"An LPB application note available on this website recommends bringing the measurement distance closer, from 30 m to 15 m (or to half the radianlength for 15.221), doing the measurement in the near field instead of the far field. The information on the LPB application note is not technically correct, but the FCC may have bought it anyway, since LPB had installed and verified some leaky coax systens, like along a highway leading to Disney World."
Yep, and for very good reason the FCC went along with it. Because at 15uV, your in the noise floor and other garbage, which the highly accurate FIM 21 will read every spike and spit of noise in the MW band and frequency of interest. To make any real meaningful measurements, a good starting point
is half radian distance of the inductive field. If the signal is reasonably close to 120uV at half radian distance, then chances are incredibly good that at the full 30m radian distance, that signal will be in the noise and clutter.
From LPB Tech Note 2 - CC System Design:
For a 540 kHz carrier current system, one would presume to move to a position 290 feet from the extremity of the AC power system and make a measurement. This is easier said than done:
A) FIM meters are calibrated logarithmically; the scale reads from 1.0 at the left (the normal 0 position) to 10 at the right. The minimum an FIM-21 will indicate is 10 μV/m, and this is at the left end (the 0) of the scale. The Commission requires any reading be in the upper third of the meter scale for meaningful accuracy.
B) Background noise on the frequency is an even more important problem in most locations. Turn OFF your transmitter and chances are excellent that the FIM will still read well in excess of 15 μV/m at 290 feet. The source of this is electrical noise on the frequency, weak signals from afar, etc. The measurement is better made by moving inwards to perhaps half the 290 foot maximum, so the signal strength reading from the carrier current transmitter has a chance of being above the noise background and within the accurate range of the FIM. At 145 feet look for a maximum of 8 X 15 = 120 μV/m. This is still not a good position on the meter scale, but it’s at least likely to be well above the background noise on the frequency.
An FCC Field Engineer once made measurements at the University of West Virginia’s carrier current station the day after LPB had checked the station. Being accustomed to working in the Radiation Field, he looked for 2 X 15 = 30 μV/m at 145 feet, but found something close to 120 μV/m. He closed the station down for excessive radiation. The manager was irate with LPB, but after a talk with the FCC man about the Induction Field, the close-down order was rescinded. This actual experience reinforces the uniqueness of the Induction Field to carrier current broadcasting.
Though the wrong thread, this is what I pointed out in the CC Journal thread, that a CC system is based and setup to function in the inductive (near) field, not radiation (far) field. Thus is pays to measure it properly..even for the FCC.
RFB
The CISPR quasi peak detector has been part of the FCC boiler plate in EMC specifications for many years, but it is not applicable to measuring the rms value of a single-frequency carrier signal, which is what the FIM is mostly used for in AM signal measurements.
The quasi peak detector is for measuring wideband, mostly impulse, noise.
The information available on the web from Potomac Instruments doesn't state anything about the detector circuit, but since the FCC says "quasi-peak", and the Potomac FIM 41 seems to be the standard, then I guess it is OK. Regardless, the measurement for a fixed frequency carrier will not vary much regardless of whether the detector is peak, quasi-peak, RMS or average, and regardless of the audio modulation level. See
for a visual example of the modulation effect.
The FIM 41 utilizes a shielded loop antenna. Such an antenna picks up only the "magnetic" part of the electromagnetic field. Shielded loop receiving antennas are known for their excellent rejection of electrical noise, which is predominantly radiated in the "electric" part of the field.
The FCC case referenced by RFB likely was not caused by inaccuracies due to "near field" or "far field" considerations, but simply by the likelihood that the noise/distant-signal level was simply higher than the transmitted signal at 30 meters. By moving the FIM to 15 meters, the transmitter signal dominated the noise/distant-signal level producing a more accurate measurement of the transmitter signal.
After my last reply, I examined the YouTube link I provided again and noticed something relevant to our ongoing concern: the minimum field strength at the receiver for a listenable signal. The video shows a field strength between 60 and 80 uV/M (depending on the antenna orientation in the video). In all cases, the transmission was "listenable", less noise at the 80 uV/M antenna orientation.
The information available on the web from Potomac Instruments doesn't state anything about the detector circuit, but since the FCC says "quasi-peak", and the Potomac FIM 41 seems to be the standard, then I guess it is OK.
I would take a wild guess and conclude it's ok with the FCC too..I mean the Potomacs are their standard equipped measuring devices. For 6 grand + that thing better tell me
a lot of things besides average/peak/Q-Peak and field strength! It should tell me the next multi-millions lotto numbers!! 😉
"The FIM 41 utilizes a shielded loop antenna. Such an antenna picks up only the "magnetic" part of the electromagnetic field. Shielded loop receiving antennas are known for their excellent rejection of electrical noise, which is predominantly radiated in the "electric" part of the field."
Yep. Another piece of standard issue equipment the field engineers get. And they sure do reject a good amount of the noise that can come from another direction besides the origin of the source signal being measured. However, even with that loop antenna pointing right at the signal source, at the maximum limit distance point..ie 30 meters or 157000/F distance, shut off the TX and that loop antenna is picking up noise from that direction it's pointing, and even enhanced by the focusing nature of the magnetic loop antenna. The field engineer who inspected my stations demonstrated this and is why they perform measurements at different distances from the source, with the TX on, and with it off. If the noise is considerable, such as buzz saw noise or image carrier noises and all the other wonderful stuff of noise found on the MW band these days, they move closer to the TX and repeat the steps. What they are looking for is at the maximum field strength distance is for your signal to suddenly take a hi-dive into the noise pool.
Yep they go through average, rms, peak and quasi-peak. Was quite interesting to observe. After the inspection, the agent allowed me to do a side by side measuring between his Potomac and my Motorola RD2012 analyzer so I could mark and set my unit to indicate the field strength to represent what the Potomac measured. Surprisingly, the displayed results between the two were only off by about 2uV.
Love my Motorola RD2012!
"The FCC case referenced by RFB likely was not caused by inaccuracies due to "near field" or "far field" considerations, but simply by the likelihood that the noise/distant-signal level was simply higher than the transmitted signal at 30 meters. By moving the FIM to 15 meters, the transmitter signal dominated the noise/distant-signal level producing a more accurate measurement of the transmitter signal."
Well noise/distant-signal level would be a far field source. Obviously to get any sort of worthwhile reading, your going to have to move to the next measuring point to get out of the noise jungle. When a CC system is properly adjusted to meet the 221 spec, the signal at the maximum distance point is supposed to drop into that noise jungle, which is compliance. CC is a near field system through and through. At the point of maximum distance specs, the near field turns into the far field and that far field signal MUST drop suddenly into the grass, and normally does on a CC system that is setup and adjusted properly. We don't care for or go for the signal to travel AWAY from the grid wire, we want the signal to travel DOWN the LENGTH of that wire as far as it can go because CC is meant to send the signal down that wire and into radios plugged into the AC power.
If the grid wire lets your signal travel down it's length and that length is 1 mile, that's great! What is NOT great is that signal emitting off of that wire further than the maximum allowed in 221 per frequency of operation, which is why a lot of CC stations prefer the low portion of the band because of the longer wavelength and increased distance allowed off the wire carrying the signal, which simply means that portables or vehicle radios can pick up the signal up to that maximum distance ANYWHERE ALONG the LENGTH of that wire carrying the signal. Beyond the maximum distance allowed, the signal must abruptly dump into that noise mess.
The power grid line is an antenna, but not in the fashion most think, and it is certainly not any sort of 3 meter vertical or piece of wire. 3 meter sticks and 10 foot long pieces of wire rely on far field reception, which is so weak that 219 setup is at the mercy of that noise/distant-signal level even more so than a CC system because a CC signal is delivered right up to the AC powered radio, and a 3 meter system throws the low level signal over some distance to reach the receiver, all the while decreasing in field strength considerably. This is the advantage of a CC system which uses near field reception instead of far field reception, and is the big difference in how it's measured.
RFB
It is true that a field intensity meter using a shielded loop antenna measures the magnetic field, but in the US, FIMs are usually calibrated for the electric field intensity (V/m). In Europe you may see FIMs (more correctly) calibrated for the magnetic field intensity (A/m).
The use of a field intensity meter that measures the magnetic field, but is calibrated for the electric field, makes sense only for measurements in the far field, but not for measurements in the near field. This is because, in the far field, there is a particular constant of proportionality between the intensities of the magnetic field and the electric field, which is 120pi, or about 377. For example, 1 mA/m correspons to 377 mV/m. No such constant of proportionality exists in the near field, and the ratio of the electic field intensity to the magnetic fiel intensity varies with the distance from the antenna, and is different for different antenna types. This is why trying to measure the electric field intensity with a loop antenna in the near field is tricky business. You have to calculate the ratio of the electric and magnetic field intensities at the distance the FIM is from the antenna for the particular type of antenna you are using.
After measuring the electric field intensity in the near field, you have to correct for the considerable electric field gain in the near field to get the correct measurement for determining compliance. The big gain in the near field gets the measurement signal out of the noise, but you have to corect for this gain. Measuremnts in the near field are really not easy.
Since RFB has been measured by the FCC it sounded from the description that the process must have been time-consuming. How long did it take?
Since I have so many frequencies, would they measure only "a station of interest" (per complaint, or detection) or would they measure all of them?
Did the FCC provide any kind of report to the owner regarding their investigation, or does the "good news" arrive silently by never receiving a NOUO letter?
Indeed, the process did span a couple hours or so, not sure as I was not paying attention to the clock but paying more attention to how the field engineer was performing the measurement. Ermi is right, the field agent spent some time punching formulas on a scientific calculator working out the difference of the magnetic field measured and the electric field measured. But the key item the field agent was looking for was the sudden signal drop at the specified distance for the operating frequency. For my station on 1670 it is 93 feet. At 93 feet from ANY grid wire carrying my signal, that signal MUST drop down to or below the noise floor, and it does.
The thing that took the most time, was finding a location where there was 93 feet of distance between one set of grid wires to another. In a community where the blocks are small and the houses are within arm's reach of each other, the power grid system is also close to one set of lines to another, making the measuring task a bit difficult, but not impossible. Right behind my studio/transmitter location is an ally and is where the power utility lines are strung out on the poles. Across the street and in that next block's ally is another set of power utility lines, and again and again. The distance between them is about 120 feet and when your signal can emit off those lines at 93 feet, well its kinda hard to find that spot in between where your going to have silence when your already within the range of the next set of power lines WAY before the 93 foot point.
There is no report card or ribbon or reward issued by the FCC after a successful inspection of a Part 15 station. The agent either tells you everything is ok, or something is not. If the fix is quick, and their day's schedule allows, they will let you fix the problem, and do the measurements again. If they cannot, they come back a few weeks later to reinspect and remeasure.
After it was all said and done the field agent went on down the road happy as a clam..as I was as well.
RFB
Why did the FCC discover you and decide to measure your signal?
"Why did the FCC discover you and decide to measure your signal?"
It wasn't a matter of discovery or request. It was due to someone locally who tuned in and thought it was a pirate operation and notified the FCC Denver office. They came out, inspected the station and went on their way to go tackle 5 licensed stations here operating without STL licenses or proper coordinates on file for the license of one of those STL's and not maintaining a proper staff or remote control of the transmitters and operating some of those transmitters WAY below their required licensed power levels and lack of required public files contents and other issues spanning 10+ years.
Oh...all the while the same ownership of said 5 stations here has outstanding NAL monetary forfeitures spanning 5+ years with another 4 stations in South Dakota also operating outside of various regulations and requirements...
...should I go on?
RFB
Your account of the FCC "looking the other way" on violations by licensed stations reminds me of a story from about 1-year ago involving either satellite radio or internet, wherein it was discovered that whichever licensed satellite service it was, was also operating UNLICENSED TERRESTRIAL MICROWAVE TRANSMITTERS to fill in the dead spots, but NO ACTION IS KNOWN TO HAVE BEEN TAKEN.
But focusing on your complainer, what kind of an odd-ball person would go to the trouble of reporting a signal they hear on the radio? People should be complaining about the lack of programming all across the dial.
I think such people are a form of "sleepers" who are planted for the purpose of "stirring it up."
"what kind of an odd-ball person would go to the trouble of reporting a signal they hear on the radio?"
It may seem strange that such a thing would occur in a large populated area where the dial is crammed with stations. But in places where the dial is not side by side station after station, especially the AM band with only 4 licensed stations on the air, and a 5th unlicensed Part 15.221 CC station suddenly appear, well the conditioned after 10 years of fear this fear that see this report that would easily call in someone carrying a Daisy waving it in a peace march to be a threat.
Actually I think it was one of the 1030 guys here who was the culprit. But it did not bother me one bit as I know exactly what I am doing and have the appropriate test gear to ensure 15.221 and 15.209 compliance. It (the station) was verified to be in compliance and that was 3 yrs ago and it has been in compliance ever since. Last check I did was 3 weeks ago on the Motorola RD2012 calibrated December 2011.
"I think such people are a form of "sleepers" who are planted for the purpose of stirring it up."
TIS what happens when social engineering goes to the extreme to make the population scared of its own shadow or fear the sound of their own house settling or the breeze blowing outside or allow themselves to be groped and grabbed for their safety.
RFB
my meter will be here next week at the latest. also scored a new (unused) iAM (not THII) still in box 100 shipped.
Nearly five years ago, I reported on this website that a signal of about 15 uV/m at 100 feet can't be heard at 100 feet at a frequency at which 100 feet is approximately the radianlength. Only a slight indication of the presence of a signal could be heard on a car radio.
So, if your signal is intelligible at one radianlength, you are probably in violation of 15.221. Something like that was reported by RFB in this thread. If your signal is just barely detectable at one radianlength, you may be OK,
[Good luck with your meter, Rev. Robert. I hope that somebody can certify the calibration for you.]
A radianlength is 47715/(frequency in kHz) meters. The range should be greater at the lower end of the AM BCB because the radianlength is longer.
A quarter wave antenna produces a field strength a few percent above 300 mV/m per kW at 1 km. Maybe an engineer at an AM station will give you more exact information about their signal strength, but that is doubtful if you don't know the engineer.
Measuring the field strength of an AM station will at least give you an indication of whether or not your FIM is working properly. This would definitely not be a valid calibration, however.