SBC

( SBC) is selling wireless telecom towers valued at $1.5 billion in the industry's largest auction yet, according to sources familiar with the plans.

Observers say the auction could tip the balance of power in the wireless-tower industry. Leading tower operators

American Tower

(AMT) - Get Report

and

Crown Castle

( TWRS) are seen as the top bidders for the towers, while smaller rival

SpectraSite

(SITE) - Get Report

is expected to bid but probably lacks the heft to win. Shares of all the possible bidders slid Tuesday, with American Tower and Crown Castle dropping 2% and SpectraSite 4%.

Here's Why

The portfolio includes between 3,600 and 3,900 towers, though location and leasing information that could be useful in valuing the assets wasn't immediately available. SBC is expected to take bids through month's end, and a winner could be determined by late next month. SBC and Crown officials didn't immediately return calls seeking comment.

The tower operators covet the SBC portfolio because, simply put, towers equal revenue. The more towers a company owns, the more income it generates, and the more growth it can put on the plate of number-hungry investors. Crown and American both operate more than 10,000 towers each, while SpectraSite operates around 4,000.

"We're very interested in the portfolio, and we're taking a hard look at it," says Jim Eisenstein, chief development officer at American Tower. "SBC has a dense footprint in a number of very attractive markets."

Diversity Courses

SBC is following in the footsteps of fellow telecom carriers, many of which began selling their tower portfolios about two years ago and using the cash to boost core operations. For the big telcos, "Towers are no longer viewed as strategic assets," says Jim McIlree, an analyst with

Tucker Anthony Cleary Gull

. "From an operator's standpoint, they're seen as cost centers."

The auctions have since helped create what is now a blossoming industry of independent tower owners. There are still many possible assets to enter the market:

AT&T

(T) - Get Report

, for instance, owns some 5,000 towers, but analysts say the company isn't likely to sell them anytime soon.

Sprint

( FON), meanwhile, is keeping its towers, though they're managed separately and, some say, may be spun off in the future, according to Mark DeRussy of

Raymond James

.

Investors have tended to be kindest to such tower owners as

SBA Communications

(SBAC) - Get Report

, which has avoided the large auctions altogether. SBA has relied on smaller deals and original tower construction, which is often cheaper than buying pre-existing towers. Hence SBA has seen its shares rise more sharply than those of American, Crown and SpectraSite, all of which have at least doubled over the past year.

But if it's cheaper to build towers than buy them, why bother with auctions at all? Because as mobile-phone use explodes, building new towers in prime locations becomes increasingly difficult. "Everyone wants a mobile phone, but no one wants to live near a tower," analyst Sean Butson of

Legg Mason

says, explaining that a massive sale of towers in prime locations is considered "a once-in-a-lifetime opportunity." He adds, though, that the SBC sale isn't a zero-sum game: "There are plenty of towers out there and plenty of demand for tower service."