They don't know they're getting it yet, but the 600 year-round residents of Block Island will receive a $300,000 gift of cable television from the federal government.

The reason: They're already the recipients of a $2 million windmill from the Energy Research and development Administration to generate electricity on their windy island, 12 miles off Rhode Island in the Atlantic.

The connection: The windmill will be so powerful that it will ruin Block Island's television reception - which, as every American knows, is an unacceptable condition of life.

"This has been at least at three-aspirin headache from the very beginning," ERDA's Donald Teague said. "The only way we can solve the windmill interference problem is to guarantee cable television for the whole island."

Teague's headache began when ERDA chose Block Island as one of three places to build 200-kilowatt windmills (ERDA calls them "wind turbines") to demonstrate the pollution-free power of wind. Culebra in Puerto Rico and Clayton in New Mexico were the other two winners of the windmill sweepstakes.

Block Island fits ERDA's windmill requirements. It's blessed with lots of year-round wind and cursed with high-priced foreign oil to make its electricity. The trouble is powerful television transmitters in Boston, Providence, New Haven, Bridgeport and New York all broadcast toward Block Island.

Remembering what propeller-driven airplanes used to do to television reception near airports, ERDA asked the University of Michigan's Radiation Laboratory for a study. It concluded that Block Island's windmill would indeed interfere with Block Islanders' TV.

"The steel blades of the Block Island windmill are to be 125 feet in diameter," ERDAs Teague explained. "When those blades are spinning that will be enough to change the signal frequency of any television picture coming into the island, especially ultra high frequency pictures."

ERDA's solution is to give the island $300,000 to build a directional antenna to pull in mainland broadcasts and underground cable to link the antenna with all the TV sets on the island.

Block Islanders will hear about their good fortune when ERDA gets around to printing the windmill's Environmental Impact Statement, which should be about six months. The windmill (and the cable television) probably won't be installed until the end of next year, partly because of the time it's taking to draft the impact statement.

If that weren't enough environmental headache, ERDA is also about to study whether giant solar mirrors it will put on top of a tower near Sandia, N.M., constitute a hazard to airline pilots.

The study is to be done with the French government, which plans construct similar solar mirrors in the south France. The mirrors concentrate sunlight and heat on a water boiler to make electricity.

"If the mirrors misfocus and don't hit the water boiler they could direct the reflected sunlight back into the air and blind a pilot in the sky," ERDA's Dr. John Vanderryn said the other day. "We and the French want to be sure there is no potential hazard here to pilots from the glare."