How dare you shine the light of knowledge in on the dark fear of the unknown, just when we were beginning to have some fun.
"Taylor, Michael" <[email protected]> wrote: Hello All, just a brief comment on this wood table business. There are 2 issues at play here. #1, the absorption of water into the wood which changes the permittivity of the structure and the subsequent effects at microwave frequencies. #2, the minerals trapped in the interstitial boundaries surrounding the cellulose fibers which impact the Permittivity of the material, also impacting higher frequencies. Some time back I conducted extensive experiments on wood tables, I replaced my "specially" constructed wood table with Styrofoam blocks. I made measurements using a well characterized broad-band noise source that I have used for years in my chamber. The data from measurements performed from 30Mhz to 6 Ghz with both the table and the foam blocks will virtually overlay each other. The differences in the measurements were so minor as to be considered well within the limit of the Uncertainty of the system, less than 1.2db (p-p) for repeated measurements made one after another of the same point source radiator. Extreme care was exercised in assuring the placement & orientation of the radiator remained the same for both tests. The "Special" table was made of triple dried Ash - mortis & tendon / glue construction - and sealed with 2 coats of Spar Varnish. A review of the properties of wood as listed in the "Wood Products Handbook" shows a wide range of mineral content and dramatically different water absorption properties for different woods. The Hardwoods of "Slow-Growth" trees have higher fiber density and lower mineral content than the softer woods. They have dramatically different water absorption characteristics also. When the correct hardwood is fully dried and then sealed with a natural spar varnish the resultant wood structure is almost RF Transparent below 6 Ghz. This cannot be said of some of the plastics & fiberglass materials currently available, many of which have poor microwave properties. I'm sure this posting will elicit a rash of responses. No threats of neutering or dismemberment please - I suggest everyone conduct there own study and arrive at their own conclusions of which materials work best for them. As for me - I will continue to use a Hardwood table - properly constructed & sealed against moisture and avoid all the hassle of a Styrofoam. If I need something tested above 6 Ghz, I'll send it out to another lab. Just one guy's opinion Michael Taylor PE, NCE Chief EMC / RF Engineer Hach Company Loveland, Colorado This message (including any attachments) contains confidential and/or proprietary information intended only for the addressee. Any unauthorized disclosure, copying, distribution or reliance on the contents of this information is strictly prohibited and may constitute a violation of law. If you are not the intended recipient, please notify the sender immediately by responding to this e-mail, and delete the message from your system. If you have any questions about this e-mail please notify the sender immediately. - ---------------------------------------------------------------- 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 - Bill You can say what you want about the South, but you never hear of anyone retiring and moving North!!! _____ Never miss a thing. <http://us.rd.yaho .com/evt=51438/*http://www.yahoo.com/r/hs> Make Yahoo your homepage. - ---------------------------------------------------------------- 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
