The symbolic centerpiece of the effort to rebuild ground zero, a 408-foot spire that brings the Freedom Tower to its projected height of 1,776 feet, could be jeopardized by a new technology emerging as an alternative to the broadcast antenna planned for the inside of the spire.

Some members of the Metropolitan Television Alliance, a collection of 11 broadcasters that has said it intends to use antennae installed on top of the Freedom Tower, have already been using multiple, low-power transmitters placed closer to street level, in lieu of a tall, single antenna. ION Media Networks, a member of the MTVA, is promoting the new technology and, earlier this month, the alliance began its own tests on a similar alternative system.

A long-term contract with the television alliance worth hundreds of millions of dollars would be a key financial component to the $3 billion Freedom Tower project. Sources close to the negotiations have said a contract with broadcasters would net about $10 million a year in annual rent and that an antenna would cost more than $20 million to build.

Following the destruction of the World Trade Center, the Empire State Building antenna became the primary antenna for the area’s major television stations, and the Conde Nast building antenna, at 4 Times Square, became a favorite backup.

The broadcast antennae are now used to transmit to televisions in the area that use “rabbit ear” antennae rather than cable or satellite technology. The industry must switch to digital transmission of television signals by February 2009, as required by the Federal Communications Commission.

In 2003, the television alliance said it intended to use a broadcast antenna on top of the tower once it was built, but at that stage the tower was projected to be completed by 2009. Now, Port Authority officials insist that the tower will be ready by 2012, but there is a degree of uncertainty in the real estate community that the goal will be met.

In the interim, the MTVA has to find another solution, and earlier this month, it began tests on the new technology that could supplant the need for the Freedom Tower antenna.

The system, and the lack of a financial motivation to build the spire to 1,776 feet, could be another setback to the Freedom Tower, which has faced a torrent of criticism over security concerns, costs, planning, and design since its conception. The tower is relying on state and federal agencies to be its anchor tenants, raising concerns about the economic viability of the project.

The leasing agent for much of the building, Cushman & Wakefield, has referred to the Freedom Tower as 1 World Trade Center, in what may be interpreted as downplaying its symbolic importance.

Without the 408-foot broadcast antenna and spire, the Freedom Tower would be cut to 1,368 feet from its symbolic height in feet of 1776, the year America declared independence. It would still be the tallest building in New York City, but it would be the exact same height as Tower 1 before it was destroyed in 2001.

A developer who runs the Empire State Building and who is a critic of the Freedom Tower plans, Anthony Malkin, said if broadcasters aren’t planning to use the antenna, it shouldn’t be built.

“Common sense would say that it doesn’t make a lot sense to build something for which the known user set is not prepared to make a commitment,” Mr. Malkin said in an interview.

The Port Authority could decide to build the spire to its planned height for purely symbolic reasons, or it could find another marketable use for the spire.

The president of the Real Estate Board of New York, Steven Spinola, said, “I seriously doubt that there will not be a need for some kind of antenna on what is clearly a perfect spot to communicate from.”

A spokesman for the Port Authority of New York and New Jersey, which owns the Freedom Tower, Steven Coleman, would not say whether the Freedom Tower’s spire and antenna would be scrapped if the television stations decided they did not want to lease it.

“We are continuing to negotiate with the MTVA,” Mr. Coleman wrote in an e-mail message, referring to the Metropolitan Television Alliance. He added that he would not speculate on the talks, which have been going on for more than a year.

A senior vice president at Richland Towers, which has tested the new technology with ION, David Denton said the new system “has the potential to become the primary source of broadcast operations in New York.”

“This is no longer an unknown,” he said. “We built it. We proved it.”

The CEO of ION Media Networks, Brandon Burgess, was quoted by Broadcast Engineering magazine as saying that the Richland Towers network “provides signal quality at least equal to that of traditional digital broadcast towers at substantially lower investment and operating cost.” Mr. Burgess could not be reached for comment yesterday.

A spokesman for the television alliance, Patrick Smith, said the organization is testing an alternative system to see if it can bridge the gap during the national switch to digital broadcasting.

“Members of the Metropolitan Television Alliance still plan to broadcast from the top of the Freedom Tower,” he said.

Mr. Denton of Richland Towers said many television stations have expressed interest in the new broadcast system, which would operate with four transmitters, with one of them potentially situated on the top of the Bloomberg LP building at 731 Lexington Ave.

Mr. Denton said the new system would cost “exponentially less” than leasing space on the Freedom Tower, but he said he could not give specific figures. The new technology also has the potential to work better with mobile television broadcasting, which is considered the future of television, Mr. Denton said.