Correct me if I'm wrong, but I think there's a couple of issues being talked about at the same time.
1) Wood absorbs moisture, and can cause surface reflections at high (>1GHz) frequencies, affecting radiated emissions measurements. One solution is to use a very thick table (80cm?) of foamed polystyrene. 2) Polystyrene has a higher permittivity than air. You no longer have reflections off the top of the table, but you have higher capacitance through the 80cm of foam to ground, affecting your measurements in a different way.. A cardboard table would be comparable to a wood table: it still absorbs moisture, it could have reflections, and since it's mostly air, the permittivity is closer to 1. Pat Lawler EMC Engineer SL Power Electronics Corp. "Price, Edward" <[email protected]> wrote on 01/25/2008 03:22:17 PM: > I don't have any data on cardboard, but it seems to me that the > cardboard should be just about the same humidity as the air next to > it. The cardboard box material is relatively thin, and permeable to the air. > Is there something I don't understand about the cardboard; will it > somehow extract or concentrate moisture from the surrounding air? > > Ed Price > [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 Daniel Roman > Sent: Thursday, January 24, 2008 6:05 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] > (Voice: +1 973-967-6485 Fax: +1 973-967-6262 > Internet: 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. > > - ---------------------------------------------------------------- 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
