Radiation Systems Inc., a $47 million company based in Sterling, Va., has named Harold Gruen, a former Fairchild Industries Inc. executive, as its president and chief operating officer.

The president's job was held by Richard E. Thomas, the company's chairman and chief executive officer. Thomas is remaining in those positions.

Robert L. Pevenstein, vice president of finance for RSI, said there was a need for additional management at the company. RSI has acquired three new companies in the past three years, he noted, but "did not increase the number of key managers needed to run the corporation."

"Management spent all their time fixing problems and had no time to think about what happens when the problems are solved," Gruen said. Division and subsidiary heads will report to Gruen as chief operating officer, while Thomas will focus on the direction of the company.

RSI makes high-technology antenna products for military and commercial use. It sells home satellite dishes, antennas for air traffic control and the military, satellite communications systems, Intelsat international satellite systems and other products.

"RSI is in a market of rapid growth and rapid change," Gruen said. "Deregulation and lucrative government contracts have opened up the telecommunications field. RSI has to decide what its future needs might be in this market."

RSI was looking for a chief operating officer "familiar with the overall telecommunications business and experienced in the military and commercial communications and electronics fields," Gruen said.

RSI had first discussed naming a chief operating officer last fall, when the company was contemplating a merger with Vertex Communications Corp., according to Pevenstein. The merger fell through in December 1986, but the idea of adding to management remained.

Gruen has known Thomas and RSI since his days with Ford Aerospace about 15 years ago, when he bought antennas from RSI. The association continued through Gruen's tenure at Aydin Corp., a $200 million subsidiary of FMC Corp., and at a division of Philco-Ford Corp. Gruen's most recent position was chairman and president of Fairchild Communications and Electronics Co. and vice president of Fairchild Industries Inc. of Chantilly. Fairchild is a $646 million company that makes aerospace, electronics and communications products for government and commercial use.

"The transition should be smooth because Thomas's experience and mine complement one other," Gruen said. "I have a background in he larger corporate point of view and am familiar with the electronics that our antennas must interface with. Thomas knows the antenna business from the inside. All our competitiveness is focused on the marketplace."

Gruen is coming to the company at a time when two of its subsidiaries -- Mark Antenna Products, which makes cellular system antennas, and Laux Communications, which makes home satellite antenna systems -- have had disappointing results. But Gruen denies that a difficult time in any particular division or subsidiary prompted the company to hire him. "It was a simple case of size. The company had just gotten too big," he said.

Gruen expects major growth in the next five years. "We want to be the biggest and best antenna manufacturer in the country, and to do that we must provide functions and capabilities beyond antennas," said Gruen. "We must install, service and maintain our antennas. We must integrate the electronic subsystems that attach to the antenna.

"But we have no plans to compete with our customers. Rather, we want to provide more capability to them," Gruen said.

In order to become "the biggest and the best," RSI will bring its separate subsidiaries and divisions under one umbrella, according to Gruen. "We will present one larger company to the customer, with the wherewithal and critical mass to take on bigger programs.

"The second step is to acquire or team up with other companies to do what we cannot do internally," Gruen said.

He said RSI has no plans to hire other management from outside the company except in specific areas where the company is lacking expertise. ASSOCIATIONS

Marshall F. Sinback Jr. has been chosen president-elect of the American Academy of Physician Assistants.

Sharon S. Blackmon, personnel director of Booz Allen & Hamilton Inc., has been elected president of the Washington Technical Personnel Forum, an organization of area personnel and human resources professionals.

The Suppliers of Advance Composite Materials Association elected David Forest chairman; Linn Matthews vice chairman; Paul Pendorf secretary, and Nick Spencer treasurer.

John H. Cammack has been elected president of the national capital chapter of the International Association for Financial Planning for 1987-88. Cammack is vice president of Alexandria Armstrong Advisors in the District.

The Armed Forces Communications and Electronics Association has elected Lucien Capone Jr., vice president of Booz Allen & Hamilton Inc., as president of its Washington chapter.

BANKING, FINANCIAL SERVICES

Riggs National Bank of the District appointed George R. Halvorsen executive vice president and deputy chief credit officer. Halvorsen had been senior vice president and head of the loan administration division of the Connecticut Bank and Trust Co. of Hartford, Conn.

MNC Financial Inc., the holding company for Maryland National Bank and American Security Bank, promoted John P. Nickles and William C. Cooper to executive vice president and senior vice president, respectively. Nickles is head of the corporation's strategic planning function and Cooper is manager of the corporate administrative service division.

Michael P. Oravec, former vice president of Bankers Trust in New York, has joined Maryland National Bank as senior vice president responsible for the business and technical plannning activities of MNB and American Security Bank's recently combined operations and data processing functions.

First American Data Services Inc., a subsidiary of First American Bankshares, promoted Avril A. DeCoteau from assistant vice president to vice president and manager of bookkeeping and retail and commercial statement processing, bank operations division.

Arthur C. Smith III has been named vice president of National Enterprise Bank's commercial loan division.

COMMUNICATIONS

The Washingtonian magazine appointed Ericka Miller, Katherine Dunbar and Steven Kaye assistant editors.

System Automation Corp. of Silver Spring promoted Harry Kaneshiro to vice president of production. He will assume the day-to-day responsibility for the operations of the company's three divisions. COMPUTER

August Automation appointed Dr. Robb Koether assistant software developer.

HOTEL, FOOD SERVICES

Guest Services Inc. of Fairfax appointed Sondra J. Gillice vice president of human resources. Gillice will be responsible for the overall management and administration of the human resources division.

Alan H. Fitzgerald has been named managing director of the Watergate Hotel. Previously, Fitzgerald was vice president and general manager of Mahogany Run Golf and Tennis Resort in St. Thomas, U.S. Virgin Islands. INDUSTRIAL, MANUFACTURING

Westinghouse Electric Corp. appointed F. Leo Wright vice president of government affairs. Wright's former position was executive director of government programs for Westinghouse. INSURANCE

Mutual of Omaha named William W. Bailey executive vice president and coordinator of national and government affairs for Mutual of Omaha and its life insurance affiliate, United of Omaha, in the company's District office. NONPROFIT

Jack O. Lanier, vice president for corporate and community development at the Greater Southeast Community Hospital Foundation, was appointed to the board of trustees of the American Cancer Society's District of Columbia division and to the board of Planned Parenthood of metropolitan Washington. PROFESSIONAL

ICF Inc. named Elizabeth Anderson and Paul Weeks to its board. Anderson is president of ICF/Clement Associates, and Weeks is the firm's first general counsel and a new vice president. William F. Hederman Jr., former director of policy analysis at the Interstate Natural Gas Association of America, has been appointed vice president for ICF.

McManis Associates Inc., an international management and research consulting firm based in the District, appointed Arlene Triplett vice president and Marco A. Monsalve and John E. Fredette senior associates.

Doug McClatchy has joined the Earle Palmer Brown Cos. as executive vice president/creative director at the agency's Washington headquarters.

Joseph S. LaValle has joined Dewberry & Davis, a Virginia design firm, as director of institutional architecture.

John M. Crawford has joined E.F. Hutton & Co. Inc. as an account executive.

The Pace Group, an architectural firm based in McLean, named George C. Toop Jr. president and chief operating officer.

Cooper-Lecky Architects has named Robert Sangine a principal in the firm and May Carr, James Cummings Jr., Patricia Donohoe, Peter Hoggan, Maury Saunders and William Thomas as associates.

Edward M. Leigh and Peter W. Sullivan joined the law firm of Anderson, Hibey, Nauheim & Blair.

Adworks Inc. appointed Chris Cotter senior art director.

Marketing Institute International Corp. named Girard T. Broaddus director of finance; William Eugene Wright manager of the market research division; and John C. Nash director of public relations.

Anthony M. Carey, Joel Z. Silver, Michael B. McGovern, Ellen F. Dyke, Michael J. Letchinger, William S. Oshinsky and Joel J. Goldberg have joined the law firm of Venable, Baetjer and Howard.

REAL ESTATE

Robert K. Wangel has transferred to the development division of Coakley Williams Inc. after a year as project manager in the construction division.

Michael Briggs, former director of education for Shannon & Luchs Realtors residential sales division, has been promoted to vice president and director of education for the company.

Spaulding & Slye has promoted Marshall H. Durston, William F. Magner and Michael G. Smith to vice presidents, Jean A. Hart to assistant vice president, and Jeffrey M. Griffin to project manager.

Long & Foster Realtors appointed Lynn Vizas branch manager in Walkersville, David W. Schmucker branch manager in Bethesda, and Jim Swisher branch manager in Parkville. Tim Dyson has been appointed area manager for Soloman's Island and LaPlata.

CAPTION;ALAN H. FITZGERALD . . . Watergate Hotel director @Harold Gruen, the new president and chief operating officer of Radiation Systems Inc. of Sterling.

. . . new Westinghouse executive