As reported our carrier current system is always in a test mode because of what I find to be a very hashy rule, 15.221, that besides talking about many different subjects all bunched together, becomes especially imprecise on the most important point, measuring the near field radiation for compliance.
If you have insomnia tonight, all you need is to follow this and you’ll be asleep in one minute…
(a) Carrier current systems … may operate …. provided the field strength levels of the radiated emissions do not exceed 15uV/m, as measured at a distance of 47.715/ (frequency in kHz) meters from the electric power line.
Levels? The word “levels” is plural. How can a single transmitting system produce numerous levels at a particular point in space?
Furthermore, where is “from the electric power line” The entire neighborhood is draped in power lines going everywhere.
It’s impossible to comprehend which exact power line to measure away from, because it will always be toward further power lines.
Christmas can wait. This is important.
A Possible Decipher?
How can a single transmitting system produce numerous levels at a particular point in space?
It can’t, but it produces different field intensity levels at different distances along, and near the a-c lines used as radiators.
If the field intensity limit given in 15.221 is met at every location along the lengths of the a-c conductors at the distances from each conductor permitted by 15.221, then likely the system is compliant.
Proving this will be difficult even with a calibrated field meter, as the field intensity permitted by compliant systems under 15.221 usually is below the r-f noise floor in an open environment.
Entangled
“It can’t, but it produces different field intensity levels at different distances along, and near the a-c lines used as radiators.”
The rule doesn’t say that in the words you’ve used, but it is likely that’s what the rule intends to say.
“If the field intensity limit given in 15.221 is met at every location along the lengths of the a-c conductors at the distances from each conductor permitted by 15.221, then likely the system is compliant.”
That’s a lost cause because “every location at the distances, etc.” is near another A-C conductor and there does not exist a location far enough from all A-C conductors for the impossible reading below the noise floor.
Reading(s) Between the (Power) Lines
For the X band this distance is about 30 meters and I can appreciate Carl’s point about locations where this distance may not exist due to the density of the lines but suppose a field strength reading could be taken at, for example, 5 meters from a line and by linear extrapolation be deemed less than the limit at 30 meters. There may be a problem since the reading would be done in the near field but could this be accounted for in the calculation?
This does raise an interesting question though for the situation Carl mentioned but without trying to “dance on the head of a pin” with this comment it should be possible to find at least some locations where the distance from a single power line could be found at which the measurement could be made.
Unfortunately, the burden of proof seems to be on the operator but it could lead to an interesting discussion with an FCC inspector.
Neil
Figured Out
Here’s what was said…
“Proving this will be difficult even with a calibrated field meter, as the field intensity permitted by compliant systems under 15.221 usually is below the r-f noise floor in an open environment.”
But that’s easy. It means that when, at the specified distance from an A-C line I have only background noise and no signal reading, THAT’S THE SPOT!