RADIO MARTI, that curious and expensive ($11 million) exercise in preaching to the converted, opened this week to decidedly mixed reviews.

Fidel Castro knew all along he would hate it, and he did. To show how much, he suspended an agreement negotiated last December which would have permitted 20,000 Cubans to emigrate to the U.S. and would have allowed us to send back the 2,700 "undesirables" he unloaded on us in 1980 when, irritated at the sight of thousands of Cubans scrambling for U.S.-bound boats, he emptied his jails and mental hospitals.

But they loved it in Miami. Jorge Mas, chairman of the advisory board of Radio Marti, says that the city's Cuban exiles are overjoyed that their country men are now "sharing freedom."

Mas, who is also the head of the Cuban-American Foundation, said on the telephone from Florida that he met a couple whose daughter would have rejoined them this week had Radio Marti not started up. The mother and father assured him tearfully, he says, that "it was a small price to pay for freedom."

Sen. Paula Hawkins, R. Fla., promptly said that Castro' s ferocious reaction just proved what a heel he is -- and justified the need for Radio Marti.

But Cubans who have been living with him for 27 years hardly needed a radio station all their own to tell them that Castro is a bad hat. They endure the repressions and the shortages. They have the block captains who turn them in to the central committee. They weep on the phone to their relatives in the U.S. Cuban mothers who have their sons brought home in boxes know that Castro is involved in foolish ventures in Angola. They see Russians in the street to remind them that they are wards of the Soviet Union.

The legislation setting up Radio Marti gives as its rationale "to promote the cause of freedom in Cuba."

"They are isolated after 27 years of communism," says Mas. "We had a survey several years ago which showed that Cubans arriving in the U.S. didn't even know the Pope had been shot or that Americans had landed on the moon."

Cubans could perfectly well have learned those facts from the Voice of America, which beams 51/2 hours of broadcasts to their island every day.

But, according to Mas, the VOA is not enough , because it gives news of all of Latin America, without special emphasis on Cuba.

He boasts that the first four days of Radio Marti -- which is named for the great Cuban hero, Jose Marti, a revolutionary and poet, who wrote eloquently against "Yankee imperialism" -- has been praised for its objectivity and accuracy. Most news and commentaries come from VOA services.

Compelling though the material may be, it is not supposed to incite the Cubans to revolt -- even though anyone who thinks about it would have to conclude that the surest way to freedom would be to throw out Fidel Castro.

"I am against the overthrow of any government," says Mas. "It would be better, of course, to negotiate, but it is impossible with Castro. He always breaks his word. Radio Marti is just an excuse for cancelling the agreement."

The idea, he says, is "to create a climate for freedom in Cuba -- when they know what is really going on, they may hold the government accountable."

But holding one's government "accountable" in the absence of the ballot box, means demonstrations and protests that could be as dangerous in Havana as in Moscow, where taking to the streets is a ticket to jail. And if intolerable conditions are the criteria for a separate radio station, isn't Chile a better candidate?

The answer is no. Chile groans under the rule of a bloody dictator, but he is anti-communist. And there are not nearly as many Chilean exiles as there are Cubans -- who went 97 per cent for Reagan in 1984. The reason the House approved Radio Marti -- while Iowa congressmen howled that Castro could retaliate by jamming Reagan's old station, WHO, and shut down a state that lives by its daily broadcasts about farm prices and weather -- was Dante Fascell, the popular Democrat from Miami, with its huge Cuban population.

At the heart of Radio Marti's murky rationale is the ineradicable conviction of conservative Republicans that the Soviets are beating us at the propaganda game. Nothing will convince the right-wing that the word is out about us and them.

And if 20,000 Cubans who know how great it is, have their dreams of coming here cancelled because of Radio Marti, it's okay with the people who dreamed it up. A right-wing dream came true in Nicaragua with the contras. Another is being realized in the Cuban broadcasts. Neither of them may work very well, but that's beside the point. They are showing the commies, and to them that's what foreign policy is all about.