I agree that wood and other high density materials are a problem as one approaches 1 GHz. I have data (taken in a previous life) that clearly shows the effect of a wooden table with a "Formica" top. This was done by placing an isotropic field sensor (ok, close to isotropic) at the center of a table and illuminating it from three meters away in a semi-anechoic chamber. Field strength was measured at each ten degrees of rotation at several frequencies up to 1 GHz. The effect at 1 GHz was in excess of 6 dB difference. The interesting part is that what appeared to be happening was low angle (below the "critical angle") dielectric reflection. The signal was strongest when aligned with the corners of the rectangular table.
Brent DeWitt Datex-Ohmeda Louisville, CO -----Original Message----- From: [email protected] [mailto:[email protected]]On Behalf Of Sundstrom Michael (NMP-RD/Dallas) Sent: Friday, November 02, 2001 7:09 AM To: 'Pommerenke, David'; 'POWELL, DOUG'; EMC-PSTC (E-mail) Subject: RE: EMC test table construction plans We use a styrofoam table here at our lab, it is a round plug of styrofoam 80cm tall and 1m across. This has worked the best for us, and it has the least reflections at any frequency we can reliably test at. I would guess that some form of hard material on top of this type of table would support 200lBs. Michael Sundstrom NOKIA TCC Dallas / EMC ofc: (972) 374-1462 cell: (817) 917-5021 amateur call: KB5UKT -----Original Message----- From: ext Pommerenke, David [mailto:[email protected]] Sent: Thursday, November 01, 2001 3:16 PM To: 'POWELL, DOUG'; EMC-PSTC (E-mail) Subject: RE: EMC test table construction plans Doug, For emissions and immunity you should not use any wood in the table. It will significantly (+/-2 dB up to 1 GHz for emissions , more above, +/-10 dB for immunity up to 1 GHz) change the test result. My experience has shown that Styrofoam is basicly the best material. There are a couple of published papers on this issue. As surface material the following worked out fine: - Foamed PVC (rather stiff, low dielectric constant due to the foamed nature), maybe 4 mm thick. - PE sheet, maybe 2 mm thick. David Pommerenke -----Original Message----- From: POWELL, DOUG [mailto:[email protected]] Sent: Thursday, November 01, 2001 11:38 AM To: EMC-PSTC (E-mail) Subject: EMC test table construction plans Hello all, I plan to construct my own insulated EMC test table for a 5 meter chamber. Seems simple enough to do and I could easily come up with something. I thought I might first ask for input from those of you in the discussion group who have experience or maybe even construction plans. Here are some features I want: 1) I will be testing products that weight up to 200 Lbs (91 kg). 2) I want to minimize metalic fastners. 3) I would like to make it a pivoting table (not motorized). 4) Height is 80 cm. 5) The surface should be replacable if it gets badly worn or scarred. I'm thinking of using hardboard. 6) Suggestions on length & width? -doug ----------- Douglas E. Powell, Compliance Engineer Advanced Energy Industries, Inc. Mail stop: 203024 1626 Sharp Point Drive Ft. 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