President Reagan's plan to launch a new Spanish-speaking radio station in Florida to broadcast "the truth" about Castro to all Cubans may hurt the very radio station on which Reagan launched his public career.

According to officials of WHO radio in Iowa, the administration's plans to set up Radio Marti--named after Jose Marti, the father of Cuban independence--could trigger reprisals by Cuba that might wipe out almost all nighttime service of the Des Moines, Iowa, AM-radio station on which "Dutch Reagan" was a popular sportscaster during the 1930s.

If the United States goes ahead and sets up Radio Marti on channel 1040, the same radio frequency on which WHO operates, many U.S. officials predict Castro will retaliate by setting up a far more powerful station on the same channel to wipe out Radio Marti's signal.

"We are deeply concerned that Cuba's announced plans to jam Radio Marti with up to 500 kilowatts of power 10 times the U.S. government's limit for American stations will destroy WHO's nighttime service, including its vital farm market reports," says Robert Engelhardt, vice president of Palmer Communications Inc., the company that owns WHO.

Radio signals travel much farther during dark than they do during daylight hours. As a result, a powerful Cuban jamming of Radio Marti could affect WHO's broadcasting signal even though the transmitters for the two stations would be thousands of miles away.

Thus, says Engelhardt, if Cuba were to try to block out Radio Marti, "such jamming will shrink WHO's coverage from a radius of 800 miles to a radius of 45 miles, from 1,700,000 square miles to 5,900 square miles--a 99.65 percent decrease."

Radio engineers say that WHO would be the only station affected by the jamming.

As a result, Engelhardt and other company officials have been busy trying to persuade the administration and Congress to use another channel for Radio Marti. Just two days ago, they met with the Presidential Commission on Broadcasting to Cuba to try to persuade them to select another AM channel or another method of broadcasting--such as FM or short-wave radio--to send "the truth" to Cuba. The commission was appointed by President Reagan to recommend a plan for establishing Radio Marti.

Additionally, notes one of WHO's Washington's lawyers, Kenneth D. Salomon, "we've tried to see that the fact that WHO will be affected by Radio Marti was brought to the president's attention. So far, we have no way of knowing if it has."

Reagan announced his intention to set up Radio Marti last September at the height of rhetorical hostility between the two countries. Accusing Castro of lying to its people for 20 years, Reagan said Radio Marti would broadcast "accurate information about Cuba" to the Cuban people.

With a $10 million authorization from Congress--which has yet to be approved--Radio Marti would operate much like Radio Liberty and Radio Free Europe, which have broadcast information to Russia and its Eastern European satellites for more than 30 years.

Initially, several U.S. diplomats in Havana opposed the idea of Radio Marti, saying it not only would generate more Cuban immigration to the U.S. but also could lead to a series of unwanted reprisals. However, commission members say that initial opposition has died down and the support for Radio Marti is "gung-ho" from administration officials.

"It is a lot easier for the U.S. to take out our feelings against Cuba with a so-called war of words rather than a war of bullets," says George Jacobs, an engineer on the presidential commission, in explaining why the program is drawing such strong support from the State Department and Department of Defense.

Commission officials say channel 1040 is the best frequency with which to carry out this verbal battle. It is the one radio frequency that won't interfere significantly with other stations in the area, commission officials say.

Commission officials are not convinced that Cuba will send out powerful signals to block Radio Marti. For one thing, they note, despite all expectations, Castro has not jammed Radio Marathon, the Voice of America station that goes to Central American countries.

WHO officials are disturbed by comments they are hearing from several administration and commission officials, and now express concern that their station will end up being just a pawn in a war of words.