President Reagan's key advisers are waiting for a positive signal from the Soviet Union that they believe could lead to a summit meeting between Reagan and Soviet leader Yuri V. Andropov within the next 10 months.

The signal, one administration official said, would have to be "substantive," which he defined as something more than the new long-term grain agreement being negotiated with the Soviets or the expanded cultural exchange program under exploration by the State Department.

This official said the Soviets could show "genuine willingness" to improve relations by demonstrating flexibility in the two nuclear-arms reduction talks in Geneva or by "stepping back" in Afghanistan or Poland or easing up on military support for Marxists in Central America.

Although the Soviets and the United States have engaged in summit propaganda aimed chiefly at European electorates, the Reagan administration's estimate since Soviet President Leonid I. Brezhnev's death last November has been that a summit in Reagan's first term is unlikely and probably undesirable.

But administration success in winning initial congressional approval of the MX missile and in lining up France and Japan behind its planned deployment of medium-range nuclear missiles in Europe has persuaded some presidential advisers that the United States now would enter a summit in a position of strength.

"It is becoming apparent to the Soviets that there is more unity on these issues in the United States and among its allies than they believed," one official said, adding that the Soviets also recognize that Reagan stands a good chance of being reelected and could be more difficult to deal with in a second term.

Administration officials believe that a summit must be held no later than the middle of next spring to avoid entanglement with the 1984 election campaign.

Reagan is described as firmly opposed to a having summit "just to chat" when there is no likelihood of agreement on important issues. But his advisers say he is ready to move quickly if the Soviets show any sign of wanting to deal.

"We are watching . . . for a sign, and there are those who think that the Soviets may give us one," one official said. "If this happens, the president would be more than willing to meet with Andropov."

The administration may try to finesse a complex and touchy issue when the United States returns to the bargaining table at the strategic arms reduction talks (START) in Geneva Wednesday.

The new U.S. proposal will call for using warheads as a measure of the nuclear arsenals of the two superpowers, as advocated by the Scowcroft commission in its recent report. Still at issue is whether the new U.S. proposal should deal with the difficult question of "throw weight," the lifting power of nuclear-tipped missiles, in which the Soviets have an acknowledged edge.

Defense Secretary Caspar W. Weinberger has pressed for inclusion of throw weight, while Secretary of State George P. Shultz believes the issue would complicate the negotiations and make agreement less likely.

Reagan reportedly is leaning toward a compromise stance in which the United States will express "an implicit recognition" of the importance of throw weight in defining equality without officially incorporating the issue in the revised U.S. negotiating proposal.

While the White House was sending administration notables to a Republican women's leadership forum in Indianapolis this weekend to tell them that the president is doing well by women, a modest package of proposals to benefit working and retired women remains deadlocked in the Cabinet.

The president said in a taped message to the Indianapolis forum that he is committed to eliminating "all traces of unjust discrimination against women." But two Cabinet meetings have failed to produce agreement on unisex insurance tables, child-support enforcement, tax credits for day care or other proposals to ease wage and occupational discrimination.

The issues have divided women in the administration, with public liaison director Faith Ryan Whittlesey questioning the wisdom of a broad-scale "women's program" and Health and Human Services Secretary Margaret M. Heckler arguing strongly that such a package is needed.

In the face of this conflicting advice, the president has done nothing. But he is expected to carry out the promise he made in his State of the Union address to support equitable pensions, and a proposal to accomplish this probably will emerge from a future Cabinet meeting.

Even on this issue, however, the administration is playing it safe. The anticipated proposal will stop short of favoring retroactive equity, one official said, because this would cause "huge financial problems" for pension plans.

Disclosure of the Week: (From presidential assistant Richard G. Darman's financial dislosure report): "Although my leave of absence from Harvard University has technically expired, I have an outstanding offer to assume a full professorship--which offer I shall treat as a conflict unless or until I decline the offer."

Exchange of the Week: ABC's Sam Donaldson to Reagan: "If you go along on the next space-shuttle flight , you can become the first spacey president."

Reagan to reporters: "Sam has spoken for all who are way out."

Reaganism of the Week: "I have just had lunch with the crew of the space shuttle. And it was quite a lunch--squeezed it from a plastic bag."