Hello all, First off, thank much for taking the time to chime in. Secondly (and very importantly), I have misstated the maximum EUT dimensions; it should read as 70x70x70 cm. This would probably change the discussion a little bit around the GTEM cell usage. I have used GTEM cells quite a bit for automotive electronic device testing, and have good understanding of its many benefits and shortcomings. I believe it is possible to "develop" a good understanding of the correlation between GTEM measurements and a SAR once one has used a GTEM for some time... By decent I mean, being able to use this small/compact chamber to perform all of the pre-compliance RE and RI measurements during the product development cycle with good confidence in the match and correlation between the data obtained in this small chamber and that gathered at a certified site during final/certification tests. Perhaps, the correlation at the lower frequencies will not be as good, but we are ready to accept this. I would offer that these tests are performed to satisfy the requirements of the EN 55022 and EN55024, and FCC radio requirements. Can one use a 3hx4wx5L meter chamber with EUT and antenna on the diagonal of the 5 x4 meter dimension to achieve this goal? I have seen a lot of literature that state the minimum size is bound to 7.6x4x3 m, and am wondering if this limit has been thoroughly studied and if it can be pushed. Thanks again for your responses. Kind regards, Oscar
On 5/3/07, Nagel Michael-amn029 <[email protected]> wrote: 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 -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Nagel Michael-amn029 Sent: Wednesday, May 02, 2007 10:40 AM To: <mailto:[email protected]> [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. 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