Derek,
in regards to the magnetic field measurement from 50 kHz to 1 GHz:
You can figure that your measurement is in the far-field when the distance is
>1/6 wavelength. At a distance of 10 meters this occurs above 5 MHz. So above 5
MHz the field will drop as 1/d and the difference between 300 meters and 10
meters is 30 dB.
Below 5 MHz what is the difference between 300 meters and 10 meters? Let's run
a NEC-2 simulation and see what we get. The antennas are 1 meter square loops
in free space. I am comparing the current induced in the two receive loops
placed 10 and 30 meters from the transmit loop.
FREQ 0.25 MHz 82 dB
0.50 69
1.00 57
2.00 44
4.00 33
8.00 30
I next placed the antennas 1 meter above perfect ground and obtained the same
results. These results can be mathematically confirmed (I will leave that up to
you) using the formulas I sent. It would be great if someone could run actual
measurements at 10 and 300 meters.
Once again we find that the FCC rules-of-thumb do not reflect reality.
Dave Cuthbert
Micron Technology
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