The Reagan administration's controversial decision to sell advanced radar planes to Saudi Arabia -- a proposal facing strong opposition from Israel and in the Senate -- is not based just on the desire to improve relations with the vital oil-producing state.

It also is of interest to Washington, government sources say, because it insures some U.S. ability to keep an electronic intelligence watch over the Persian Gulf region, is likely to mean a small but continuing U.S. military presence in Saudi Arabia, and reflects a recognition on the part of the Saudis that they have responsibilities for helping in the security of the region.

The big planes -- called AWACS, for airborne warning and control systems -- basically are flying radar stations and command posts that, from an altitude of about 30,000 feet, can look across borders, scan the horizon out to about 250 miles, and spot any potentially threatening enemy planes.

In a way that probably was not foreseen seven or eight months ago, the AWACS have become a central facto in U.S. policy in the Middle East.

Last October, the Carter administration sent four of three planes, and some 400 U.S. military personnel to opeate them, to Saudi Arabia on a "temporary" basis when the nation feared that its oil fields might come under attack if the new war between Iran and Iraq spilled across the Gulf. It now appears, however, that those plans and U.S. personnel will stay in Saudi Arabia for several more years until the Saudis begin receiving their own planes late in 1985, if the sale is approved in Congress.

While the Saudis eventually want to man and operate their own planes by themselves, sources estimate they will continue to require perhaps 400 to 500 U.S. specialists to help them for some time, even after the new planes arrive.

The proposed sale of five of these planes to the Saudis has become the most controversial part of a large arms deal proposed by the White House that also wound sen aerial refueling tankers and additional equipment to improve the performance of U.S.-built F15 fighters already sold to the Riyadh government.

The Israeli government objects to the AWACS sale, believing these planes would jeopardize the security of the Israeli air force should any new battle break out with any of Israel's Arab neighbors. Their proposed inclusion also has produced sharp controversy in the Senate, with forecasts, even by Reagan supporters, that the entire arms deal might be defeated in Congress.

It is clear that the Saudi -- this country's largest oil supplier and a relative moderate among the oil-producing states -- want the AWACS planes and are making the arms package a test of American goodwill. It also is clear that the administration is trying hard to construct what it sees as the need for a "strategic consensus" against a perceived Soviet threat, involving countries fromm Pakistan through the Middle East.

Administration officials say "it defies the imagination" to believe that the AWACS planes would be used in any war against Israel, pointing out that the United States have given iron-clad security guarantees to Israel and that Washington is also doing "everything it can" to strengthen Israeli intelligence.

The continued physical presence of the planes and the U.S. military personnel in Saudi Arabia, these officials maintain, enhances the U.S. ability to meet any external threat to the region.

There are some 40,000 Americans in Saudi Arabia but only about 500 are military personnel, not counting the extra 400 to 500 for AWACS. Other U.S. military groups help train the Saudis, while a Corps of Engineers detachment is helping the Riyadh government build army bases and airfields capable of accomodating forces for larger than those in the Saudi military.