The Navy is complaining that Congress is forcing it to spend at least $85 million -- and possibly as much as $300 million -- on a radar system it doesn't want but which is built by a company on Long Island, home ground of Rep. Joseph P. Addabbo (D-N.Y.), chairman of the House Appropriations defense subcommittee.

Navy leaders contend in interviews that Addabbo and his allies have kept the Navy from getting competitive bids. They said Congress, with a heavy assist from former staff members of its military committees who now work for defense contractors, is forcing them to award a contract to the Sperry Corp. to develop a radar for the Navy's FFG7 frigate.

Navy officials engaged in the radar fight portrayed the Sperry proposal as "a classic buy-in." One official predicted the $85 million Sperry estimate will balloon to $300 million.

The controversy is an example of the behind-the-scene pressures through which lawmakers often get defense contracts for firms in their districts under the smokescreen of highly technical language. In this case the smokescreen is the carefully tailored specifications that only Sperry's proposal meets.

The plum for Sperry is hidden in this language by House-Senate conferees in a compromise defense authorization bill for fiscal 1983:

"Of the amount authorized in Section 201 for the Navy, $15 million is available only for the phased array radar improvement program for the Mark 92 fire control system."

That, and other congressional directions, Navy officials said, assures that only Sperry's unsolicited radar proposal can be bought by the Pentagon.

Sperry's Gyroscope Division at Great Neck, N.Y., would do most of the work on the radar. Although outside of Addabbo's Queens County congressional district, such a multimillion-dollar contract would be an economic lift for the whole area. The record shows Addabbo has been fighting strenously for the contract.

Sperry has hired former staffers of the congressional military committees to advance its radar proposal.

Don Lynch, until last year a staffer on the Senate Armed Services Committee, confirmed yesterday that he has been working for Sperry on the radar contract and has hired two other old congressional hands to help him: Ralph Preston, formerly Addabbo's chief staff aide on the defense appropriations subcommittee, and Frank Slatinshek, formerly staff director of the House Armed Services Committee.

Although both the House and Senate armed services committees approved the language tailored to Sperry's interest, correspondence between Addabbo and Navy Secretary John F. Lehman Jr. show that they have been the principal combatants on the issue.

"An X-band radar only upgrade does not provide a cost-effective gain in military capability," Lehman wrote Addabbo on July 9 in opposition to the Sperry proposal. "To thus constrain radar upgrade to X-band-only would preclude consideration of other attractive phased array technologies in alternative frequency bands."

He described an alternative to Sperry's recommendation that "would provide the significant increase in combatant capability that we are seeking" to improve the Mark 92 fire control system. "Not acceptable," Addabbo replied. "The committee directs that the Navy immediately take all necessary steps to comply with its stated objectives for upgrading the Mk92. Further delay will not be tolerated."

Lehman responded that "I remain convinced" the congressional prescription is the wrong one, but said he would seek a formal proposal from Sperry.

A Sperry spokesman yesterday declined to comment until after checking with his corporate executives. A Sperry consultant said the Navy is resisting the upgrading program because it does not want to take funds from warships bigger than the FFG7, a light destroyer.