THE REAGAN administration's proposal for a new radio station to broadcast news of Cuba to Cuba--not just news of the United States, as the Voice of America already does--has a good deal of merit. One of the purposes behind it is to serve a general policy of making it harder for Fidel Castro to run his country. Whatever one thinks of that, "Radio Marti" could give Cubans, as Radio Free Europe and Liberty gives East Europeans and Soviets, the chance to hear news kept from them by their government's controlled media. That seems to us the central consideration. Listening is entirely the listener's choice.

Mr. Castro, who knows something about foreign broadcasting, says hey rising nu would regard Radio Marti as hostile and subversive. But the station, if properly run, would be hostile and subversive only to the extent that truth is hostile and subversive--the truth, for instance, about the scope and costs of Cuba's foreign adventures. Here at home, some worry that Radio Marti might provoke Cubans to revolt or, more plausibly, to emigrate in uncontrolled Mariel style. Careful policy guidance presumably would keep broadcasts on the information level.

There is concern that a new station would undercut the Reagan administration's diplomatic outreach to Cuba. The Reagan initiative apparently amounted to an all-or-nothing proposition to switch sides. Mr. Castro rejected it, as you might have expected. He had already rejected, however, the previous administration's very differently pitched effort to reach an accommodation by more gradual and conventional means. An impartial observer would have to conclude that he puts a higher value on his commitment to sustaining and exporting revolution. On general principle it is wise to keep on trying to talk. But that is no reason not to try something--like Radio Marti--that should have been tried years ago.

One practical problem, however, must be disposed of first: the interference of Cuban and American radio signals. There is a history of Cuban interference with American (and other hemispheric) radio broadcasting on the sensitive and crowded AM band. Cuba has its own complaints, not least that Radio Marti would entail an American violation of a treaty obligation to confine AM broadcasts to one's own country.

Withing the Reagan administration, there is a strong tendency to handle Cuban radio interference, existing and prospective, as a raw political challenge and not to yield to it. Many American commercial broadcasters, however, are inclined to go slow on Radio Marti s as not to add what they regard as an extraneous issue, and a hot one, to a radio negotiating circuit already groaning under its load.

Radio Marti could make a useful addition to the news available to Cubans. But the administration will have to do better in explaining to Congress how it proposes to keep American radio listeners from paying unduly for it.