Ask Dr. SETI ®
Charles Osborne, K4CSO
This month's hot topic is an FCC Notice of Inquiry called "Broadband Over Power Lines" [FCC ET Docket No.03-104]. Most of the time I ignore FCC NOI and NPRM (Notices of Proposed Rule Making) that are clearly going to generate a maelstrom of negative comments. They usually disappear for six months and get reintroduced after radical changes. But let's not trust that to happen in this case. See http://www.arrl.org/news/stories/2003/04/30/2 for further info on how to file comments with the FCC electronically.
You see the FCC has touted BPL as a potential way to get high speed Internet to the masses. Cable-TV and DSL are only available to citizens located close to cities and high speed points of presence for the phone company. But everyone has AC power lines. So the FCC thinks this will be the answer to getting the Internet to more people who are still on slow speed dial-up. Politically this sounds great. Technically it has ominous implications. Early tests indicate it could raise the noise floor +30 dB above galactic background at HF. You won't be doing much JOVE / Solar 20MHz work, or even regular ham radio under those conditions. The chances for regular ham radio stations to interfere backwards into such a network is also enormous.
One overlooked problem is feeding the bandwidth into your area. Every transformer constitutes a block to high speed signals. So somewhere close to the end users, even higher bandwidth needs to be available to be divided out. Sounds like a job for fiber. Yet the locales that this is really targeting probably don't have fiber or they'd have DSL. This may be yet another idea that works at small scales, but becomes a real mess, system engineering wise, when widely deployed. Give it ten years, and 2.4 GHz wireless could also be an example of that.
Ever walked by your PC monitor and picked up buzzing interference on a ham handie talkie, scanner, or AM radio? Monitors have steadily become faster and higher bandwidth devices themselves. But as their bandwidths passed 100 MHz the SVGA cables started looking like good antennas. Ferrite beads on the cords today help make them pass FCC Part-15 emission specifications.
But what if the antennas become power lines, distributed over your whole neighborhood via everyone's AC wiring? The local pole to house wiring could look like a pretty good HF antenna. Anything more than a tenth wavelength or with significant VSWR will start to radiate signal. This is very obvious to those of us who have designed RF circuit boards, finding that compact circuits, matched impedances, and ground planes for shielding are needed to keep the signals from jumping from one trace to another or radiating. Aside from using all underground wiring to get to the house, there's very little we can do to stop a 1 MHz or 10 MHz signal from radiating from house wiring. In fact there are significant regulations on computers and appliances to keep them from radiating back out into that same AC wiring at low frequencies.
In an odd twist the FCC turned down a request to create a new ham band at 136 kHz due to pressure from the power utilities who already use VLF to send control and monitoring signals between high tension line substations over the powerlines. The utilities were afraid that a few hundred 50watt ham stations scattered across the country would interfere with their substation controls. Guess they weren't consulted about BPL. With millions of potential interferers coast to coast, once they figure this out the utilities will likely also be on the FCC's back for even proposing the idea.
Cable TV systems constantly fight broken shields, unterminated lines, or poorly crimped F connectors turning the cable into an antenna. When this happens, FAA airport approach frequencies, and local television stations are jammed by the cable system signals on those frequencies. With BPL the AM radio band could easily become unusable for over the air reception of all but the strongest signals.
Digital signals are insidious for making harmonics. A 48 MHz PC clock crystal can easily be received all the way to 10 GHz and beyond. So what's to make the BPL low frequency signals and all their harmonics stay on unshielded wire? There are products out there specifically made to use house wiring as a hidden ham HF antenna. In the case of BPL, by some mystery the FCC thinks it will not cause harmonics and excessive radiated interference. Tests show significant effects even in the 327 MHz radio astronomy frequencies.
The evaluation tests on typical house wiring indicate that one would have to be 219 km away to drop below the ITU-R RA769-1 recommended signal levels in the radio astronomy bands at HF. House wiring looks like a -30 dBi antenna. Worse, if everyone in your neighborhood is on this system, the noise power from many sources will add. The overall effect the FCC is shooting for is that everyone's noise floor gets degraded just a little. But with radio astronomy being down in the -200 dBm range, the desired to undesired signal to noise ratio will be at a disadvantage.
I'm hoping some of you will send comments to the FCC in response to this NOI. Do not call them idiots, or make irrational "You can't do this!" statements. They are the FCC. They can do it, with a simple stroke of the pen. Arguments saying it will violate Part-15 limits have been met with FCC comments like "Well, we'll change part 15." So expect anything that provides: "economic stimulus", "looks good politically" to the most constituents, and adds "competition" to the market place to get a speedy route through the regulatory process. We have until mid June for your well thought out, cleverly articulated comments to reach the FCC.
Disclaimer: The opinions expressed in editorials are those of the individual authors, and do not necessarily reflect the position of The SETI League, Inc., its Trustees, officers, Advisory Board, members, donors, or commercial sponsors.
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this page last updated 31 May 2003
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