"Kunde, Brian" <[email protected]> wrote:
The IEC/EN 61000-4-3:2006 standard says in section 7: "NOTE 2: At higher frequencies (e.g., above 1 GHz), tables or supports made >from wood or glass reinforced plastic can be reflective. So, a low dielectric constant (low permittivity) material, such as rigid polystyrene, should be used to avoid field perturbations and to reduce degradation of field uniformity." 1. Note the words "can" and "should". They are not "will" and "shall". So I assume this is not an absolute if a practical alternative cannot be found. 2. From what I understand, "rigid polystyrene" is Styrofoam, as earlier mentioned. However, is the white plastic foam sheets, or Instapack, or other rigid foam materials still considered "polystyrene"? Does anyone know if these alternatives are any better than wood in regards to its permittivity? If the material is substantially foam, the relative dielectric will be close if not very close to air. (foamed metal does not count) 3. Is the permittivity of a materials affected by its thickness? Is a few inches of wood worst than a couple meters of Styrofoam? thickness, volume, density all count. A large block of styrofoam dissolved with acetone and then dried to sheet leaves a large stain. Various woods have various dielectrics and absorb various amounts of moisture depending on the environment. (Wood moisture is measured with an RF meter.) Styrofoam is not known to absorb much moisture, soak it and it will get a little damp. 4. The standard mentions "glass reinforced plastic" which I assume is fiberglass. What about PVC, Delran, or Formica? To handle the weight of our products our test tables are constructed from a combination of PVC legs and supports and a wood top with Formica. All these have substantial density and noticable dielectric well above air. 5. I saw a chart that compared the permittivity of wood, Styrofoam and several other types of material but for the life of me I can't locate it. Either it was in one of the trade magazines or someone posted it on this email group. If anyone has that chart or knows its location, can you send it to me? If I recall, I thought all the material listed in the chart was unacceptable except for Styrofoam. Did I read/remember this correctly? That chart or one like, lists the permittivity of the bulk materials. Convert styrofoam back to styrene (acetone trick above) and it too may have a noticable number. Modifiy most any of the materials to a a very thin layer, low density for the volume be measured and it too will have a low relative dielectric number. Foamed wood resembles cardboard boxes. But there is still the hydroscopic issue to deal with. 6. Is there a test procedure that can be used to qualify your test table? For example, run your unified field test with your test table in the field. If a unified field can be achieved, then your table is OK to use. Try this one, build a very large two plate air capacitor. One with just enough space to slide the table in between the two plates. Maybe the chamber floor can be one plate and a suspended metal plate at about 0.8 meters above the floor for the other. Now the real trouble starts - use this capacitor as part of a transmission line. Drive one corner with a tracking generator and receive on the diagonal corner. I'll leave the attachment method and frequency range up to the imagination. Peak hold on the air gap, then roll in the test object - a table. If it still looks like air, no change. Otherwise, can the difference seen be a problem? 7. If I wanted to test the permittivity of a material myself, how would I do this? See #6 for a crude relative estimate. Scaling the size of air capacitor can be useful for testing various materials. (ps. the construction might have resemblenc to a TEM chamber or Triplate test station, each which can be used to the same measurement. 8. Several people have commented about wood table absorbing moisture from the air. Is this the problem with wood? Can't wood be sealed to now absorb moisture? Can wood be used if it is dry? We test in a chamber and our entire lab has humidity controlled to 30-50%RH. In a controlled environment, can wood table be used? Wood has been used most of the time. It the different types of wood and absorbed moisture that has been blamed for the differences between labs. - Bill Thanks, The Other Brian _____ From: [email protected] [mailto:[email protected]] On Behalf Of Daniel Roman Sent: Thursday, January 24, 2008 9:05 AM To: [email protected] Subject: RE: Table for Measuring Emission for Desktop Equipment If you can't use wood for a table, why would cardboard be ok? I would imagine that it would also absorb moisture from the air and cause attenuation at high frequencies. Regards, -- Dan Roman, N.C.E. Regulatory Engineer Dialogic, Inc. 1515 State Rt. 10 Parsippany, NJ 07054-4538 *mailto:[email protected] <mailto:[email protected]> *Voice: +1 973-967-6485 Fax: +1 973-967-6262 Internet: http://www.dialogic.com <http://www.dialogic.com/> _____ From: [email protected] [mailto:[email protected]] On Behalf Of Price, Ed Sent: Thursday, January 24, 2008 8:53 AM To: [email protected] Cc: [email protected] Subject: RE: Table for Measuring Emission for Desktop Equipment I am fortunate to have our company's shipping department located immediately next to my lab. I often use empty cardboard boxes to build non-conducting platforms. It's surprising how much weight a taped box can support. There are several densities of urethane foam, but there is also a white foam made from polyethylene (or maybe polystyrene). This stuff is so strong that you can't hardly make a dent in it with your thumb. Finally, there is the foam which is used for foam-in-place packing. This stuff is poured into a box like a bubble-bath, and then cures to a hard foam. Fill an empty cardboard box with this foam, and you can get a very tough support structure. Ed Price <mailto:[email protected]> [email protected] WB6WSN NARTE Certified EMC Engineer & Technician Electromagnetic Compatibility Lab Cubic Defense Applications San Diego, CA USA 858-505-2780 (Voice) 858-505-1583 (FAX) Military & Avionics EMC Is Our Specialty _____ From: [email protected] [mailto:[email protected]] On Behalf Of Kunde, Brian Sent: Thursday, January 24, 2008 5:23 AM To: Grace Lin; Bill Stumpf; [email protected] Cc: [email protected] Subject: RE: Table for Measuring Emission for Desktop Equipment We have products that are benchtop models that weigh over 400 lbs. We have to sit the product on the test table with a boom lift and roll it into the chamber. How can we do this with styrofoam? The Other Brian _____ From: [email protected] [mailto:[email protected]] On Behalf Of Grace Lin Sent: Thursday, January 24, 2008 7:44 AM To: Bill Stumpf; [email protected] Cc: [email protected] Subject: Re: Table for Measuring Emission for Desktop Equipment Ari and Bill, Thank you so much for sharing your knowledge and experience, especially for the photo sent by Ari. Is there any other material good for high frequency measurement? Some of our products weight 75 lb. For styrofoam block, I probably need to build a solid 1.5mx1mx0.8m styrofoam block. This would be interesting. Best regards, Grace On 1/23/08, Bill Stumpf <[email protected]> wrote: Agreed! Commonly known as "Blue board" or "DOW board", this type of closed-cell insulation works best for high-frequency testing. You can usually find it at lumber yards. Be careful of glues as they often have a dielectric characteristic themselves. We end up replacing our top layer of foam often since it does get chewed up... Bill Stumpf _____ From: [email protected] [mailto: [email protected] <mailto:[email protected]> ] On Behalf Of [email protected] Sent: Wednesday, January 23, 2008 9:22 AM To: [email protected] Subject: RE: Table for Measuring Emission for Desktop Equipment We have a great table that is made of styrofoam. The material is about 10 cm thick, originally intended for thermal insulation of buildings. Look from local hardware store. The sheets can be glued together to get a stronger table. Only drawback is the soft surface. Have a thin polycarbonate, etc. sheet on it or just be careful. Good luck! Ari Honkala _____ From: [email protected] [mailto: [email protected] <mailto:[email protected]> ] On Behalf Of ext Grace Lin Sent: 23. tammikuuta 2008 14:39 To: [email protected] <mailto:[email protected]> Subject: Table for Measuring Emission for Desktop Equipment Dear Members, Could someone recommend the sources for a 1mx1.5mx0.8m table (or materials to make a table) that is good for high frequency measurement? According to the latest 61000-4-3, a wooden table is not good for taking measurement above 1GHz. A friend mentioned styrofoam block. He doesn't know where to find it. Thank you and look forward to hearing from you. Best regards, Grace - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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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 - Bill You can say what you want about the South, but you never hear of anyone retiring and moving North!!! _____ Be a better friend, newshound, and know-it-all with Yahoo! Mobile. <http://us.rd.yahoo.com/evt=51733/*http //mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ> Try it now. - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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