I agree with you but Derek is correct in his statement. The standard leads
you to believe that the ground plane is prohibited. The exact statement is:
"For measurements at frequencies below 30 MHz no artificial ground plane
shall be used."
I could be wrong but I suspect that this is often (if not always) ignored.
Bob Heller
3M EMC Laboratory, 76-1-01
St. Paul, MN 55107-1208
Tel: 651- 778-6336
Fax: 651-778-6252
=========================
Ken Javor
<ken.javor@emccom
pliance.com> To
Sent by: EMC-pstc <[email protected]>
owner-emc-pstc@ie cc
ee.org
Subject
Re: Radiated emssion measurement
06/28/2005 11:21 below 30MHz
PM
Strictly conjecture on my part; I have no specific knowledge of the
standard cited.
1) Below 30 MHz, there is not much opportunity for constructive/destructive
interference, because path length differences between direct and reflected
rays are a small fraction of a wavelength (on a 3 meter site). Also
measurements below 30 MHz are often made with an electro-statically
shielded loop, and that measurement is supposed to be less site dependent
than an electric field measurement. That means a ground plane is
unnecessary, not prohibited.
2) The size of an OATS or indoor site is tied to the antenna-EUT
separation. For an OATS, the clear area is an ellipse whose major diameter
is twice the antenna-EUT separation, and the minor diameter is (if memory
serves) รท3*antenna-EUT separation. I don't know of a reason for the 80 MHz
cut-off unless this is a really old standard from the days of pyramidal
foam absorber instead of ferrite tile. Then it would be obvious why 80 MHz
was the lower frequency.
3) Below 30 MHz, and using a loop I don't think the ground plane should
make that much difference, but laying absorber on the ground plane also
won't make much of a difference: ferrite tile absorber is only rated down
to 20 or 30 MHz. At lower frequencies its performance drops off below the
nominal 20 dB it is rated from 20/30 - 1000 MHz.
From: Y W Leung <[email protected]>
Date: Wed, 29 Jun 2005 11:40:20 +0800 (CST)
To: EMC-pstc <[email protected]>
Subject: Radiated emssion measurement below 30MHz
Dear all,
In EN300330 radiated emission measurement, Annex A 1.1 mentioned
about for measurement (on an outdoor test site) of frequencies below
30MHz, no artificial ground plane shall be used. But for 30MHz or
above, a conducting ground plane shall be used. Is anyone know the
reason why the ground plane is not needed for frequency below 30MHz?
There are two other options test site for the test, one is the indoor
site of min. 6m x 7m with a barrier of absorbent material in front of
the wall, but this site is only for frequency over 80MHz.Why the
sized needed so big and the frequency is over 80MHz (not 30MHz)?
Another option is use anechoic chamber, but just for frequency over
30MHz.
I have an idea how about I use a fully anechoic chamber (absober
placed on the floor between the EUT and receiving antenna) instead of
the outdoor test site without ground plane, any difference between
them?
Thanks advance for any comment or idea.
Regards,
Derek.
????????
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