On August 11, 2004, Bob Gonsett of Communications General Corp. wrote to Dane Ericksen (Chairman, SBE FCC Liaison Committee, a member of the SBE Board of Directors, and a senior engineer at Hammett & Edison, Inc.) and asked about his level of concern toward BPL - Broadband over Power Lines - as it pertains to over-the-air low-band VHF television and FM broadcast interference in particular. Bob cited as an example a case where AT&T had been granted experimental authorization WD2XIZ to test BPL in the 1.705 - 80 MHz range. Following is Dane's response.

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The answer to your question lies in the lower frequency of the proposed

BPL spectrum: 1.705 MHz. Why is the lower frequency not 1.0 MHz or 0.5 MHz?

Because even the FCC realizes that RF couples into non-twisted pairs, that

non-coaxial overhead power lines will radiate, and that energy below

1.705 MHz would cause massive noise interference to reception of AM broadcast

signals. I am not aware of any footnote to Maxwell's equations that says

propagation makes a step-function change at 1.705 MHz, so it seems to me that

the FCC has already admitted, in a de facto way, that RF coupled onto power

lines is going to radiate. Also, remember all the interference that mid-band

cable signals caused to aeronautical frequencies in the 108–137 MHz and

225–400 MHz aeronautical bands, when cable systems started using those

frequencies? And that was radiation from shielded coaxial cables! It

was such a problem that the FCC had to require cable systems using mid-band

and hyper-band frequencies to make annual leakage measurements (the CLI, or

cumulative leakage index, rules) and to use frequency offsets on some cable

channels.

Noise-like BPL interference will be yet another insult to over-the-air

VHF low-band TV broadcasting. Because of the "fail gracefully" nature of NTSC,

it will probably be tolerable in some cases, although certainly annoying,

especially in weak signal areas.

I suspect that the problem will be much more serious for VHF low-band

DTV operations, with a supposed 28 dBu threshold (that's a mere 25.1 uV/m;

the scientists at the Arecibo Observatory would probably get excited about

such low levels, but I find that signal level not very credible for an

over-the-air broadcast signal). While viewing a degraded analog signal will

tell a technically astute viewer legions about the nature of the problem, a

DTV tuner's response to virtually all reception problems is blue-screen

squelch. So, you need test equipment to figure out the nature of the problem.

I expect that a lot of BPL interference will go undiagnosed. Allowing BPL at

the same time the Commission is trying to roll out DTV is the best way I can

think of to ensure that VHF low-band DTV never works.

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