NAB Test Shows Most Wireless Modulators Exceed Part 15 Limits

The NAB tested 17 wireless FM modulator devices currently on the market and found that 13 of them (76 percent) exceeded field strength limits set by the FCC -- six by as much as 2,000 percent (10 dB) and one by 20,000 percent (20 dB). The association sent letters to both FCC Chairman Martin as well as to Senate Commerce Committee Chairman Ted Stevens and Co-Chairman Daniel Inouye notifying them of the results of the tests.

These devices have the potential to interfere with licensed FM broadcasts (and Channel 6 TV broadcasts if they operate out-of-band). NAB used the study to raise questions about and express concern with proposed legislation to allow unlicensed device operation in the TV spectrum.

As I've pointed out in previous articles, there is a loophole in current Part 15 regulations that allows unlicensed devices to emit spurious signals that have been demonstrated to cause significant interference to TV reception. In response to the MSTV video showing the interference, supporters of unlicensed devices in TV "white-space" have said manufacturers would not sell devices that emit such spurious signals, even though technically they could under current rules. However, at least one paper supporting these unlicensed devices urged the FCC to adopt stricter interference rules for them. The obvious question raised by the NAB study is if manufacturers are unable to meet simple field strength limits for analog FM modulators, can we expect them to follow the complex procedures (channel monitoring, databases of current TV stations, etc.) and tight spurious emissions limits required to avoid causing interference in the congested TV broadcast spectrum?