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 Previous: President Haynie Wins Third Term, Other Officers Re-Elected |  Next: AMSAT-NA ECHO Satellite Moves Another Step Closer to Launch |
By Gary Pearce, KN4AQ
January 20, 2004
North Carolina amateurs and a utility company come together to learn about interference issues related to the Internet delivery system.
Editor's Note: [April, 20, 2004] |
 From left, Bill Godwin and Will Roberts, AA4NC, of Progress Energy; Wake County EC Tom Brown, N4TAB; North Carolina Section Technical Specialist Frank Lynch, W4FAL, and Gerrett Durling of Amperion check out the display of a portable spectrum analyzer during BPL testing. [Gary Pearce, KN4AQ, photo] |
Progress Energy of Raleigh, North Carolina fulfilled its promise to North Carolina Amateur Radio operators when it recently invited several area hams to observe its Phase II Broadband Internet over Power Line (BPL) trial in a rural subdivision south of the city.
The invited hams included Wake County Emergency Coordinator Tom Brown, N4TAB; North Carolina Section Technical Specialist Frank Lynch, W4FAL, and myself in my capacity as a North Carolina Section Public Information Officer. Network engineer Bill Godwin represented Progress Energy and engineer Gerrett Durling represented Amperion, the equipment manufacturer. Also present for the tests were two hams who are employees of Progress Energy: Don Duckett, N9MN, and Will Roberts, AA4NC.
A half mile of feeder
The BPL trial consisted of a half-mile of overhead power line feeder along a highway, and a few dozen homes passed by buried power lines carrying the BPL signal. The BPL equipment had been installed and connected, but no customers were on-line yet; the neighborhood was brand new, and was isolated from other residential areas by farm fields and a wooded stream.
Durling explained the system to us. The overhead feeder
brought the broadband Internet connection to the neighborhood via BPL. An
802.11-based wi-fi system provided a bridge from the feeder to the neighborhood
underground system, where it was again transported on the power line with BPL.
There was a "repeater" in an above-ground pedestal every block or so, and most
of those were connected to 802.11 wireless access points. The access points
provided the Internet connection to the residences; customers would not be
connected to the BPL system directly from their ac outlets. Customers who sign
up for the test will need to have 802.11 wi-fi transceivers connected to their
computers or home networks.
 Gerrett Durling of Amperion stays in contact via cell phone with the Amperion Network Operation Center while Frank Lynch, W4FAL, transmits on 10 meters. [Gary Pearce, KN4AQ, photo] |
We hams were particularly interested in the spectrum used for the BPL legs of the system. Durling explained that each run of BPL, from repeater to repeater, used two blocks of spectrum, about 3.5 MHz wide for the downstream signal and 2.5 MHz wide for the upstream signal, for a total of 6 MHz. At each repeater, a different block of spectrum was required. The blocks could be anywhere between 2 and 50 MHz, although Amperion preferred to use spectrum between 8 and 30 MHz. The specific spectrum chosen for any line segment depends on the characteristics of the line and the need to avoid frequencies that would interfere with other services in the area of that line, including Amateur Radio. The blocks could be as close as 100 kHz apart, or could be as far apart as opposite ends of the available spectrum.
Monitoring for interference
The overhead feeder segment in this trial used spectrum blocks around 25 MHz and 29 MHz. As I tuned across the band I was to monitor, I wondered what I would hear. I had the volume cranked up, sensitive to every pop and squawk from the speaker. There is variety in the sounds of the various BPL systems, so I wasn't sure what to expect. Once I began hearing it, though, the Amperion system's multiple-carrier sound was unmistakable as it rose out of the background noise.
We listened in that area with mobile ham equipment, driving on the road adjacent to the power line, and received strong S9 signals in the immediate vicinity of the line. The signal fell off quickly when driving away from the line and was only faintly audible at about 400 feet. The BPL signal appeared as a series of carriers about 1 kHz apart and completely filled each block of spectrum. Some of the carriers were modulated by slow clicks, but the system was not carrying data to homes. A ham at a home station just under a mile away reported hearing the 10-meter BPL signal at about S6 using an 80-meter dipole antenna. A ham with a large multi-element antenna on a 100-foot tower five miles away did not hear any BPL signal.
We amateurs also listened for noise coming from the underground BPL segments. We found spectrum used near 10 and 15 MHz--including a strong beat note against WWV at 15 MHz. The BPL signal was weaker and only audible within about 100 feet of the above-ground pedestal. The hams using mobile stations to monitor heard no BPL signals between pedestals. A home station with a larger antenna would have a better chance of hearing the underground signals anywhere in the neighborhood.
Next we tried a brief transmit test, with results reported by the Amperion Network Operation Center (NOC) via cell phone. The NOC sent data through the overhead feeder, and looped it back to measure throughput. A 5-watt FM signal on 29.6 MHz from a mobile parked directly under the overhead segment stopped throughput for the duration of the transmission. However, a 100-watt FM signal on the same frequency only caused a momentary blip in throughput. A 100-watt CW carrier on 40 meters had no effect on data throughput. This was just a quick test, and the contradictory results shouldn't carry too much weight.
Amperion's BPL equipment is frequency agile. The NOC can quickly move any above-ground repeater to another block of spectrum, or notch a few hundred kHz of spectrum in a block, by remote control. During this test, though, they were not able to move the overhead segment away from the 10-meter band, due to the schedule of the NOC staff. There could be a little domino effect when one repeater is moved, since adjacent repeaters must use different blocks of spectrum. This could conceivably cause one ham to start receiving interference if his local spectrum block was moved to accommodate an interference problem of a ham on the other side of the neighborhood.
 An Amperion extractor system situated on a power pole. The flat panel box holds an antenna and additional hardware which extracts the Internet data from the power line and transmits a 2.4 GHz "wi-fi" signal to the neighborhood "repeater" nodes. [Gary Pearce, KN4AQ, photo] |
Conclusions
I felt that the discussions with Durling showed that Amperion is concerned with interference to hams and other spectrum users. Durling said his company is especially interested in the report due from the NTIA sometime early this year, adding that NTIA has spent several weeks monitoring Amperion and other companies' installations. One question not addressed was ionospheric propagation--all of the discussion was about the effect of local signals--but it is possible that BPL signals, which have the characteristics of QRP HF ham transmissions, could reflect off the ionosphere and appear hundreds or thousands of miles away, causing interference. The energy of one isolated trial area might not have much impact, but a mature nationwide system with hundreds or even thousands of installations using the same spectrum blocks could be a very different story.
It's too soon to draw many firm conclusions from this one test observation. Amperion's tests show that their signals meet Part 15 guidelines, although those guidelines were drawn more to measure the leakage from a computer cabinet than from a long unshielded wire. Amateur observation of this and other BPL trials show that, Part 15 limits notwithstanding, the radiation is clearly enough to cause harmful interference in the immediate vicinity of the power line, which would violate Part 15, whether the signal is within radiation limits or not.
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Amperion's equipment appears to be frequency agile enough that, in the hands of a utility or service provider who cared enough and was responsive enough, interference problems to individual hams could be cured on a case-by-case basis, assuming that systems were not designed to avoid the ham bands altogether from the beginning. Not all BPL systems--or Internet Service Provider subcontractors--may have this flexibility, or level of customer service.
Even with frequency flexibility, fitting 3.5 MHz wide blocks of energy in between ham and other bands would be a delicate jigsaw puzzle. The good news is that Progress Energy and Amperion are clearly aware of and concerned about interference to spectrum users, especially Amateur Radio, and they are considering ways to eliminate it. While the "BPL industry" has taken a general public stance that there is no interference problem (see FCC ET Docket No. 03-104, Reply Comments of the Power Line Communications Association, in Adobe PDF format), Progress Energy and Amperion have not made those statements and there was no evidence of that attitude at this meeting. The technology is still young and evolving, with interference mitigation a clear goal in next generation designs. Is the strange noise you're hearing interference from a BPL system? Probably not. These days, the bands are full of computer generated noise. Some of it may be hard to distinguish from BPL system sounds, except for one thing: when BPL signals are present, they occupy a broad chunk of spectrum--hundreds of kHz. If you're hearing noise at just one spot on the dial, it's probably not BPL. Plus, BPL installations are so rare right now that it's quite unlikely you are hearing one.
If you suspect an interference problem from a power line, you should contact the utility itself first. From there, ARRL can help you as you interact with the utility to resolve the problem. More information on power line noise can be found on the ARRL Web site at www.arrl.org/tis/info/rfi-elec.html.
Gary Pearce, KN4AQ, is an ARRL Public Information
Officer in the North Carolina Section. He can be reached via e-mail at kn4aq@arrl.net.
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