NAB and MSTV (Association for Maximum Service Television) filed comments in response to the FCC Notice of Proposed Rule Making to modify rules affecting the power level and emissions (including duty cycle) of Lojack transmitters operating on 173.075 MHz. The NPRM, issued in response to a petition for rulemaking filed by Lojack Corp., would allow SVRS (stolen vehicle recovery system) base stations to increase effective radiated power from 300 to 500 watts and allow Lojack to increase the output power of mobile vehicle locator transmitters, or VLUs, from 2.5 to 5 watts. Lojack requested the power increase to offset the reduction in the bandwidth from 20 kHz to 12.5 kHz for 173.075 MHz transmissions.
MSTV and NAB noted that the NPRM seeks to relax several important rules designed to prevent interference to reception of TV Channel 7. In their comments, they warn, "It is critically important that the commission protect the viewing public's ability to receive over-the air television programming, including emergency information, news, and entertainment programming. Lojack's proposals would create a serious risk of interference to this important public service by, among other things, combining unjustified power increases with dramatically intensified spectrum usage, all while eliminating critical Channel 7 interference studies. The commission should preserve the Channel 7 study requirement. These studies protect the ability of local television broadcast stations to make their service available to the viewing public. They should not be disregarded, especially in light of Lojack's desire to increase power, intensify the spectrum's use for purposes other than the recovery of stolen vehicles, and use digital emissions whose interference risks have yet to be tested and established."
MSTV and NAB's comments include an engineering exhibit) prepared by Hammett and Edison. In the exhibit, H&E states, "it is impossible to gauge the impact of a possible 3 dB power increase, a 7 dB duty cycle increase, and the switch from FM to digital modulation by an interfering 173.075 MHz SVRS signal to a digital TV Channel 7 signal without detailed laboratory measurements of the susceptibility of consumer-grade 8-VSB receivers to the new SVRS signals." H&E continues, "Thus, until comprehensive laboratory measurements testing the new digital SVRS and VLU signals against a reasonable universe of consumer-grade 8-VSB DTV receivers has been performed, no one, not Lojack, not the FCC, nor broadcasters, can evaluate the interference potential of the proposed changes."
The comments and the exhibit also raise concerns about flaws in the original interference study, Lojack's use of 10 dB polarity isolation when several Channel 7 stations are using circularly polarized antennas, and the problems with Lojack's proposed use of filters (which would likely have unacceptable group-delay characteristics for DTV reception) on consumers' receivers to resolve interference problems.
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