Sounds to me as if they're being doubly cautious here. First they warn you
not to make measurements in the near field. Then they use the 40dB per decade
extrapolation factor to account for measurements taken in the near field (or
perhaps the cross-over point between the near and far fields). If you look in
a fields book, the equations for the electric field strength indicate that in
the extreme far field, the electric field decays at a rate of 1/r, where r is
the distance. This is where the 20dB per decade comes from by the relation of
20log(r1/r2). Since a decade is defined as the ratio of 10:1, 20log(10) is
20dB per decade. Getting back to the equation for electric field strength, the
electric field decays at the rate of 1/(r*r) in the midrange region (somewhere
between near and far field). This is where 40 db per decade comes from. The
squared term in the logarithm expression comes out front as 2*20(r1/r2).
Furthermore, in the extreme near field, the electric fi!
eld decays at a rate of 1/(r*r*r). This equates to 60 dB per decade. In this
case, the cubed term comes out front as 3*20log(r1/r2). If you look at Figure
3-1 and 3-2 in GR-1089, they switch from 20dB per decade to 60dB per decade at
15.9 MHZ, which is the crossover point from near to far field at 3 meters if
you assume the crossover point to be lambda/pi.
>>> <[email protected]> 01/21 6:51 AM >>>
Tha paragraphs are from FCC Part 15.
=======================================
---------------------- Forwarded by Robert E. Heller/US-Corporate/3M/US on
01/21/2000 05:49 AM ---------------------------
Robert E. Heller
01/21/2000 05:49 AM
3M Product Safety, St. Paul, MN 55107
76-1-01
EMC Laboratory Fax: 651-778-6252
To: [email protected]
cc:
Subject: Question on 15.31
I have not heard back fron the FCC on the following question. Does anyone
have any insight into the answer?
Thanks for your time,
Bob Heller
=============================
---------------------- Forwarded by Robert E. Heller/US-Corporate/3M/US on
01/21/2000 05:46 AM ---------------------------
Robert E. Heller
01/19/2000 09:52 AM
3M Product Safety, St. Paul, MN 55107
76-1-01
EMC Laboratory Fax: 651-778-6252
To: [email protected]
cc: Roger D. Kuhn/US-Corporate/3M/US
Subject: Question on 15.31
Mr. LaForge, I have a question concerning paragraphs 15.31(f)(1) and
15.31(f)(2). Paragraph 15.31(f)(1) concerns measurements above 30 MHz and
at the bottom of the paragraph has a distance extrapolation factor of
20dB/decade. Paragraph 15.31(f)(2) concerns measurements below 30 MHz and
at the bottom of the paragraph has a distance extrapolation factor of
40dB/decade.
Why are there different extrapolation factors above and below 30 MHz?
Thank you,
Bob Heller
3M Company
651-778-6336
[email protected]
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