| T O P I C R E V I E W |
| eliquist |
Posted - 12/30/2019 : 11:35:00
I'm surprised that I was unable to find good info on measuring in the
near field in web searches, given how tricky but important the subject is.
Wikipedia and the OSHA document it cites give some general clues but no practical info.
Does it apply to home and office and cellular stuff or only really relevant to high-power industrial things?
Documentation of some meters says they are only valid in the far field, and I suspect it may be true of others that neglect to say so.
But if near-field measurements can't be made with a conventional meter, and near-field is one wavelength, then 60Hz measurements can't be made from less than thousands of miles away, clearly absurd.
Even 10MHz signals couldn't be made anywhere within a normal size home.
The definition of near field may depend on antenna length.
House wiring typically extends for dozens of feet, could be hundreds if all branches are added up, but is connected to utility wiring that extends many miles, so what's the length to put in the formula?
How far off are RF near-field measurements with consumer meters likely to be?
Will such measurements be too low, too high, or can it go both directions?
Do we really only need to worry about the reactive near field or also radiative?
Are there differences in near-field behavior with different types of sources or at different frequencies including above GHz?
Can there be a deadzone in the near field where interactions cancel each other?
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| 6 L A T E S T R E P L I E S (Newest First) |
| rfrazier |
Posted - 02/25/2020 : 22:55:11
Hi eliquist,
I posted some extensive information on near field far field in this post:
https://www.gqelectronicsllc.com/forum/topic.asp?TOPIC_ID=8856
Here are the highlights as I understand them.
First, near field is 3 wavelengths, not 1. Within that range, there is not a coherent relationship between the electric field and the magnetic field which will ultimately become a radio wave assuming we're talking about radio. So, in essence, you CANNOT measure radio waves in the near field because a radio wave doesn't exist as such. You can measure the electric component, or the magnetic component, but you cannot mathematically combine them to get a number for RF because the relationships are unpredictable. Note most electric field and magnetic field meters cannot measure at common RF frequencies.
When, you run the numbers, you get the following examples of common measurements in order to be 3 wavelengths away:
A wifi router at 2.4 GHz - you must be 15" (inches) away to measure RF accurately.
A cell phone at .6 GHz (600 MHz) - you must be about 5' (feet) away to measure RF accurately.
An AM radio tower at 600 KHz (1000 X lower frequency) - you must be about 5,000 feet away to measure RF accurately.
So, what happens if you move your meter closer than the limits above, say to the router? You will certainly see a trend of the reading going up, and that trend is certainly true, and that radiation will affect your body more if you're closer. But, the NUMBER will be invalid and inaccurate.
So, what you said is true at 60 Hz, you are ALWAYS in the near field. You CANNOT measure 60 Hz radio, even if such a thing could exist. In the near field, you can ONLY measure the electric field component, or the magnetic field component. You may draw health conclusions about each. But you cannot say anything valid about a radio field in the near field, other than it's getting bigger closer to the source. Interesting article here about ELF (Extremely Low Frequency) radio - requires a 30 mile long antenna. https://en.wikipedia.org/wiki/Extremely_low_frequency
In my experience, with a Safe and Sound Pro II professional grade RF meter, any time you're within about 5 feet of any common wireless device, the radio field will be over 1000 uW/m^2. According to Building Biology standards, 1000 uW/m^2 is extreme, and should be remediated immediately. It is well above the level where harmful biological effects start occurring over time.
If you hold your phone 4" from your body rather than 5', you will be exposed to substantially more radiation. We just cannot quantify the number unless we switch over and start talking about electric and magnetic fields. HOWEVER, most electric field meters and magnetic field meters top out at 400 KHz. So, you really cannot measure those components of a radio signal that way for our common sources. That's why, if you want a valid number, you need to stay in the far field.
Having said that, I don't have a problem showing a client the meter going up and up and having an alarm going off as I get closer to his cell phone to make a point. I just cannot claim the number was accurate. But, if it was in the extreme zone when it was in far field, you can bet the radiation is more extreme as I get closer.
Also, in that other post, I discussed a number of other factors which make it very hard to get a valid RF number, aside from near field far field.
Hope that helps.
Ron
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| eliquist |
Posted - 02/25/2020 : 13:52:16
| "and near-field is one wavelength" - No,near field means CLOSE (ie. Near) to
| the transmiting element,cable or antenna.
| "Do we really only need to worry about the reactive near field or also
| radiative" - the radiation is causing the reaction,no?
No, when it comes to electromagnetic physics, "near field" has a
particular meaning, it is not just the common meaning of the
word "near" + the common meaning of the word "field".
Ditto for "reactive near field" and "radiative near field".
For an introduction to the subject check out
https://www.ic.gc.ca/eic/site/smt-gst.nsf/eng/sf09976.html#sA.0
which notes that in the near field electromagnetic fields are
unpredictable and can vary drastically from one inch to the next.
| all electromagnetic detecting meters will measure near field,but perhaps not
| as accurately as far field.
Correct.
| "How far off are RF near-field measurements with consumer meters likely to be"
| - Near field normally refers to electric or magnetic fields ,if you are "near
| field" to an RF transmitter,then the meter is likely to be more accurate than at
| a distance,and you will be cooked.
No, many meters only measure the electric or magnetic field and derive the other assuming a constant linear factor, which works in the far field but not in the near field. If a near field is overwhelmingly magnetic, an electric meter will be drastically wrong in its magnetic "measurement", and vice versa.
Measuring farther away may result in anomalies if there are objects interfering with the field or other sources, but that has nothing to do with accuracy. But in the reactive near field, the field itself has irregularities as well as potential interactions between the field and the mere presence of the meter and/or the measurer's body, which do affect accuracy.
I haven't been able to find much practical info, but highly technical docs like https://pdfs.semanticscholar.org/c9cf/7df85e45cb1777e5bee125e9ec03be2f5255.pdf indicate you need to take hundreds or maybe thousands of measurements and feed them into complicated computations to get an accurate picture of the field from near-field measurements. It also has cool color graphics showing complicated fields indicating that not only near fields but also far fields can be complicated with both hotspots and dead zones, so if you only take 1 measurement you may completely miss the existence of a field blanketing most of the area, and if you don't take measurements in many locations you are liable to miss the peak hotspot.
| Starting on page 19 of the 1984 document there is a discussion of the
| measurement and effects electric fields near power lines.
Page 19 only says the relationship between electric and magnetic fields is different in the near field from the far field. Nowhere does it say anything about how they differ or provide any practical info on measuring.
Are there other forums better suited for these kinds of technical questions?
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| Bill D. |
Posted - 01/06/2020 : 14:03:15
quote:
Originally posted by eliquist
Nowhere in that WHO document did I find anything about near field physics or
measurement, nor anything about RF frequencies, much less anything relating to
any of my questions, which remain unanswered.
Starting on page 19 of the 1984 document there is a discussion of the measurement and effects electric fields near power lines. All of that relates to the near field effect, of course. |
| paul |
Posted - 01/06/2020 : 13:39:31
@ eliquist - you did ask a lot of questions at once.
"and near-field is one wavelength" - No,near field means CLOSE (ie. Near) to the transmiting element,cable or antenna.
"But if near-field measurements can't be made with a conventional meter" - all electromagnetic detecting meters will measure near field,but perhaps not as accurately as far field.
"The definition of near field may depend on antenna length" - No,see above.
"How far off are RF near-field measurements with consumer meters likely to be" - Near field normally refers to electric or magnetic fields ,if you are "near field" to an RF transmitter,then the meter is likely to be more accurate than at a distance,and you will be cooked.
I wonder if you have read and perhaps misinterpreted a recent comment by JewelHanda here -https://www.gqelectronicsllc.com/forum/topic.asp?TOPIC_ID=8457
"Do we really only need to worry about the reactive near field or also radiative" - the radiation is causing the reaction,no?
"Are there differences in near-field behavior with different types of sources or at different frequencies including above GHz" - Yes,but all are damaging to living cells.
"Can there be a deadzone in the near field where interactions cancel each other" - maybe,but good luck with that one.Probably reflection ,diffraction will get you anyway.
I hope this answers some of your questions.
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| eliquist |
Posted - 01/06/2020 : 09:36:57
Nowhere in that WHO document did I find anything about near field physics or
measurement, nor anything about RF frequencies, much less anything relating to
any of my questions, which remain unanswered.
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| Bill D. |
Posted - 12/30/2019 : 17:51:33
This document may address some of your concerns. I am not aware of anything comparable. "Extremely Low Frequency Fields
Environmental Health Criteria Monograph No.238" https://www.who.int/peh-emf/publications/elf_ehc/en/
The 1984 version may also be of help. https://apps.who.int/iris/bitstream/handle/10665/39376/9241540958_eng.pdf?sequence=1&isAllowed=y |
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