Note the phrase ... At 1 MHz, all our fields are below IEEE safety guidelines.

However, the MIT experiement was done at 10 MHz ... which has lower safety guidlines.

Question ... do you have a cite for the H field disapating under the d cubed rule?

 

Note the phrase ... At 1 MHz, all our fields are below IEEE safety guidelines.

However, the MIT experiement was done at 10 MHz ... which has lower safety guidlines.

Question ... do you have a cite for the H field disapating under the d cubed rule?

At 10 MHz the fields are also below the safety standards you have posted as well. Check it out for yourself. At 10 MHz the MIT guys had values below than what your graphs depict to be safe.


As for your question: No I do not. But I found this by a simple google search:

My question for you: Where did you get those graphs from?

 

Table 9 is from IEEE standard C95.1, the calculation for Power Density is from the same book.

The Graph is IEEE C95.1 graphically and is from the Nato Standard I posted earlier.

Now, according to your chart ... magnetic fields follow the inverse square law near the source, but follows the inverse cubed law further out. In any case, it becomes expotential weaker.

 

At 10 MHz the fields are also below the safety standards you have posted as well.

At 10 MHz, the standard for the E-field was 82.38 v/M. MIT measured 185. The H-field standard is 1.63 A/M. MIT measured 21.

We have a difference of opinion there ....

 

At 10 MHz, the standard for the E-field was 82.38 v/M. MIT measured 185. The H-field standard is 1.63 A/M. MIT measured 21.

We have a difference of opinion there ....

You are correct that the chart depicts MIT's demonstration @ 10 MHz as "not safe". However, the, your chart and graph are contradicting each other. The graph does not say that. The chart does. According to your graph, at 10 MHz, the approximate value I got for Magnetic Field Strength is ~147 A/m and the Electric Field Strength as ~552 V/m. These are way above what MIT got, meaning its totally safe. I'm confused as to which is the correct source for acceptable exposure levels.

 

You are correct that the chart depicts MIT's demonstration @ 10 MHz as "not safe". However, the, your chart and graph are contradicting each other. You are correct that the chart depicts MIT's demonstration @ 10 MHz as "not safe". However, the, your chart and graph are contradicting each other.

The chart states for 3 to 30 MHz ... E field is 823.8 / fM ... fM = Frequency in MegaHertz. H field is 16.3 / fM. If you interpolate the chart you will see they agree.

 

Now, according to your chart ... magnetic fields follow the inverse square law near the source, but follows the inverse cubed law further out. In any case, it becomes expotential weaker.

I think that is misleading.
The magnetic field strength of a single wire (one polarity) is an inverse square law. This is not the same inverse square law as RF power, it is simply yet another relationship that depends on the square of the distance.
The RF inverse square law everyone keeps alluding comes from a very simplistic fact - for a given amount of energy being sent into space in it will spread out evenly over the surface of a sphere. It is simply power divided by surface area. If your antenna is directional you simply have a fraction of a spherical surface giving you a proportional constant so it doesn't change the r^2 behaviour.

As soon as you get a loop of current, which you always do (especially with a coil), what you have is a magnetic dipole, whose field strength drops off as 1/r^3, an inverse cube law. This cube law comes from the fact that for any piece of current in the loop there is current of the opposite polarity on the opposite side of the loop. They cancel as seen from a distance, hence making the attenuation faster. This has nothing to do with energy being sent out into space.

An infinitely long current has a magnetic field that drops off with 1/r. If it is infinitely long then as you move further away you are 'seeing' more current, making the attenuation slow.
A segment of current has a magnetic field that drops off with 1/r^2. As you move away there is no more current to discover. It drops off like the E field from an electric point source.
With the loop, as you move away you see current of the opposite polarity, reducing the field even faster with 1/r^3, just like an electric dipole source.

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/curloo.html#c1

In their derivation you see dB, this is the differential field from a short segment of wire. Notice the r^2
Notice the denominator of:
(z^2 + R^2)^3/2
R is the radius of the loop, z is the distance from the loop. Notice that if z^2 >> R^2 you have pure r^3 drop off.
The MIT loop is 30cm radius. Anything further around 1m+ fits this approximation pretty well (1m^2 + 0.3m^2)^0.5 = 1.04m

 

An infinitely long current has a magnetic field that drops off with 1/r. If it is infinitely long then as you move further away you are 'seeing' more current, making the attenuation slow.

Infinitely long current? Are you talking wavelength?

 

Zero,

Do realize that is a log-log graph and not a linear graph?

 

Infinitely long current? Are you talking wavelength?

No, I mean an infinitely long wire in terms of physical size, or equivalently a situation where you're much closer to the wire itself than the ends of it.

 

Zero,

Do realize that is a log-log graph and not a linear graph?

Yep. I would need the actual graph on the software that made it to do a regression analysis and find out the exact values. For now, it seems you may be correct that @ 10 MHz they may have exceeded the safe exposure limits, but I doubt there's any significant harm actually done because they actually put people in between the transmitter and receiver during the MIT experiment. It's probably just a little more exposure than what a typical cell phone or equivalent provides.

 

You just better not have silver fillings while standing between transmitter and receiver.

Hotlips

 

I would need the actual graph on the software that made it to do a regression analysis and find out the exact values.

Use whatever software you wish to use. The formulae's are in table 9.

but I doubt there's any significant harm actually done because they actually put people in between the transmitter and receiver during the MIT experiment. It's probably just a little more exposure than what a typical cell phone or equivalent provides.

In all power density measurements I've seen, it follows E^2/377 * H^2/377. 377 is the figure for air. And naturally, these measurements are only valid in the "far-field". We all agree these experiments are in the near field, less than 1/2 lamda. In short (wavelength) antenna theory ... within the near-field the H-field dominates. This is not some 3.3 W cell phone.

At 1.63 amperes, the people standing between the coils should not exceed the 0.1 hour limit. MIT's experiment exceeded that by a bunch (21 A). Exceeding that would be harmful, according to IEEE/ANSI and a host of other agencies. There is no standard for maximum per year like there are with ionizing radiation.

You would have to track exposure time and have it documented in the persons health record so 30 years from now, if you have some wierd health issue, you'll remember what you did when. Significant harm is relative.

I do not see where they got such a low power level with the E and H numbers they stated.

 

Use whatever software you wish to use. The formulae's are in table 9.



In all power density measurements I've seen, it follows E^2/377 * H^2/377. 377 is the figure for air. And naturally, these measurements are only valid in the "far-field". We all agree these experiments are in the near field, less than 1/2 lamda. In short (wavelength) antenna theory ... within the near-field the H-field dominates. This is not some 3.3 W cell phone.

At 1.63 amperes, the people standing between the coils should not exceed the 0.1 hour limit. MIT's experiment exceeded that by a bunch (21 A). Exceeding that would be harmful, according to IEEE/ANSI and a host of other agencies. There is no standard for maximum per year like there are with ionizing radiation.

You would have to track exposure time and have it documented in the persons health record so 30 years from now, if you have some wierd health issue, you'll remember what you did when. Significant harm is relative.

I do not see where they got such a low power level with the E and H numbers they stated.

I have no idea man. I just stated what they documented on paper. That 3.3 Watt radiation was probably in near-field exposure, because they measured how much their transmitter radiates in the field area it set up. I don't know much about safety standards and all, but those tables you posted are not really clear if they're for near field or far field radiation, and I'm not too sure if the graph and the chart are related either.

 

The E & H numbers they posted are the maximum rms values they calculated at a distance of 20cm. To calculate the radiated power you'd need to know the entire distribution and be in the far field.
Also notice that the maximum Poynting vector they calculate is not equal to E x H, since those maximum values did not exist in the same location in space.

3.3W in radiation cannot be in near field exposure because by definition it is being radiated.

 

zero ...

there are discrepancies between the supplemental and the original document that was in the science journal ... which is online also.

The E & H numbers they posted are the maximum rms values they calculated at a distance of 20cm.

Which values are we to believe ... the supplemental information or the original article? The original article has the E-field at 20 cm 1.4 kV/m and the H-field at 8 A/m. And the original article has the E-field of 210 V/m, H-field of 1 A/m at 1 meter. This seems reasonable (I haven't put it to the calculator) if you belive the inverse square law or the inverse cubed law comes into play and believe the near-field is as chaotic as we read.

The problem with calculating stuff is the next logical step is to measure it. As the original article shows in the graphs, they juxtaposed calculations and measurments, as it is human nature to do those things. Granted their first datapoint was at 75 cm, but all the measurements were within 15 meters or lamda/2.

I'll leave you with a quote from the original article

...that it should be possible to reduce the values cited above for the electric and magnetic fields, the Poynting vector, and the power radiated so that they fall below thresholds specified by general safety regulations [e.g., the IEEE safety standards for general public exposure. ]

 

The table on page 4 of this thread, which has the calculated 185V/m and 21A/m values, is for a 'capactively loaded loop' while the actual experiment was done using 'self resonant coils'.

The near field isn't exactly chaotic it's just asymmetrical and dependent on location in all directions. It's a complicated summation of several terms which depend on angles and various powers of distance.

From Electromagnetic Compatibility by Clayton Paul:

 

Your correct Ghar, they are talking about Capacitive loading.

 

guys i am trying a different approach......
1. create a high magnetic field using only a single coil( keep secondary aside for a time)
check that field by placing a iron in that field and experiment with the distance and input
power.
2. than i will try for transferrin electricity by placing a secondary coil on the other side

auy sugestion......????????