Clearwire said today its sales more than doubled in the third quarter, sending its stock up 18 percent in morning trading.

The WiMAX operator posted revenue of $332 million in its preliminary third-quarter results, a 126 percent increase over the same period last year.

Subscriber numbers were also rosy, with net wholesale adds of 1.9 million. The new customers are expected to bring Clearwire’s subscriber base to 9.5 million.

Clearwire said cost cutting efforts and customer growth helped reduce its losses, but said its cash reserves fell to $700 million by Sept. 30, from $848 million at the end of the second quarter.

The results still have to be finalized, and Clearwire’s full third-quarter earnings are expected to be released in the coming weeks.

Sprint’s plan to deploy its own LTE network and phase out devices running on Clearwire’s WiMAX service by the end of 2012 has worried investors. Sprint is Clearwire’s largest wholesale customer, and its loss could have a major impact on revenues.