Ford Aerospace & Communications Corp., one of the nation's largest defense contractors, said yesterday that it plans to expand its regional research and development operations with a new high-tech facility in Howard County, which eventually would provide 800 new jobs.

Ford Aerospace signed a contract Monday to build a 400,000-square- foot facility on 47 acres of an 180-acre industrial park soon to be completed on the Anne Arundel-Howard County line, according to Mark Miller, public affairs manager for Ford Aerospace. The move would make Ford Aerospace one of the county's biggest employers, he said.

However, the expansion would also mean a loss of jobs for Anne Arundel. For eight years, the California and Washington-based subsidiary of Ford Motor Co. has had its local operations in the Parkway Industrial Center in Anne Arundel.

Company officials began considering expanding into nearby Howard when they realized the current facility would be inadequate to meet their future needs.

Under the expansion, three-quarters of the 650 current employes would remain in the original facility, while the rest and any future workers would move to the Parkway Corporate Center, which is being developed on an adjacent parcel of land.

Jeff Stone, Anne Arundel's director of economic development, said yesterday that he was pleased by the company's decision even though it meant a net loss of jobs for his county.

"Howard's gain is the region's gain," Stone said.

"Essentially, we liked the state of Maryland and we've had a very good experience in Anne Arundel," Miller said. "When we wanted to expand, our primary concern was being close to the existing site." Miller added that Howard County officials had worked hard to make the project work.

Plans call for construction on the project to begin next year, with an initial phase to be completed by 1989. The entire project is not expected to be finished until 1993, Miller said.

Yesterday's announcement followed months of negotiations between Howard and Anne Arundel planners and state transportation officials. Although officials in Howard had long been confident that the project would go through, by late yesterday they still had not received word that the contract had been signed.

Among the issues that had prevented the deal from being closed before this week was concern over the amount of traffic the project would generate on Dorsey Road, an already crowded artery in Anne Arundel.