There are reasons why I limited the use of a GTEM cell, being pointed out by Gert.
The conversion algorithm from GTEM to OATS models the EUT as an ideal dipole. As for the EUT size: Cables, if present, are part of the EUT and do count for the dimensions. This works for EUTs which are smaller than the wavelength of the highest frequency to be measured, if your EUT is larger you will (obviously) deal with directivity issues. To make it short: As long as you deal with a small EUT (battery powered), you get good results in a GTEM cell with the limitations which have been pointed out. If the EUT is well known and you want to see changes during modifications, it is possible to push the limits further (frequency, size) and to use cables and externally powered EUTs. The FAR is certainly a good instrument, but when it comes to higher frequencies, there are some points to consider: - No height scan in FAR means that you only capture the horizontal radiation pattern, which does not mean that other lobes with a high gain outside the horizontal plane do not exist. - When your EUT is large compared to the wavelength of the frequencies measured, you will have to deal with an irregular radiation pattern and the measurement time will certainly be high when you want to capture all lobes (not only the horizontal ones). - If you consider a height scan, you will either have to point the (horn, logarithmic) antenna constantly to the EUT or you deal with an antenna gain depending on elevation (as the EUT moves out of the main lobe of the antenna pattern) and you will certainly lose sensitivity at higher elevations. I agree that the FAR is close to ideal when you want to have an environment without reflections, but there are implications for the measurement time/completeness. It would be good to know more about the EUT of which we only know the approximate volume. I am just afraid that 'decent' does not correlate with a tight budget. Best regards, Michael Michael Nagel Senior EMC Engineer Motorola GmbH ECC Embedded Communications Computing Lilienthalstrasse 15 85579 Neubiberg/Muenchen - Germany Ph.: +49-89-9608-0 Fax: +49-89-9608-2376 e-mail: [email protected] info: http://www.motorola.com/computers 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 From: Gert Gremmen [mailto:[email protected]] Sent: Mittwoch, 2. Mai 2007 22:22 To: Powell, Doug; Nagel Michael-amn029; [email protected] Subject: RE: Smallest yet Practical EMC Chamber 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 - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc This message, including any attachments, may contain information that is confidential and proprietary information of Advanced Energy Industries, Inc. The dissemination, distribution, use or copying of this message or any of its attachments is strictly prohibited without the express written consent of Advanced Energy Industries, Inc. - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________
