Dave,
We have run the measurements at 300 m, 30 m, and 10 m and found the fall
off of 1/R^3 apply because the magnetic field is being measured using a
loop antenna. The far field E field rule does not apply for a magnetic
field.
Regards,
Frank de Vall
Sr. Engineering Manager - Compliance
Assa Abloy ITG and HID Corporation
drcuthbert@micron
.com
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ee.org cc
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Subject
06/22/2005 08:29 RE: 標題： RE: Near field H-field
AM measurement.
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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