Ending a month of congressional silence on an arms-sale deal that sparked a monor furor this summer, 21 senators yesterday introduced a resolution to block the proposed sale of seven airborne radar and battle management systems to Iran.

The resolutions's principal sponsor is Sen. John C. Culver (D-Iowa), leader of the congressional opposition that in July forced President Carter to withdraw temporarily his offer to sell the planes.

Coming only a week before the deadline for congressional action on the proposal, and in the absence of a complementary House resolution that would be needed if Congress is to block the sale. Culver's resolution stands little chance of stopping the $1.2 billion transaction.

But Culver served notice in a statement prepared for the Congressional Record that he will probably try again to block the sale when the administration makes a promised review of the security risks involved a year before the first plane is scheduled for delivery in 1981.

The plane that is the subject of the controversy is a modified Boeing 707 jet carrying Westinghouse-built radar with the usual and highly useful capacity to look down at tartets beneath it. It is called the Airborne Warning and Control System (AWACS). teh planes cost $126 million apiece.

The Pentagon has said AWACS is being sold to Iran, a nation critical to the West's supply of Middle Eastern oil and that shares a 1,200-mile border with the Soviet Union, for defensive purposes only.

Culver noted yesterday that AWACS can still exert an unprecedented multiplier effect on the U.S. built offensive fighter planes Iran already has. Pentagon officials counter that no use AWACS offensively in battle would leave Iran naked from a defensive standpoint.

That administration has guaranteed that it will remove from the Iran-bound planes the sensitive coding and communications gear that was the major source of concern in Congress and the Central Intelligence Agency in July.

Nevertheless, Culver said in his statement, the "proposal is as risky and unacceptable now as it was in July, and we must reiterate our opposition."

"Coding equipment is not the distinguishing feature of the AWACS the most revolutionary development in radar since the invention of radar" as the Air Force claim, is the advanced look down radar with its built-in electronic countermeasure capabilities . . ."

Noting a recent article in a West German newspaper reporting that the Soviet Union has offered $4 million to any pilot who delivers a West German F-4 Phantom to the Warsaw Pact countries. Culver asked, "If a 20-year-old aircraft with a 10-year-old radar is worth $4 million to the Soviets, what would be their offer for an AWACS?"

Pentagon spokesmen scoff at Culver's arguments. First, they note, the slow-flying AWACS would carry a crew of 17 Iranians, all of whom would have to cooperate in a defection. "i don't see how that would be possible," said Erich Von Marbod, second-in-command of the Pentagon's arms-sales agency.

As for a russian theft of AWACS, Von Marbod said. Iranian security has never been compromised on any of the advanced U.S.-built weapons it posseses.

And even if the Russians got an AWACS, he said, it would be a "practical impossibility" for them to make any use of it in their effort to develop a look-down radar.

Von Marbod said the ground-based radar system Culver prefers to AWACS for sale to Iran is a greater danger to U.S. security. In 1981, when the AWACS sale is scheduled for consummation, AWACS will be almost 10 years old, he said, while the ground-based system - which includes a look-down for mountaintop use - will just be coming into production.