Clearwire Corp., the unprofitable company building a high-speed wireless network across the U.S., rose the most in almost two weeks after Crest Financial bought a 7.9 percent stake and said the shares are undervalued. Overland Park-based Sprint owns about 50 percent of Clearwire.

Crest Financial may seek to add directors to the board, though it has no agreement with Clearwire yet, the Houston-based investment firm said in a filing on June 1.

Clearwire shares rose 4.8 percent to $1.21 in late morning trading. The stock climbed as much as 6.5 percent earlier in the session, marking the biggest intraday gain since May 22.

The rally follows a 40 percent decline in the stock price this year, dragged down by concerns that the company doesn’t have enough money to build out its network. It had $1.4 billion in cash and short-term investments as of March 31, an amount that isn’t sufficient to fund cash-flow shortfalls and interest expense through 2013, according to Dave Novosel, a senior bond analyst at Gimme Credit LLC in Chicago.

Sprint, the third-largest U.S. wireless carrier, is an investor in Clearwire and relies on the company’s network. Sprint holds a 50 percent financial interest and a 47.1 percent voting interest in Clearwire