WASHINGTON -- Microsoft on Monday gave a simple reason why its prototype for beaming high-speed Internet service over unused television airways failed a government test: the device was broken.

The Federal Communications Commission said on July 31 that the device did not reliably detect unoccupied spectrum and could interfere with other TV programming and wireless microphone signals.

On Monday, Microsoft sent the agency a letter explaining that a subsequent test determined the equipment was defective.

Representatives for Microsoft and other technology companies met with FCC engineers last week and determined the device "was working improperly and an internal component was broken," Microsoft's managing director for government affairs, Jack Krumholtz, said in a statement on Monday.

"This accounted for the FCC's aberrant test results," Krumholtz said.

An FCC spokesman declined to comment on the matter.

Microsoft said in an FCC filing that it sent a duplicate device that was functioning properly, but that the agency never tested it.

Microsoft is part of a coalition of companies that wants to beam high-speed Internet service through unoccupied TV channels, also known as "white spaces."

The coalition submitted two prototype devices, one developed by Microsoft and another developed by Phillips Electronics.

The coalition said the Philips device was able to detect both TV and wireless microphone signals in a laboratory setting.

The FCC's engineering office plans to hold a hearing Thursday to provide an overview of the tests and consider suggestions for further evaluation of the devices.

While FCC Chairman Kevin Martin said he wants the white spaces to be utilized, he's facing resistance from TV broadcasters, who fear that it won't work and would cause problems with TV programming and with a federally mandated transition from analog to digital signals in early 2009.

The coalition said it's confident the FCC will be able to designate the white spaces for high-speed Internet service, which would be accessible and affordable especially in rural areas.

According to its timetable, the FCC could adopt rules for operating unlicensed devices in the white-space spectrum by October and start certifying similar devices that meets its technical requirement. In any case, no devices would go on sale before the digital TV transition in February 2009.

In addition to Microsoft and Philips, the technology coalition includes Google, Dell, Hewlett-Packard, Intel and EarthLink.