The Loudoun County Board of Supervisors, over strong citizen objections, yesterday narrowly approved an application for a 140-foot microwave tower to be built on rolling farmland on the Catoctin Ridge, near the county's Waterford Historic District.

Supervisors voted 4 to 3 to grant United States Transmission Systems Inc. permission to build the long-distance tower on a 530-acre dairy farm owned by Lily G. Brown, 1 1/2 miles east of Waterford. The firm was given permission to install a cellular mobile telephone antenna and two accessory buildings of about 250 square feet each.

Loudoun has several similar towers, according to Supervisor Frank I. Lambert (R-Catoctin), who supported the proposal.

Area residents who spoke at two recent public hearings vigorously opposed a new tower, saying they were fearful of possible health hazards from a microwave tower. They said that the tower might interfere with bird migration and that they were troubled by the effect it might have upon tourism and property values. Its location near the Waterford Historic District and its compatibility with agriculture in the area were also concerns of the residents.

"What is in it for the county?" asked Mouncey Ferguson, one of several residents who spoke before yesterday's vote. Ferguson said he lives within a mile of the tower site.

In response, the county has said that microwave transmissions from the tower will be well within the safety standards of the American National Standards Institute. Officials said the tower site is not in a historic zone; that the county's other microwave towers -- including one near the Oatlands plantation -- have disrupted neither tourism nor property values; that negative effects on bird life are unproven and that agriculture will not be harmed.

Voting in favor of granting the special exception and commission permits required to operate a public utility in an agricultural zone were supervisors Lambert, Andrew R. Bird III (R-Sterling) and Steve W. Stockman (I-Broad Run) and Board Chairman Frank Raflo (D-Leesburg). Supervisor Thomas S. Dodson (D-Mercer) was absent.

Lambert said yesterday he believed that county law enforcement and rescue workers are exposed to more radiation from walkie-talkies than Waterford residents will receive from the tower.