LISBON -- Determined to get what he says Portugal is due, Prime Minister Anibal Cavaco Silva is going to Washington next week to press his case in person for increased compensation for the strategic U.S. air base in the Azores.

Cavaco Silva's trip, which aides said is to begin Monday, is part of a Portuguese campaign to convince American officials, particularly in Congress, that Portugal should receive increased U.S. aid to guarantee continuation of what is now broad public and official acceptance of Lajes Air Base.

"I am concerned about public opinion," Cavaco Silva said in an interview last week. "I would like to maintain a public opinion different from that in Spain."

The Spanish government, responding to a strong popular current, forced the Reagan administration to agree last month to withdraw all 72 American F16s from Torrejon Air Base near Madrid.

Unlike Spain's demand for removal of the F16s, which was fundamentally political, Portugal's disagreement with Washington has centered on the amount of U.S. aid provided Portugal in compensation for the base. Portuguese observers, including leftist militants, said there is no evidence of significant public resentment over the presence of the base.

Cavaco Silva said that, during negotiations for a new base agreement in 1983, Portugal was led to expect the aid level would rise to more than $200 million a year and remain there. But after reaching $207.9 million in 1985, the first U.S. fiscal year of the accord, aid instead has dropped to $188.9 million in 1986, $147.4 million in 1987 and $117 million in 1988.

"The expectations are not being met," Cavaco Silva said. "This is clear. This creates problems for my government, political problems and also {affects} the capacity to help the regional government in the Azores."

Cavaco Silva said a visit here this month by Defense Secretary Frank C. Carlucci did not produce a sufficient response to Portuguese demands. Carlucci announced creation of a task force to increase Portugal's ability to benefit from the Southern Regional Amendment, a measure that grants surplus U.S. military equipment to some North Atlantic Treaty Organization allies.

"This does not mean we have found a solution to the disagreement over compliance with the spirit of the agreement," Cavaco Silva said.

He described efforts to get more benefits from the Southern Regional Amendment as the "military side" of what must be done. Still to come, he added, are negotiations on economic support funds for government projects and, in particular, development of the Azores islands.

Cavaco Silva said Portugal's desire to remain a reliable NATO ally led it to undertake the negotiations without invoking a formal review of the base agreement, but that this remains a possibility if the talks drag on without result. Under terms of the accord, which runs until 1991, Portugal became entitled to demand a formal review as of Feb. 4.

"We shall continue to talk, but we have not excluded the mechanism in the agreement," he declared. "We cannot continue to live on unfulfilled expectations."

Asked whether his government would be willing to wait until the next U.S. fiscal year for an aid increase, he responded: "We can't wait until the next budget year. We don't want to wait. We cannot wait, politically."

The Reagan administration has argued that it did its best to obtain more aid, but that Congress cut back its requests as part of general foreign aid reductions. The administration's request for Portugal this year, for example, was $207.5 million until Congress trimmed the aid budget.

Cavaco Silva and other Portuguese officials have refused to accept that argument. The Azores base accord renewed in 1983 was a government-to-government contract, the prime minister insisted.

At the same time, Cavaco Silva plans to meet with congressional leaders as well as administration officials while in Washington, officials said.

Cavaco Silva also sought to underline what Portugal says is the increased importance of Lajes Air Base since the treaty on intermediate and shorter-range nuclear missiles was signed in December by the United States and the Soviet Union. Lajes is all the more necessary as a refueling point with the increased emphasis on conventional forces created by the treaty, he said.

The base for years has been a regular stop for military aircraft crossing the Atlantic. It also is a jumping-off point for U.S. submarine-hunting aircraft and was a key staging point for the U.S. military airlift to Israel during the 1973 Middle East war. The facility lies on Terceira Island, about 1,000 miles off Portugal in major transatlantic air and sea lanes.