But the question was "decent" RE and RI measurements !
With or without cabling connected ?
I will present a discussion with cabling connected, as this
is most often the case and has not yet been discussed.
Decent, that is from a technical point of view, or from
a standards point of view. That is NOT the same.
Standards focus on reproducible measurements with
a taste of tradition ("we always did it like that").
Technically, the best way of evaluating fields and creating fields
is the real free field, without any reflection at all.
TEM cells do generally underexpose cabling, especially
at lower frequencies (due to size << lambda)
OATS and SAR's do underexpose low frequencies for horizontal
polarization and small EUT's, up to -21 dB worst case (and that
includes height scan!)
The only DECENT choice from technical EMC point of view
is the fully anechoic room (FAR) with 3 meter antenna EUT distance
or (if pre-compliance and using a small antenna) 1 meter
antenna EUT distance. (so minimum 3 meters longest size)
Beware with short distances to walls , reflections
tend to be stronger and the risk of nulls in frequency response
rises.
But FARS are far from popular in standards (but gaining attention).
>From a standardization point of view a decent chamber is
a 3 meter or 10 meter OATS or SAR, with the FAR as good
second.
So two choices remain for a really low cost solution:
- Small FAR (fully anechoic room) from technical point of view
- Small SAR (Semi Anechoic room) from a standard point of view
(G)TEM and other cells do a good job in reproducibility for
(very) small EUT (automotive especially) or in
radio/tv test set ups, and can create high field strengths
at low power cost.
For height scan purposes.
4 meter height scan, is required only for 10 meter SAR/OATS
2 meters (that is 1 to 2 meters) will do for 3 meters SAR/OATS
For the FAR no height scan is required.
For those interested I can provide a set of diagrams
>from R&S showing optimum response as function of antenna height
for 3 and 10 meter SAR.
FAR/SAR construction in a nutshell:
Use ferrite tiles 5.3 or 6.7 mm glued to steel
or composite ferrite/cone solution.
Ferrites can be tuned to broader frequency response
by separating them from reflecting wall using insulating
spacers (at the cost of efficiency of course)
Cones are required only above 1 GHz and only partially
with high directivity antenna's (horn antenna's)
Well, this is not a very balanced opinion, I know,
but it might create some new discussions ........... ?!!!
Gert Gremmen
Ce-test, qualified testing bv
Van: [email protected] [mailto:[email protected]] Namens Powell, Doug
Verzonden: woensdag 2 mei 2007 21:37
Aan: Nagel Michael-amn029; [email protected]
Onderwerp: RE: Smallest yet Practical EMC Chamber
Seems to me that the smallest practical EMC chamber is going to be a TEM
Cell...
Regards,
-doug
From: [email protected] [mailto:[email protected]] On Behalf Of Nagel
Michael-amn029
Sent: Wednesday, May 02, 2007 10:40 AM
To: [email protected]
Subject: RE: Smallest yet Practical EMC Chamber
Hello!
> What is the smallest chamber size that one can use for making decent RE
> (e.g., compliance levels of +/- 6dB in the low range and +/- 4dB in the
> upper range to the calculated NSA) and RI measurements in a 3-meter antenna
> distance and for a 70 cm3 test/EUT volume?
It is difficult to provide a straight answer.
You EUT looks relatively small (although the volume by itself can have strange
shapes),
so let's presume that you EUT is small and battery powered and you weren't
aiming for 20GHz, a GTEM cell
could do the job up to 1GHz if you stick with IEC 61000-4-20, if the largest
dimension of your EUT
is less than 6 inches, you can easily go up to 2GHz, more depends on the
directivity of your EUT.
But you want to go up to 20GHz...
> Would you have contact the information for a reasonable source for 10x10x6.7
mm Ferrite Tiles?
> The same for the absorber cones; I am aiming to use the chamber up to the 20
GHz.
If you want to have a large useful space and good performance, hybrid
absorbers seem to be the best solution.
Ferrite tiles will work good on the lower frequency ranges, but when it comes
to the multiple GHz range, pyramid
absorbers have better performance.
For the minimum size I cannot tell a number, but if you look into that
document from Frankonia:
http://www.frankonia-emv.com/data_sheets/shielded_rooms_anechoic_chambers.pdf
you might find something useful. Other suppliers like ETS-Lindgren may have
similar things.
Best regards,
Michael
Michael Nagel
Senior EMC Engineer
Motorola GmbH
ECC Embedded Communications Computing
Lilienthalstrasse 15
85579 Neubiberg/Muenchen - Germany
Vorsitzender des Aufsichtsrates: Fred Irwin
Geschäftsführung: Roland Duerr, Dr. Christoph Hollemann, Heinrich Korte,
Ralf Gerbershagen (Vorsitzender)
AG Wiesbaden, HRB 16024
VAT/USt.-ID: DE 113866163
Ph: +49-89-9608-0
Fax: +49-89-9608-2376
e-mail: [email protected]
info: http://www.motorola.com/computers
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