Astronomers in Hat Creek, Calif., are planning today to switch on the first elements of a giant new array of radio telescopes that they say will greatly extend the investigation of natural and unnatural phenomena in the universe.

When the Allen Telescope Array, as it is known, is complete, it will consist of 350 antennas, each 20 feet in diameter. Using the separate antennas as if they were one giant dish, radio astronomers will be able to map vast swaths of the sky cheaply and efficiently.

The array will help search for new phenomena like black holes eating each other and so-called dark galaxies without stars, as well as extend the search for extraterrestrial radio signals a thousandfold, to include a million nearby stars over the next two decades.

Today, 42 of the antennas, mass-produced from molds and employing inexpensive telecommunications technology, will go into operation. “It’s like cutting the ribbon on the Nina, the Pinta and the Santa Maria,” said Seth Shostak, an astronomer at the Seti Institute, in Mountain View, Calif., who pointed out that this was the first radio telescope ever designed specifically for the extraterrestrial quest.

The telescope, named for Paul G. Allen, who provided $25 million in seed money, is a joint project of the Radio Astronomy Laboratory of the University Of California Berkeley, and the Seti Institute. “If they do find something, they’re going to call me up first and say we have a signal,” Mr. Allen said in an interview, adding, “So far the phone hasn’t rung.”

Describing himself as “a child of the 50s, the golden age of space exploration and science fiction,” Mr. Allen, a founder of Microsoft, said he first got interested in supporting the search for extraterrestrial intelligence after a conversation 12 years ago with Carl Sagan, the Cornell astronomer and exuberant proponent of cosmic wonder.

When the idea later arose to build a telescope array on the cheap, using off-the-shelf satellite dish technology and advanced digital signal processing, Mr. Allen was intrigued. “If you know anything about me,” he said, “you know I’m a real enthusiast for new unconventional approaches to things.”

Telescopes, including radio telescopes, have traditionally been custom-built one-of-a-kind items. The antennas for the Allen array are stamped from a mold. Mr. Allen’s family foundation put up the money to get the first part of the array built, with other contributions from Nathan Myhrvold, formerly of Microsoft and the chief executive of Intellectual Ventures in Bellevue, Wash., among others.

Leo Blitz, director of the Radio Astronomy Laboratory, estimated that it would take three years and $41 million more, depending on the price of aluminum, to complete the array. The full array, astronomers say, will be useful not just for science, but also as practice for a truly giant telescope known as the Square Kilometer Array, which would have a combined receiving area of a square kilometer and which astronomers hope to build in Australia or South Africa in 10 or 20 years.

Dr. Blitz said the main advantage of the Allen array for regular radio astronomy was the ability to obtain images of large swaths of the sky, several times the size of the full moon, in a single pointing. At low frequencies, he said, the full array could map the entire sky in a day and night and do it again the next night.

“This has not been possible before,” he said.

In its partial form, Dr. Blitz said, the array is already almost as fast, and much cheaper to run, than larger telescopes.

The speed should make it possible to catch transient events, like radio bursts from colliding black holes, that might last only a few hours, while the mapping ability should enable astronomers to search for lumps of gas without stars, the so-called dark galaxies predicted by the prevailing models of cosmology.

The search for extraterrestrial intelligence has lived on the kindness of strangers since Congress canceled a NASA-sponsored search using existing radio telescopes in 1993, only a year after it had begun. The Seti Institute, which was to have conducted a search of nearby stars under contract to NASA, raised money from Silicon Valley and revived the search as Project Phoenix, using existing radio telescopes.

Project Phoenix was finished three years ago, having checked some 750 stars for signals, Dr. Shostak said. While that might sound like a lot, he said, “it doesn’t impress anybody who knows how many stars there are in the galaxy.”

There are some 200 billion stars in the galaxy, and a significant fraction of them have planets. Estimates of the number of intelligent civilizations in the galaxy have ranged from one (or none, if you are particularly discouraged about human affairs) into the millions.

Dr. Shostak calculated that the full Allen array would be able to detect a signal from as far as 500 light years that is only a few times more powerful than what can now be sent by the Arecibo radio telescope, a 1,000-foot-diameter dish in Puerto Rico that is the world’s largest (although it is in danger of being shut down to save money). That translates to about a million stars, which he said was getting into a promising number. Dr. Shostak described the expanded search as looking for the needle in the proverbial haystack with a shovel instead of a spoon.

Anyone out there and broadcasting, for whatever wacky alien reason, would also have to be broadcasting right at Earth. But advanced civilizations, Dr. Shostak said, would be able to tell there was life on Earth because of the oxygen in our atmosphere.

“We’ve been broadcasting that for 2.5 billion years,” he said.

The first thing Dr. Shostak and his colleagues plan to do with the newly operational 42-antenna array is to survey a strip across the center of the galaxy. There will be several billion stars in the field of view, but they will be very far away, 10,000 to 50,000 light years, so any signal would have to be huge to be detected. But who is to say that among galactic civilizations there are not a rare few with tremendous capabilities?

“I’ve never begrudged aliens any power in their transmitter,” Dr. Shostak said.