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FCC Approves BPL Despite Interference Concerns
goosman writes "The ARRL is reporting that the FCC has approved revised Part 15 (unlicensed services) rules to specifically regulate the deployment of broadband over power line (BPL) technology. The Commission adopted a Report and Order in ET Docket 04-37 when it met in open session today. At the same time, three members of the Commission, including Chairman Michael K. Powell, specifically mentioned the concerns of Amateur Radio operators at the open meeting and expressed either assurances or hope that the new BPL rules will adequately address interference to licensed services."
The twisted lines you saw and the twisted lines that would supposedly stop interference are two different things:
- A standard power cable is a long steel cable with an outer layer of aluminum strands twisted around it serving as conductor.
- A "twisted pair" of lines are two lines, corresponding to the signal line and the return path, that are twisted around each other. The idea is that if the same signal travels in both directions, the lines will each generate an equal but with opposite sign EM field (which is what causes interference), and these two fields will effectively cancel eachother out.
Unfortunately, if you twisted power lines, you'd need insulation tens of centimeters if not meters thick to prevent arcing. High voltage lines can run up to 400kV, and standard insulators between those and the cable towers are a good 3 or 4 meters long...
Jw
Parent poster is correct, and well explained. Here's another way to look at the twisted-pair concept.
Any electrical circuit forms a loop; you can trace the current going out from the power source, through the load, and back to the other side of the power source.
For an electric power transmission line, this "loop" is the wires on the left and right sides of the power-line crossbar (OK, not all lines look like that, but the principle is the same). You can trace an imaginary line down one side of the power line and back on the other, enclosing a loop 12 feet wide and many miles long, with enormous area. This is one reason power lines are a bad idea for carrying RF signals; they make a GREAT antenna.
For radio interference, the area enclosed by this loop is an important factor; reduce the loop area, and you reduce the radiated interference. The DIRECTION of the current in the loop also counts; a clockwise loop radiates with a phase opposite that of a counterclockwise loop and can cancel it out if the two are right next to one another.
Now imagine twisting the two wires around each other; you get many very tiny loops with alternating CW/CCW directions of current flow in the loop; their net radiating effects cancel out.
Interesting note: Cross-country power lines ARE in fact twisted pairs, to prevent another interference type. At every Nth tower, you'll see the lines cross over so the left-hand line goes to the right. This results in loops of a half-mile length or so; useless for shielding from RF, but VERY important for protecting the grid from geomagnetic storms, where the Earth's magnetic field is pushed around by solar wind. Making the net loop area zero prevents the transmission line from acting as a giant DC generator and blowing out the switchgear, causing major blackouts (this happened in Canada in the 1970s, IIRC).
"My strength is as the strength of ten men, for I am wired to the eyeballs on espresso."
It won't work that way. If BPL interferes with ham radio, the number of operators will decrease below the crticical mass necessary to provide emergency communications, worldwide.
Here's why:
BPL produces interference across the entire spectrum of "high frequency" (3-30Mhz) radio, and a little above and below in fact. The HF frequencies have special properties (on this planet, at least) of being reflected around the world by the ionosphere. A tiny sliver of these frequencies are used by amateur radio operators, but there are litterally thousands of other kinds of licensees worldwide.
BPL power lines radiate this interference, and when the ionosphere is highly reflective, the interference will be sent around the world. Since the FCC denied the request to have the BPL systems transmit identification, there won't be any way for anyone to identify which BPL installation is causig interference, since it might be halfway around the country, or halfway around the world.
There are BPL systems that don't use HF radio waves, but in all the rush to "Step 3: Profit" these technical issues have been ignored, and the comlpanies with the best lobbiests have won.