A Fairfax County judge ruled yesterday that an anti-tax group did not collect enough valid petition signatures to force a voter referendum, but left unresolved whether the group would be able to use the signatures in a future effort.

Circuit Court Judge Thomas J. Middleton ruled that Citizens for Sensible Taxation fell 778 names short of the 40,589 valid signatures needed to force a vote this summer on whether Fairfax should change its form of government.

Middleton did not rule on whether Frederick T. Dykes, one of the group's founders, would be allowed to file the same petitions with additional signatures in a new referendum drive, or whether the existing petition is now invalid.

That issue probably would be argued in court if Dykes tries to refile the petition.

"We've already gotten about 50,000 people to sign once, and if we have to get 50,000 signatures again, we'll do it," Dykes said Wednesday.

His wife, founder Marcia Dykes, said her group has not decided whether to try to refile the petitions. Frederick Dykes said the group may try to refile the petitions to force a referendum in November or in November 1991, which would coincide with the next Board of Supervisors elections.

One of the group's prime objectives was to oust the board in a special election by this November, a year before its regular term expires. The group hoped such a show of strength would persuade state and local lawmakers to enact laws limiting individual real estate property tax increases to 5 percent a year, the group's ultimate goal.

Middleton's ruling means the supervisors will not face an election this year.

Petition signatures from 10 percent of the county's 405,888 registered voters, or 40,589 people, are needed to force a referendum. The county registrar, after disallowing about 8,000 signatures becasue they were illegible, duplicates or did not belong to registered voters, found that the group fell 778 signatures short.

Marcia Dykes said state and county officials threw up impediments to the referendum, which further angered taxpayers and invigorated her group. "If anybody thinks this whole thing is dying or dead," she said after Middleton's ruling, "they ain't seen nothing yet."