BY NANCY SAN MARTIN

  WASHINGTON - Transmitters in Cuba are jamming the signals of at least four U.S.-based television stations owned by Iranian Americans who are critical of the Tehran regime and use satellites to transmit programs to Iran, according to broadcasters and a private U.S. firm that has pinpointed the source of the interference.

  All the transmissions affected so far are beamed from Los Angeles -- which has a large population of Iranian exiles -- by privately owned stations that oppose Iran's theocratic government, officials of the four stations said.

  U.S. government officials said they are still trying to determine whether three other satellite broadcasts, transmitted by the Voice of America from Washington to Iran, are also being disrupted.

  ''We simply don't know if our signals are being jammed,'' said Joe O'Connell, a spokesman for the Broadcasting Board of Governors, which is in charge of the U.S.-funded VOA broadcasts.

  IMPROVING QUALITY

  News of the Cuban jamming came as U.S. authorities revealed they have been studying ways to enhance TV Martí broadcasts to Cuba by using analog satellite
  transmissions -- rather than digital transmissions currently being used -- that are more difficult to jam and more easily captured by the estimated 10,000 to 15,000 satellite dishes on rooftops across the island.

  ''In the future, things might change,'' O'Connell said, adding that his office recently ran some experiments on improving satellite transmission to the island. ``The whole idea was to find effective ways to enhance TV and Radio Martí to Cuba.''

  The disruptions of the California broadcasts were first detected on July 5. Broadcasters have since been unable to get their programs to reach Iran, where the ruling
  Muslim clerics have sought to bar access to media not controlled by the government.

  ''We never thought that in a country such as the United States we'd be suppressed from freedom of speech,'' said Kourosh Abbassi, a spokesman at Azadi Television, which promotes political change in Iran. ``Seeing as we have no ax to grind with the Cuban government, they must be in cahoots with the Iranian government.''

  ANTI-AMERICAN

  Cuba has long been friendly with the equally anti-American Iranian regime, even selling biotechnology used to manufacture medical products to Tehran in the late 1990s
  that a Cuban defector alleged in 2001 could be used to produce biochemical weapons.

  The jamming, first reported by NBC News, began as the Washington-based VOA began broadcasting a new Persian-language television program, News & Views, to Tehran as Iranian students launched a series of street protests against their government.

  FCC officials confirmed that they were looking into the jamming reports.

  John McCarthy, a spokesman for Loral Skynet, the operator of the Telstar-12 satellite used by the California broadcasters to beam their signals to Iran, told The Herald that the company has identified the source of the jamming, but declined to discuss further details.

  However, according to a letter from the Loral Skynet made public by Azadi Television, a privately owned U.S. transmitter location company was able ``to provide an ellipse of the most probable location of the source of the interference, which it identified as being in the vicinity of Havana, Cuba.''

  Efforts to reach Cuban officials for comment were unsuccessful.

  It is presumed that Iran is using jammers in Cuba, 90 miles off the U.S. coast, because they are physically closer to the Telstar satellites.

  The other affected stations are Channel 1, Pars TV and National Iranian TV, which began its broadcasts to Iran three years ago.

  The U.S.-funded VOA uses two satellites for its transmissions to Iran, Telstar 12 and another that covers the Middle East, Europe, Russia and North Africa.

  BALLOON IN THE SKY

  TV Martí relies primarily on a regular TV signal, not a satellite, broadcast from a balloon tethered 10,000 feet above Cudjoe Key in the Florida Keys. Those transmissions are easily blocked by the Cuban government.

  But TV Martí, with little fanfare, has also been broadcasting since 1990 on satellite through New Skies 806, the Latin America net portion of the U.S. International
  Broadcasting Bureau. IBB, a U.S. government agency, is the parent agency of the Office of Cuba Broadcasting in Miami.

  TV Martí's satellite broadcasts are widely available in Latin America via cable services. But its digital service requires Cubans with satellite reception dishes to use a
  hard-to-obtain converter, O'Connell said.

  Regular, analog, satellite transmissions to the island have not been used in the past because the number of satellite receiver dishes in Cuba has been limited. But over the past five years there has been a boom in Cuba's black market for satellite receivers that U.S. officials hope to tap into.

  U.S. officials have been struggling with Cuban jamming of TV and Radio Martí for a decade. In May, the Pentagon deployed a special airplane to test improvements in transmissions to Cuba, using a technology meant to break through the ''wall'' of Cuban jamming efforts and make the U.S.-operated stations more effective at reaching Cubans.

  TV Martí costs about $11 million a year. About another $15 million goes to Radio Martí.