Dear All,
Does anyone know how to measure the level of the water table around a
OFTS/OATS? I believe the answer is in the radiated emission measurements.
This thread made me to recall my experiences with snow in the North of
England. I have found out that ( if the snow does not start to accumulate
on the top of the enclosure,) the presence of snow on the ground is akin
to a shifting ground plane (variable 'ground water table' levels) which
affects the 'null' points. So we have to be careful to interpret results
if measurements are done at say some restrictive heights. On the whole I
have measured the deviation from the original radiated emission
measurements (when the ground was not soaking wet and without snow) and
compared the results of the radiated emission measurements at certain
frequencies (especially around the nulls ) in my short study and have found
the difference to be very substantial! The maximum difference between
sample means are about 23 dB around the frequencies where at one time was
the nulls (found during fair weather). Having accounted for the effect of
temperature changes on the EUT the conclusion was the shifting ground water
table, which was not readily measurable as snow pile up on the grass and it
became more complicated as they starts to melt the next day. The peak
radiated emissions outside the nulls are relatively stable.
I was using a smallish 'outhouse-wardrobe sized' enclosure to protect the
EUT. BTW, the OFTS (aka OATS) site is a certified site. :-) Of course
the measurements for the certification was done during fair weather!
Tim Foo
"Darren Pearson" <dar...@genesysibs.com>
12/14/01 06:01 PM Subject:Re: Enclosed OATS facilities
Hi
On the subject of OATS, I need to get one set up in the UK.
Can any one recommend a company that will install an compliant OATS in the
UK?
Regards Darren
"Ehler, Kyle" <keh...@lsil.com>
12/14/01 05:53 AM
I have the 'dome' (well, my employer does). Actually, it is an 8 meter
dia. radome, white fiberglass throughout. With door, HVAC and rotating
floor that serves as ground plane.
We pipe in fibre optics for PC host to EUT control from a receiver shelter
located 50 meters perpendicular to the elipse. It is part of our $5M
facility.
It looks like a giant golfball sitting on a large, grounded, concrete pad.
-not the sort of place to be in a raging electrical storm...
If we could simplify the RE emissions data collection, it would truly be a
golden opportunity. Worthy of the expense to upgrade. Would this serve as
a really large, spherical GTEM or more like a spherical magnetic field
antenna, with the EUT on the INSIDE?
Food for patents...
geor...@lexmark.com
12/13/01 05:39 AM
Chris,
I like your innovative thinking! When I once managed our acoustics lab, we
had both a semi-anechoic chamber and a reverberation chamber. The SAC was
similar to those for EMC, in that the walls were designed to absorb all
sound waves striking them, so that sound pressure at a point near the DUT
could be measured.
The reverberation chamber sides (6) were to reflect all sound waves
striking them so the total noise energy emitted from the DUT could be
measured.
This might suggest that your "igloo" EMC chamber not include the initial
absorber layer. With a good conductive inner liner, all EMC energy would
eventually be conducted away by this liner, and measured with a low
impedance meter between the liner and ground. It really does not matter
how many reflections take place, as all wave energy must eventually leave
via the conductive path.
The problem is how to fully electrically isolate this conductive
"collector" from additional outside EMI. Perhaps a very thick di-electric
(low C) between the collector and an outer conductive layer to absorb
in-coming EMI.
If you ever build one that works, just allow me to visit you at your new
house in the Bahamas, bought with your profit from the "Igloo Chamber".
George
Subject::Enclosed OATS facilities in snow country
"Jim Conrad" <jc...@shore.net>
12/13/01 09:25 PM
We plow the snow away from around the building. Unfortunately the building
is up on a small rise but it does make it easy to get the snow line below
the ground plane.
Jim
"Chris Maxwell" <chris.maxw...@nettest.com>
12/13/01 04:45 AM
Hmmmm,
This conductive layer of snow reminds me of a daydream/ thought experiment
that I had for measuring emissions...
What if you put a DUT inside a chamber that looked like a hemisphere. The
chamber would be hollow (otherwise, how would the DUT get in). The chamber
"skin" would be a sandwich with a thin layer of absorber on the inside and
a good conductor (conductor 1) then a dielectric then another good
conductor (conductor 2) on the outside.
Why these layers?
The inner layer would offer just enough attenuation to reduce reflections,
while letting some energy get to the conductor 1 behind it.
The conductor 1 layer would effectively be a "integrating measurement
antenna" which picks up and integrates all emissions from the DUT.
The dielectric layer would insulate conductor 1 from conductor 2. (maybe
this layer would need to be RF absorbant as well, not sure).
The conductor 2 layer would be grounded all the way around and would serve
to block ambients.
What would happen?
Would conductor 1 capacitively couple to the DUT such that a simple swept
RF voltage measurement between the DUT and conductor 1 would show the total
interference produced by the DUT?
Who's with me? Let's go to K-mart and get:
A large dome tent.
About 50 square yards of tin foil
Some Tokin flexible ferrite stuff **
A DUT.
An RF voltmeter/spectrum analyzer and a stub cable. **
**probably not available at K-mart...maybe Wal-Mart?
Might make a fun experiment, or maybe give the neighbors the idea that
you're building an escape pod to the mother ship.
Any immediate pitfalls that can be foreseen by the collective gurus?
Chris Maxwell | Design Engineer - Optical Division
email chris.maxw...@nettest.com | dir +1 315 266 5128 | fax +1 315 797
8024
NetTest | 6 Rhoads Drive, Utica, NY 13502 | USA
web www.nettest.com | tel +1 315 797 4449 |
Bill Owsley <ows...@cisco.com>
12/13/01 03:09 AM
The IBM Greenock lab had an OATS with a very steep roof line, sharp A
frame, and the heat to keep folks warm inside kept the lower edges somewhat
free of snow. I don't recall any concerns, because the snow was not on
top.
- Bill
geor...@lexmark.com
12/13/01 02:06 AM
Once upon a time, before we were spun off from IBM, and I was the EMC
manager here, I faintly recall that the IBM Boebligen lab in
Germany had an OATS facility. I also faintly recall that snow on the
rooftop did impact the measurements needed.
Note that OATS structures are normally constructed with non-conductive
materials, e.g. wood, plastic, etc. A layer of snow represents a plane of
conductive material, albeit not a great conductor.
However, these are memories from the distant past. Surely there are some
still using OATS facilities where winter snow is a problem.
George
plaw...@west.net (Patrick Lawler)
12/13/01 01:40 AM
I saw some photographs of an enclosed OATS facility in an area subject to
snow.
How does snow accumulation on the roof affect performance measurements?
Does it affect the NSA figures?
Is the effect significant enough that attempts are made to keep the roof
snow free? Or does the normal attempt at keeping the inside test area warm
enough for people take care of snow build-up?
plaw...@west.net
(Patrick Lawler) To: EMC-PSTC
<emc-p...@ieee.org>
Sent by: cc: (bcc: Wan Juang
Foo/ece/staff/npnet)
owner-emc-pstc@majordo Subject: Enclosed OATS
facilities in snow country
mo.ieee.org
12/13/01 01:40 AM
Please respond to
plawler
I saw some photographs of an enclosed OATS facility in an area subject to
snow.
How does snow accumulation on the roof affect performance measurements?
Does it affect the NSA figures?
Is the effect significant enough that attempts are made to keep the roof
snow free? Or does the normal attempt at keeping the inside test area warm
enough for people take care of snow build-up?
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