Sprint Nextel has moved to protect one of its most valuable assets - access to a big chunk of spectrum - just as it is preparing to become a more aggressive force in wireless, with the backing of SoftBank of Japan.
The company, the No. 3 cellphone provider in the United States, disclosed on Thursday that it had offered to buy a stake in Clearwire from its founder, Craig O. McCaw, the cellphone pioneer, effectively giving it majority control of the struggling broadband company.
Sprint already relies on Clearwire to handle data demands for some of its customers, and the smaller network's big block of wireless spectrum could be useful in building out its own next-generation cellphone network.
“We believe it is a strong signal that Clearwire's future could likely be increasingly aligned with Sprint's strategy,†Michael Rollins, an analyst with Citigroup, wrote in a note to investors.
Sprint and SoftBank announced on Monday a deal that involves the J apanese telecommunications giant buying a 70 percent stake for $20.1 billion as well as providing an immediate cash infusion into Sprint.
While the Clearwire maneuver is not directly connected to that deal, it is a signal of intent from the American cellphone company, whose recent ambitions to challenge the market leaders Verizon Wireless and AT&T have been limited by a debt-laden balance sheet.
Sprint has already been converting much of its existing infrastructure into a Long Term Evolution network, which uses a faster data technology used by the newest smartphones.
A portion of Clearwire's spectrum, which is similar to the radio band that SoftBank uses, could eventually be turned into an LTE highway for devices that work on the networks of both Sprint and its Japanese partner.
While the agreement with Mr. McCaw appeared to come together with surprising speed, Daniel R. Hesse, Sprint's chief executive, said in an interview on Thursday that his compan y had already made it known to its partners in the company, which include Intel and Comcast, that it would be interested in buying their stakes if they were willing to sell.
Sprint already owns more than 48 percent of Clearwire, but the agreement with Mr. McCaw's investment vehicle, Eagle River Holdings, buying class A and class B shares from it for $100 million, will push that stake to more than 50 percent. Under the terms of agreements among the Clearwire investors, Eagle River must offer the other companies the right to buy into a portion of its shares.
Both Intel and Comcast have received that offer, giving them 30 days to decide.
Perhaps the most important aspect of the agreement with Mr. McCaw, from Sprint's perspective, is that the board seat held by Eagle River would be filled by Clearwire, adding a third independent director on the 13-member board.
Sprint would still control seven board seats. But having more independent voices could keep Cle arwire more aligned with the network operator's interests. Other strategic investors in Clearwire have also named directors. Agreements among the strategic investors require that certain important business decisions be approved by 10 of Clearwire's 13 directors.
“Sprint would sleep better at night knowing that the board was still controlled by Clearwire,†said Craig Moffett, an analyst with Sanford C. Bernstein.
Shares of Clearwire tumbled 10.18 percent on Thursday, to $2.03, as investors' hopes for a more conventional takeover were dashed.
Mr. Hesse stressed that the Eagle River transaction was not required for the closing of the SoftBank deal, and that he was not currently planning to further change Clearwire's governance.“Our interest is aligned with the public's,†he said.
But Mr. Hesse acknowledged that his company had previously been constrained in the strategic moves that it could make because of its heavy debt load and limited cash. †œIt's been very tough sledding for us because we have been the poor kid on the block,†he said. “We have seen a number of opportunities pass us by over the years.â€
Last month, Time Warner Cable said it planned to sell its 7.8 percent interest in Clearwire. Sprint passed on buying that stake at the time, but Mr. Moffett said that the company would have loved to have reached a deal if possible.
Mr. Hesse may now be feeling emboldened after having struck the deal with SoftBank. The Japanese company plans to inject $8 billion directly into Sprint as part of its effort to take control of the American carrier, which can be used for corporate moves like acquisitions.
It isn't clear whether - or when - Sprint will try to buy out its remaining partners in Clearwire, though a person close to one of the other strategic investors said that any decision would ultimately come down to the price.
Neither Mr. Hesse nor Masayoshi Son, the founder and chief execut ive of SoftBank, would comment on other moves, including potential takeovers. People briefed on the matter have said that Sprint had considered bidding for MetroPCS, a smaller American cellphone service provider that plans to merge with T-Mobile USA.
Mr. Son has intimated that he envisions Sprint as a potential vehicle to buy other service providers. The enlarged T-Mobile has been suggested as a possible target. In an interview on Thursday, he emphasized that his ambition is to create one of the world's biggest mobile Internet companies.
And Mr. Hesse said on Thursday that he still anticipated being a part of the deal-making in the American cellphone service industry.
“This is a scale game,“ he said, adding that he believes regulators would favor transactions that fortified competitors to Verizon Wireless and AT&T, which currently tower over the rest of the industry.
But first their transaction must pass muster with American regulators, including the Justice Department and the Federal Communications Commission. It must also be reviewed by the Committee on Foreign Investment in the United States a government panel that determines whether a deal would pose a threat to national security.
Neither Mr. Hesse nor Mr. Son said that they expect any problems with the committee review, and that they have not heard from any legislators regarding potential issues with Sprint having a foreign owner.
And while a joint venture of SoftBank obtains network equipment from Huawei and ZTE, two Chinese telecommunications equipment makers criticized by the House Intelligence Committee earlier this week as being threats to national security, Mr. Son said that he is mindful of keeping the American government happy.
He said that Huawei and ZTE currently supply only a small part of his company's total equipment, and that he is mindful of the multiple government agencies that currently have contracts with Sprint. “We will re spect whatever the American government wants,†he said.
A version of this article appeared in print on 10/19/2012, on page B4 of the NewYork edition with the headline: With Cash From Deal With SoftBank of Japan, Sprint Goes After Bigger Stake in Clearwire.