# EMI measurments in practice - contradicting V/m, A/m, W/m2

Discussion in 'Physics' started by Janis59, Jun 1, 2018.

1. ### Janis59 Thread Starter Active Member

Aug 21, 2017
586
93
Just I begun to meditate why it is so?
If I measure EMI field then it MUST be that 1 W/m2 corresponds to 20 V/m and 0,05 A/m and 1/1,257 uTeslas.
But making a thousands of measurements with at least 10 different instruments I NEVER get a situation where these formulas are true.
For example, I am sitting 0,5 m off my computer and 3-axis measure device shows E=2,7 mV/m H=7,0 uA/m and F=0,023 uW/m2.
If the first reading be right, then other two must stay H=0,0067 uA/m and F=0,135 W/m2
If the second reading be right, then first and last ought stay rather similarly as previous scenario
If the third is only right, then ought stay E=0,46 V/m and H=0,0011 uA/m

If to step up to the greater power kingdom, say 1 km zone around megawatt-scale tower, the figures are in scale of hundreds V/m and about 1 W/m2 but all three components of EMF fields never pass together, more over, I may step one step forward and E is scaling out whilst H is totally lost, two steps aside and it happens contrary, H dominates and E is tiny. If look at power density, the situation is not better, one step and it shows 5 W/m2 , two steps left, and it shows 0,1 W/m2.

What the heck is happening there? Damn Marsians with their fokuses-pokuses?? Near zone standing waves?? Multiple mirroring (fringe pattern) from earth roughness/bushes/grass leaves?? But it happens everyeveryeverywhere, so I feel it stays somewhere into deepest nature of EM waves. Why mamma Google are not full of explanations for such quirky fact?

2. ### bogosort Active Member

Sep 24, 2011
238
132
I'm not sure where those conversion factors come from, but the discrepancy isn't surprising. We can't measure the EM field directly, and measuring it indirectly is problematic. Consider that the magnetic field strength (A/m) is dependent on charge velocity, which is always measured with respect to some frame of reference. So, depending on the frame, the velocity can be anything -- including zero, in which case the magnetic field is an electric field! Likewise for the electric field. Compounding the problem is the practical fact that any so-called EM meter will use different sensors to measure the E-field and the B-field, each of which will necessarily be tuned to some particular measurement range. Combining these measurements into a final flux (W/m^2) may be somewhat helpful to give an overall sense of what's going on, but it won't be accurate in any sense of the word, especially over a broad range of E- and B-fields.