Senate Minority Leader Howard H. Baker Jr. (R-Tenn.) has called a press conference for 10:30 this morning to announce his conditional opposition to the strategic arms limitation treaty, according to informed sources.

Associates of Baker said yesterday that he will announce his opposition to SALT II in its present form and will spell out some specific recommendations for changes in the treaty.

In January 1978, Baker announced his conditional opposition to the Panama Canal treaties. After those treaties were amended, Baker decided his conditions had been satisfied, and he voted for the treaties. The Carter administration and the Soviet Union both have said that SALT II cannot be altered substantively.

Baker expected to be a candidate for president in 1980 although he has not formally announced, holds a potentially decisive swing position in the Senate's SALT debate.

In another SALT development, Sen. Jake Garn (R-Utah), a hard-line critic of the agreement, proposed yesterday an "alternate treaty" that would force both superpowers to eliminate 650 strategic weapon launchers from their arsenals.

Garn said he knew his proposal would be unacceptable to the Soviet Union, President Carter and his Senate colleagues, but he said he wanted to make it public as an example of what a genuine arms control agreement might look like.

Garn, who hopes to play a leading role in the Senate debate on SALT II, insisted that his alternative treaty was not an attempt to scuttle SALT entirely. Pressed by reporters, however, he admitted that he would prefer to have no SALT treaty at all than to see the Senate accept the one signed in Vienna by President Carter and Soviet leader Leonid Brezhnev.

He also admitted that Senate adoption of even part of his proposal might lead to Soviet rejection of the treaty. That prospect didn't bother him. If the Soviet Union sincerely is interested in strategic arms limitation, Garn said, it would agree to reopen negotiations toward a treaty providing for reductions, not increases, in the superpowers' arsenals.

SALT II, Garn said, "legalizes the biggest arms race in the history of this world," and he said it will give the Soviet Union strategic superiority.

In a long prepared statement, Garn called for drastic changes in U.S. defense policy, including targeting of more Soviet military installations, opening the way to a U.S. "first strike" capability, renewed attention to defenses against incoming enemy missiles (which largely are banned by the SALT I treaty) and expensive modernization and expansion programs for the U.S. military.

But Garn insisted repeatedly that he would like a SALT agreement that puts real restrictions on Soviet and American forces. He aligned himself with dovish critics of SALT II, naming specifically Sens. George McGovern (D-S.D.) and Mark Hatfield (R-Ore.), and saying he agreed with their criticisms of SALT II as being too permissive.

He said he would propose a "sense of the Senate" resolution that would send SALT II back to the president for further negotiation, and wouls spell out the kind of treaty he believed those new talks should produce.

Garn predicted the Senate would reject this idea, but he said that he would offer the major points of his new treaty proposal as amendments to SALT II in the Senate debate.

Garn's proposal calls for staged cuts in the superpowers' arsenals, starting at the level foreseen by SALT II but cutting deeper at the end of 1981 and deeper still by the end of 1983. Instead of SALT II's limit of 2,250 on strategic nuclear weapon launchers, Garn calls for a limit of 1,600.

He also proposes a formula for limiting both the Soviet Backfire bomber and the U.S. FB111 bomber, many of which are based in Britain. Garn suggests that a Backfire would be counted as the equivalent of three-fourths of a strategic launcher, and an FB111 as one-half. This is one of the individual ideas Garn promised to introduce as an amendment to SALT II.

Garn also proposes-and says he will offer amendments to incorporate-new sublimits on large missiles in the Soviet arsenal for which the U.S. now has no equivalent.

On verification matters, Garn suggested both on-site inspections and an absolute ban on encoding any radio messages sent back to Earth by test rockets. This idea is opposed by the Pentagon.

Garn said he also would offer amendments to permit testing of all kinds of long-range cruise missiles and to remove any restrictions on the way the United States might base its new MX missile or transfer U.S. strategic technology to North Atlantic Treaty Organization allies.

In a conversation after his press conference, Garn said he had changed his personal position during recent years and now accepts the idea that U.S. interests can be preserved if there is genuine Soviet-American parity in strategic arms. Garn said he used to think only U.S. superiority would do.

In his prepared statement Garn called for "parity plus" but did not explain the term. CAPTION: Picture, SEN. JAKE GARN . . . prefers no SALT to the one signed