites are more reliable than undersea cables, which have experienced failures for a variety of reasons.

The Reagan administration has asked Comsat and AT&T to discuss how much communications traffic should travel by satellite and how much by cable as the FCC considers removing current rules.

AT&T, which backs removing set limits because of a proliferation of competition in telecommunications, said it would not switch all of its traffic to undersea cables if given the choice.

"AT&T is committed vigorously to support Intelsat," said AT&T spokesman Herb Linnen. "It's been our policy for many years not to put all of our eggs in one basket. ... We need to have diversity in our facilities." Some locations are also not economically served by undersea cable, he said.

Nevertheless, Comsat's Perry concedes that eventually revenue from Intelsat communications traffic will decrease as competition intensifies. Growth in other sectors of Comsat's business is expected to offset losses, he said.endqua