If the Reagan administration begins propaganda broadcasts to Cuba aimed at undermining Fidel Castro's government, Cuba will respond with stepped-up transmissions that would block commercial programming in the United States, according to Deputy Foreign Minister Ricardo Alarcon and other Cuban officials.

In an emerging radio war between Washington and Havana, said Alarcon, "those who will suffer the consequences will be those in the middle: the North American [U.S.] radio transmitters and radio listeners."

The Cubans say the potential Achilles' heel of the Reagan administration's plans to establish an anti-Castro station called Radio Marti lies in the political clout of U.S. radio advertisers and commercial stations that could be affected by retaliatory Cuban jamming.

The State Department has maintained that, "deliberate, Cuban-caused damage to U.S. broadcasting in violation of international agreements should be considered an unfriendly act to which we should respond. The technical means exist to do that."

But the Cubans say that in any confrontation over the airwaves they inevitably must come out ahead since their radio stations are government-operated and there are no internal pressure groups -- such as those faced in Washington -- to undermine decisions.

"If Radio Ciudad de Habana is blocked, for instance, nothing happens," Alarcon said. "But if a show . . . sponsored by General Electric is blocked, then General Electric will pull its advertisements off the air."

Conservative Republicans long have advocated government-sponsored broadcasts to Cuba similar to Radio Liberty and Radio Free Europe broadcasts to the Soviet Union and Eastern Europe. In September, the Reagan administration began taking steps -- even though legislation is required -- to create a station to be called Radio Marti after 19th century Cuban independence fighter Jose Marti.

Castro has said since that the Cubans are "prepared to give a suitable response" to "subversive" Radio Marti broadcasts.

As the bill funding Radio Marti comes up for probable vote in the U.S. Senate this week, Cuban officials are taking an even more unequivocal line in light of what they see as the Reagan administration's attempt to soft-pedal possible Cuban retaliation.

In testimony before the House of Representatives on May 10, Assistant Secretary of State Thomas O. Enders said, "We are seriously concerned about Cuban damage to all U.S. stations and, indeed, when Cuba threatens any U.S. interest. But we cannot allow Cuban threats of outlaw behavior to dictate our foreign policy."

Enders added, however, that "the truth is that we do not know for certain what Cuba will do to interfere with U.S. radio." He suggested that Castro might decide to employ low-level local jamming of Radio Marti, such as that used in Havana against the Voice of America, which would have little effect on U.S. commercial broadcasting.

Alarcon put aside such speculation: "Definitely what I mean by response is that you will be able to listen to Cuba in the United States a good deal more easily than now."

In the context of increasingly tense relations with the Reagan administration, Alarcon said in an interview, "the United States can launch an atomic bomb against Cuba, and Cuba cannot launch one against the United States. But radio transmissions are something else."

Cuban broadcasting already interferes with such U.S. stations as WIOD and WQBA in Florida, possibly on purpose in the case of the latter, a Spanish-language station.

Meanwhile, many U.S. commercial stations are heard each evening in Havana. "This is imperialism at its best," complained one Cuban official. "They say, 'To hell with those Indians down there, we in the United States will have 4,641 stations in the AM band.' "

U.S. studies indicate that as many as 200 commercial U.S. stations are potentially subject to interference from powerful new 500 kilowatt transmitters here -- 10 times as powerful as the maximum allowed in the United States -- if the Cubans choose to use them in that way. The most vulnerable, because it broadcasts on the 1040 kiloherz band proposed for Radio Marti and thus is a likely focus of Cuban jamming, would be WHO in Iowa -- where Ronald "Dutch" Reagan once worked as a sportscaster.

Cuba's big transmitters were planned by the Cubans before Radio Marti was an issue and probably would affect some U.S. stations in any case, but Cuban officials note that the impact on American broadcasting could be made greater or could be minimized through technical negotiations.

Some attempts were made with little success to resolve the problem last fall before and during the regional AM broadcasting conference in Rio de Janeiro. The Cubans say the Sept. 23 announcement of plans to build Radio Marti effectively scuttled further talks.

"On our part there was a willingness to talk," said one Cuban official. "But you cannot sit down and negotiate with a club on the table," he added, meaning Radio Marti.

[The administration's bill is scheduled to come before the Senate Foreign Relations Committee for possible vote on Thursday. The outcome is expected to be close, according to a staff member. A report from the 11-member Presidential Commission on Broadcasting to Cuba, submitted yesterday, said the radio should "avoid harsh, strident or obviously ideological presentations or concepts to which the average Cuban cannot relate." It also listed alleged Cuban failings that should be the focus of the broadcasts.]