Arlington's estimated 20,000 cable TV subscribers can expect a $ 1 increase in their basic monthly service rate Jan. 1 unless the County Board blocks the rate rise sought by Arlington Telecommunications Corp (ARTEC).

ARTEC, which two months ago won an extension of its franchise until 1995, has informed the county that it plans to raise its basic charge from $ 9.95 to $ 10.95 a month to keep up with inflation. The increase, the second in the last 18 months, will go into effect automatically unless the five-member board rejects it.

The rate increase has the support of the county staff and County Board Chairman Stephen Detwiler. "It's a business judgment that they feel is necessary," Detwiler said yesterday, "They have to market the service at that price."

Brian Ferguson, chairman of the Arlington Cable Television Advisory Committee, said yesterday that his group likely will urge that the board hold a public hearing first before allowing the company's request to go through.

"I think ARTEC ought to show that it needs it," Ferguson said. "They got the extension as a remedy for their financial problems and yet they have done nothing with the extension to refinance."

John Evans, ARTEC's executive vice president, could not be reached for comment yesterday. In his recommendation to the County Board, Arlington Community Affairs Director William Hughes said the rate increase was justified because it is "approximately related to the inflationary index."

County Board member Dorothy Grotos, who cast the sole dissenting vote on the extension of ARTEC's franchise until 1995, was doubtful yesterday that a majority of the board would seek a public hearing on the rate proposal.

"Judging from past performances, what ARTEC wants, ARTEC gets," she said. Although the company had indicated it would seek a higher rate two months ago, "I didn't think they meant this soon in the future," Grotos said.

ARTEC, which won the first large cable TV franchise in the Washington area eight years ago, had asked for an extension on its Arlington franchise in order to obtain long-term financing on about $ 6 million in short-term debts. The 1995 expiration date added four years to the franchise, which last July had already been extended by the County Board to 1991.