Submitted in this thread by "RFB" on November 7, 2012 - 11:43: ...But I will guarantee you this...if his signal is a few microvolts above 15 at 154 feet, the FCC isn't going to stop the press and break out the violation pad. ...
I agree with that, RFB. But let us do a little more math to see what the field at 154 feet might be for the scenario reported in this thread.
This particular carrier-current system reportedly produced a signal indication of 63 dBµ on a Tecsun PL-380 receiver tuned to its frequency, when located 1,000 feet away from the nearest a-c conductor that this c-c system was driving.
In an earlier post in this thread, I reported that a known field intensity of about 500 µV/m produced a signal strength reading of 27 dBµ on a Tecsun PL-310, on a frequency only 20 kHz away from the reported frequency used by this c-c system. The PL-310 uses the same microchip as the PL-380.
The 36 dB difference between these two indicated voltages is a ratio of 63:1. In other words, the PL-380 was indicating a field intensity from that c-c system at that location of 63 x 500 µV/m = 31,500 µV/m.
If the field from this c-c system is 31,500 µV/m at 1,000 feet from the nearest a-c conductor, then the 1/r field at 154 feet from that a-c conductor is 1,000/154 x 31,500 µV/m = 204,545 µV/m.
Probably most would agree that this exceeds the 15 µV/m limit of §15.221 for a distance of 154 feet by a lot more than "a few microvolts."
Part 15 Rules appear simple, but both their underlying physics, and compliance to those Rules are far from simple to grasp and implement -- especially for "newbies," and maybe even for some who are not.
But then, those are the people who can most benefit from the posts in this thread.
The calculations presented by rich are highly interesting in the difficult science of measuring for carrier current compliance, but let's explore further.
If I read correctly, and it's possible I may have overlooked something of what was said by Bruce in his description, but I think he said he was measuring "distance from the pole". But that doesn't really explain where the overhead wires were located.
We may have a situation here, where because of Bruce's eyesight condition he may not have been able to ascertain the location of the overhead wires, or, as could happen with anyone, be absolutely sure that the overhead wires were actually part of the carrier current path.
Therefore, his reading might not have been the required "distance from the wires" that have significance in grabbing a meaningful field reading.
"Probably most would agree that this exceeds the 15 µV/m limit of §15.221 for a distance of 154 feet by a lot more than "a few microvolts."
I don't think it's "probably", more like all would agree it's over the limits.
But let me make a point that Bruce is experimenting and learning as he goes. Since Bruce is most likely not going to have precise measuring gear, and the added fact the coupler being used is home built and under test, and that Bruce is not leaving this testing signal on for hours and hours, but long enough to test and measure, I would think that "probably most" would look at the field strength reported as part of the experimentation process that Bruce is currently undertaking and that it is not "broadcasting" to any degree.
Your information will help Bruce along with the development of his home built CC coupler. I'm confident that when Bruce has it perfected, it will meet that limit just fine.
Continue experimenting and measuring Bruce.
RFB
That guy must have thought I
was nuts. (And now I have to
go back out there and measure
the sidewalk.)
He must have seen me walking
down the street putting that
PL-380 next to the phone poles.
But the weirdest part of all of
this was going down to my friend's
house and actually hearing my station.
I don't know if I told the story completely.
We were talking in her kitchen and she said,
"well... why don't you turn on your radio
and see if you can get your station?"
So I did and there was nothing. I was
disappointed but not too much. In Part
15, this is a regular occurrence!
Then I started walking toward her living
room. And she told me that she heard a
little bit of music. It was very weak.
I still didn't think it was my transmission.
But as I got more into her living room,
it got louder. I kept walking toward the
strongest signal. I got to a "sitting
window." I guess that's what you call it.
It was a corner sitting window, so the
flat area was a big corner and so was the
window. Bull's eye. The signal was the
strongest there. And that's the side of the
house where the power line connects,
although up higher on the side of the
house, of course.
I set the Grundig S-350 on the big window
sill and we went back into the kitchen and
she and I talked for a while.
The signal wasn't perfect, but it was
completely listenable. And we talked,
and listened to it for about 30 minutes.
Then I went home. I am fortunate to
have some good friends in the neighborhood
on this busy main street.
We all brought up our kids together here.
This took place before I did mods to the
coupler last night. So the signal there
is probably stronger now. However,
I won't be going back there for a while.
I don't want to wear out my welcome. i
will probably go back in a few weeks.
So it was house number 14, probably
about 800 or 900 feet away.
What a great experience!
Bruce, W 60 HZ
I just caught some of your comments that
I didn't see before.
I'm about to go to bed.
But I will have a few responses tomorrow.
Thanks a lot you guys!
Bruce, W 60 HZ
P.S. It's funny, I have a friend who really
likes the callsigns: W117VAC or W120VAC
or W125VAC.
It's all alphabet soup to me!
(a) Field strength levels of radiated emissions from carrier current systems may not exceed 15 uV/m, as measured at a distance of 47,715/(frequency in kHz) meters from the electric power line.
Here's my interpretation.
15uV is not the reading being sought at X distance.
15uV is to be DIVIDED by the distance, which is derived from the distance portion of the formula. In other words, there are two formulas.
At my frequency of 970kHz the distance formula gives me 161.3 feet from the electric lines, (49.19 meters) and when this distance is divided into 15 uV the result is .304 uV.
The expression "15 uV/m" is a division operation.
15uV is not the reading being sought at X distance. The expression "15 uV/m" is a division operation.
The unit of measurement "µV/m" refers to the radiated r-f voltage existing between two points in space separated by one meter.
In carl's case, FCC §15.221 permits a field strength not exceeding 15 µV/m between two points in space separated by one meter, at a distance of 161.3 feet from the nearest a-c conductor driven by the c-c transmitter.
But as mentioned earlier, a 15 µV/m field* in the AM broadcast band is well below the ambient r-f noise level in almost all radio listening environments. So if a c-c system can usefully be received on an AM receiver over 100 feet away from the nearest a-c line, probably that c-c system is non-compliant with FCC §15.221.
* let alone a 0.3 µV/m field
A forward slash between two figures in mathematics conventionally signals a division operation.
If 15 uV/m is not a division problem, but the following figures, 47,715/frequency in kHz is a division operation, it would be the case that the FCC is using non-standard and confusing symbolic language only understood by insiders.
Most part 15ers, as hobbyists, are made outsiders by the dancing language in the Part 15 Rules, written partly for the comprehension of the unschooled hobbyist, but at intervals written for trained insiders making the practice of the part 15 hobby treacherous and unlikely to be compliantly achievable.
rich is not "anti part 15". The Part 15 Rules are anti hobbyist.
By the way, my near field looks pretty
tight.
My dog Hoover got out into the busy
street, which is bad. The only way to
get him is to open the car door and
then he thinks he's getting a ride.
My dad and I did this in the driveway,
Hoover jumped in, and we had him contained.
(Thankfully, it was almost disaster.)
Then we decided to go around the block. We
went right out of the driveway.
I heard the signal on 1020 for a few houses.
(The signal rides a power line on the right
side of the street.)
We drove around the block. (The signal abruptly
vanished when we turned right to go around.)
We went around and there was nothing. Then we
came around toward the house from the
left. I didn't hear anything until we got
into the driveway.
Gee, I hope nothing's wrong. I'm going to have
to go out to the poles again.
Bruce, W 60 HZ
Everyday I am learning new things about the poles along the street. There is a pole in front of every house in some cases, and in other cases there is a pole at every other house.
I notice one house, two doors from here, that has no electric lines from the pole. It must be getting power from the rear or perhaps underground.
Houses on the other side of the street mostly get power from this side of the street, as there are no poles over there, except an occasional pole here or there.
Are power wires not connected to my same transformer of any interest?
I went out again.
The readings on the poles were lower
today, but still very very good.
It's gonna change all the time.
Best Wishes,
Bruce, W 60 HZ
By studying CC at the pixel level I'm slowly gaining a sense of what's happening.
Trips in the automobile listening to a silent carrier are informative, because I can listen for that moment when sputter noise starts to overtake the signal, and there is a quick drop-off at that point, which seems like the "edge" of the CC signal.
By increasing power from 1/4W up to 1Watt there is no doubt the near field increases according to every indication, i.e., field strength meter, peak as viewed on a spectrum analyzer....
But interestingly the "edge" only increases by a small amount.
I will continue trying to attract a clear explanation of how to decipher the actual part 15 rule for determining the outer boundary, rather than some virtuoso math with no followups to further dialogue.
By now readers are reaching for the mouse, but I'll be back.
I will continue trying to attract a clear explanation of how to decipher the actual part 15 rule for determining the outer boundary, rather than some virtuoso math with no followups to further dialogue.
The specific followups apparently sought by the poster of the above clip might more readily appear if that poster had provided, or could provide technical justification for others to post such followups.
The math posted previously in this thread is not virtuosic, just that taught in primary school.
Learning when such (simple) math is appropriate to analyze/predict r-f propagation is a different matter, however -- usually requiring many years of study and experience.
"Learning when such (simple) math is appropriate to analyze/predict r-f propagation is a different matter, however -- usually requiring many years of study and experience."
There's your virtuosity. And bravo.