Clearwire posted some rosy fourth-quarter numbers yesterday, but a spike in churn and a weaker-than-expected forecast indicate the company is not out of the weeds just yet.

The WiMAX provider more than doubled its sales and narrowed its losses, posting positive adjusted earnings before income taxes, depreciation and amortization (EBITDA) for the first time ever.

Revenue increased to $361 million, from $175 million the previous year, but its net loss widened to $236 million, from $128 million in 2010.

EBITDA finally swung into the black at $22.4 million, from a loss of $265.6 million last year.

Momentum from its strong fourth quarter isn't expected to carry over into 2012, however.

Clearwire warned that its full-year revenue for this year could come in between $1.15 billion and $1.25 billion, a likely decline from full-year 2011 sales of $1.25 billion.

Its 2012 adjusted EBITDA losses are forecast to come in between $250 million and $350 million, compared to its $313 million adjusted EBITDA loss last year.

The weak outlook prompted a drop in Clearwire's shares in late Wednesday trading, and its stock price was still down more than 10 percent today.

Clearwire makes the majority of its money off wholesale services sold to Sprint, which uses Clearwire's network to offer dual-mode 3G/WiMAX smartphones, tablets and USB modems to its customers. Sprint subscribers account for about nine out of 10 Clearwire customers.

The arrival of the iPhone at Sprint last October increased Clearwire's wholesale churn rate as customers flocked to the hugely popular smartphone, which does not use its network.

Wholesale churn more than doubled to 2.9 percent, from 1.4 percent the previous year, while churn among retail customers inched up one-tenth of a percentage point to 3.9 percent.

"Wholesale churn increased in the fourth quarter, both on a year-over-year basis, as well as quarter-over-quarter, as our largest wholesale partner began offering an iPhone for the first time," Sprint CFO Hope Cochran said during a Wednesday earnings call. "While churn may be elevated due to demand for such an iconic device, we do not believe this metric is relevant."

The fixed terms of its contract with Sprint for the next two years also shields Clearwire from the impact of the iPhone, Cochran said.

However, the WiMAX provider has been scrambling to find new wholesale customers after Sprint announced last year it would use its own LTE service instead, eventually limiting its use of Clearwire's network to supplementary capacity.

The FCC's decision to block LightSquared from deploying its wholesale LTE network could be good news for Clearwire as companies hungry for mobile broadband look for a new source of connectivity. Clearwire has already signed up two of LightSquared's customers: FreedomPop and Simplexity.

"We expect to make significant progress toward signing new wholesale customers in 2012," president and CEO Erik Prusch said during Clearwire's call yesterday.

Clearwire added fewer new customers during the last three months of 2011 than it did during the same period in 2010.

The number of net new wholesale customers dropped 36 percent to 904,000, and it lost 31,000 net retail customers, bringing its subscriber base to 10.4 million.

Sprint's decision to cap the amount of WiMAX data on data-only devices like tablets and USB modems affected ARPU, Cochran said, but the decline was offset by an overall increase in traffic from smartphones. Wholesale ARPU rose to $6.34, while retail ARPU increased to $46.69.

Clearwire recently raised the funding it needs to start building a TD-LTE overlay network that will be used by Sprint and other companies to beef up their own LTE services.

Prusch said that Clearwire was in discussions to include up to 8,000 sites in the first phase of its LTE plan. The first 5,000 sites are expected to be on air by June 2013. Construction is expected to begin around the end of March.