Hi, guys and gals,
Well, it was me that done it, with the help of 160,000 hams who have helped
make sure ARRL has resources to do this sort of thing. Thanks to all who
helped make it happen. I couldn't have done it without you.
In a number of lists, a number of questions have come up.
Q: The video is a bit rough. Putting a radio in a car and driving around is
not very high tech. Why did you do it this way?
A: ARRL has submitted a 120-page paper to the FCC. Appendix C of that paper
calculates the interference potential from Part 15 radiated emissions limits.
This video study accomplished just what it intended. It demonstrated that
systems whose emissions are at the FCC emissions limits have a significant
impact on HF communications. When the video was put together last Friday, the
reply comment date was August 6, so ARRL did a fast job that was just
sufficient to that task, if accompanied by explanation. With the extended
timeline, there is more time to do a better job of editing a "non-ham" version,
with an A/B comparison showing non-BPL areas and just a bit of voiceover on the
video, as well as the expected written explanation. But ARRL expected that the
video be understood by hams "as is" and wanted to get this information into the
hands of the amateur community soonest.
Q: How representative of what you found was the video?
A: I ran about 3 hours worth of video, and believe that what is on the version
on the web is a fair representation of what interference levels that I found.
If the S meter levels were S5, then that is what we showed. If we found
interference on 5.3 MHz, that is what was shown. The systems were visted at
different times of day, so in some cases, with most people at work, the usage
of the system was light. In doing two long drives showing the levels not
changing much with distance, I think the video communicates that I did not go
looking for hot spots. The system in an area of underground wiring did exhibit
less noise level than the systems with BPL signals on overhead lines. In a few
cases, the stronger amateur signals were copyable, although it sure wasn't
pleasant copy. The ones on the tape were actually S9, and would have been Q5
except for the noise.
Q: Was that noise from your car?
A: Only at a level well below what was being observed. The 89 Subaru has a
computer, but it is off when the ignition switch is off, and most of the test
were done parked, with the engine off. When in motion, I have up to an S5 or
S6 ignition noise, but I only made tests in motion where the BPL noise was much
higher. I am an experienced HF mobile operator, so I have a good understanding
of what to expect in mobile installations, and what was seen on the tape wasn't
normal mobile "noise." Nor was I parked next to the 7/11 with the microwave
running. What was observed was pretty consistent in the test areas, especially
the two where I took a good long drive down the road.
Q: Why does it sound like ignition noise or power-line noise?
A: The very fast pops heard are the keep-alive pulses from the modem, or brief
bursts of download. Note that even under those circumstances, an occasional
longer burst is heard. Unlike auto ignition, which would be very regularly,
these were almost random in timing. I have also heard these systems with
longer downloads, and that ignition-noise sounding popping is light compared to
how strong it can get on the particular systems involved. The system with the
stronger buzzy sound is from a larger number of modems, on a different system
at a different time of day. It is much more irregular than power-line noise
and when analyzed, lacks 60- and 120-Hz peaks. I also noted that as I drove
within the area, it was constant along a long stretch of road, something that
doesn't happen with power-line noise, and when I drove outside the test area,
it diminished smoothly.
Q: Some of the noise was only S4 or so. That isn't that bad.
A: It sure would be for this QRPer. In a quiet location, the ambient noise
level is 10s of dB lower than S4. Also, in some cases, the short pulses really
didn't get the S meter swinging. You may note that the S meter jumps a few
units during the louder bursts.
Q: My power line noise is stronger than that. Why is this a problem.
A: If you have S9 power-line noise, you are suffering noise that is clearly
harmful interfence. You should contact Mike Gruber, W1MG@arrl.org and ask him
to help. With ARRL's help, the FCC has sent over 30 companies a custom version
of the "power-line letter."
Q: Did the noise blanker help?
A: Sometimes. I observed that on some frequencies, the noise blanker did
something, while on others, it did nothing at all. In any case, a noise blanker
is not a panacea, and when I listened to a signal and used the noise blanker,
it clearly resulted in a muffled and distorted signal. And, as traffic useage
increases on the network, it would be so continuous that a noise blanker would
have less and less effect.
Q: Why are some of the receivers tuned outside the ham bands?
A: There are no standards for the spectrum used by BPL systems. In each case,
I tuned where the signals were. Some were very heavy in the ham bands, others
were heavy somewhere else. In some cases, I tuned to 5.3 MHz, a "round" number
near our new allocation, or to WWV.
Q: Why are there not widespread reports of interference from these systems?
A: These trials are relatively small. In some cases, they have as few as a
dozen homes. The larger are several hundred. In many cases, there are no hams
at all in the trial area and if there are, they may not be active on HF, or
active at all. Hams a few blocks away are apt to hear the signals, but may not
know what they are and, if S4 or so, may not be too agressive at trying to
track down the source.
Although this video is not as polished as I would like, it clearly gets the
point across. It is not intended to be absolute measurement, although ARRL has
the capability to make reasonable field-strength measurements. It is intended
to show the interference potential to amateur stations in the region, and it
does just that. Though I was close to the power lines, on the road, this is a
legitimate HF amateur installation and with its modest antenna, it is not much
different than I estimate would be heard with a Yagi 50 feet back, at a height
of 40 feet or so.
73,
Ed Hare, W1RFI
ARRL Lab
225 Main St
Newington, CT 06111
Tel: 860-594-0318
Internet: w1rfi@arrl.org
Web: http://www.arrl.org/tis
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