La Cruz, Costa Rica — Besides the mounting guerrilla raids along the Nicaragua-Costa Rica border, another sort of conflict is heating up here: a war of the airwaves. In the 31/2 years that the Sandinistas have been in power in Nicaragua, they have stepped up radio and television broadcasts of their leftist views in this border region to a point that many Costa Ricans feel they have lost control of their own country's airwaves. Many worry that their children are being influenced by the Sandinistas' leftist message.
So the administration of Costa Rican President Luis Alberto Monge is calling for a ''rescue of radiophonic sovereignty in Costa Rica.''
In a letter to President Reagan last November, President Monge asked for $20 million for radio and TV broadcast equipment to be used in ''the conquest of the hearts and minds of Central Americans in the cause of democracy, freedom, and social justice.''
Already reeling from the worst economic crisis in its history, Costa Rica has also asked Canada, the International Fund of Agricultural Development, and others for help.
Here in La Cruz, a high, windy city of about 11,000 that is only 12 miles from the Nicaraguan border, five Nicaraguan radio stations are heard, in contrast with three Costa Rican stations.
Until late March, many of the 400 TV sets in La Cruz did not receive any Costa Rican television - but three Sandinista TV channels came in loud and clear.
Fully a third of Costa Rica's national territory is dominated by Nicaraguan broadcasting, according to Costa Rican sources. And by 1985, Nicaragua may have 105 medium-wave radio stations with a potential 1,074 kilowatts of power - more than double Costa Rica's projected 50 stations with 509 kilowatts - if the Nicaraguans continue at their present rate of expansion, these sources say.
''What worries us most is the education of our little ones,'' says Jose Antonio Loaicega, a resident of La Cruz. ''All the Sandinista programs are about communism with people in uniforms carrying guns.''
The militant, uniformed, rifle-toting images that appear on TV screens here and throughout Costa Rica's northern zone are jarring in an antimilitaritist country whose Constitution bars formation of an army.
''I'm a teacher and I can tell you, the children are influenced,'' says Luz Maria Vega, a La Cruz mother of two. She says her children have begun to ''act out war.''
''Instead of playing cops and robbers, they play contras and Sandinistas,'' she says. They ''march like they're carrying guns and they yell, 'Patria libre o morir (free country or death),' '' she says. Felix Sanchez Gallo, La Cruz city manager, says, ''The militarism isn't healthy - we're not accustomed to it.'' He says he does not allow his children to watch Sandinista TV.
President Reagan referred to the radio problem in his visit here in December. And in March he wrote Monge a letter pledging support for ''combatting foreign broadcast signals that present a false image'' of the region. Reagan made no commitment to Monge's $20 million request, but he said he would authorize a team of broadcast specialists to visit Costa Rica to survey the country's needs.
Armando Vargas, Monge's spokesman and the country's minister of communication and information, publically tries to tone down any worry about a war of the airwaves. But the message seems to come through anyway.
''The issue is not that Nicaraguan radio and television are heard and seen in Costa Rica, it's that Costa Rican radio and TV are not,'' he says. ''We feel that all Costa Ricans should have access to Costa Rican broadcasting.''