Precisely.
And Figure 2 hasn’t changed, in terms of the general layout of how
table-top equipment is laid out in a shield room, since well before
MIL-STD-462 (1967), going back to at least MIL-I-6181B, from 1953. Note that
MIL-I-6181B implies the existence of MIL-I-6181 and MIL-I-6181A, but I’m too
lazy to dig through them to ascertain if the set-up in those was similar or
not. And MIL-I-6181 was just one of several Service-unique specifications
covering EMI limits and test methods, and they were all quite similar. In
particular, all the testing of tabletop equipment was performed on ground
planes in shield rooms, and the ground plane was directly attached to the
shield room wall behind it.
As an aside, if you look at shielded enclosures of the pre-MIL-STD-461-era,
they tend to be remarkably similar in size: close to twenty feet wide, only
about eight feet deep and high. The reason for that was that dipole antennas
were used from roughly 30 MHz to 1 GHz, and a 30 MHz dipole is 5 meters long,
and there was a required clearance between the dipole tips and the walls. But
the dipole only needed to be one foot removed from the test sample, not one
meter, as per MIL-STD-461. There was no technical need for absorber, and no
requirement. And of course the dipole could only be deployed horizontally, so
eight feet sufficed for depth and height.
So from 1953 – 1967 everyone understood how to do this test, and all the
figures in MIL-STD-462 (1967) simply carry this practice forward: the ground
plane is backed up to the shield room wall behind it. This is figures 1, 2,
CE01-1, CE02-1, RE04-2, and the other tests just showed details of the set-up,
with the ground plane up against the shield room wall either implied or
explicitly stated.
Because there was no requirement for absorber in 1967 and earlier, absorber
would have been the exception, not the rule. So the major change with respect
to previous practice in MIL-STD-462D (1993) was the mandatory use of absorber,
and as Ed has pointed out, that is going to drive either a deeper ground
plane, or longer bond straps between the ground plane and wall or floor than
previously necessary, but it doesn’t change the fact that in the vast
majority of chambers, to make the room’s footprint as small as possible,
that means the ground plane backs up to the wall.
The point of all the above, and my previous posts on the same subject, is that
what is being asserted to be an error or inconsistency is something which has
been accepted practice for well over a half-century.
Ken Javor
Phone: (256) 650-5261
________________________________
From: Cortland Richmond <k...@earthlink.net>
List-Post: emc-pstc@listserv.ieee.org
Date: Tue, 30 Aug 2011 09:07:06 -0400
To: <emc-p...@ieee.org>
Subject: Re: RE102 Vertical Ground Plane
Hard to make a shielded room without walls.
Cortland Richmond
KA5S
(opinions mine,not my employer's)
On 8/30/2011 4:52 AM, rehel...@mmm.com wrote:
And Figures 2 and 3 of the Figures 1 through 5 clearly show a vertical
back
plane with the tabletop horizontal ground plane bonded to it (or at least it
implies a vertical back plane). It is either a vertical back plane or the
floor back plane bent up at a 90 degree angle. If there wasn't any confusion
since 1993 then there should have been. : )
The equipment I have is tabletop equipment to be mounted below the
water
line (i.e., below deck). The reference is to Figures 1 through 5 (all of
them), not just Figure 1.
Bob Heller
St. Paul, MN 55107-1208
Tel: 651-778-6336
Fax: 651-778-6252
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