In message <54ffbe24c443bc45b8deb804650d2d2b0592a...@ct11exm64.ds.mot.com>, dated Tue, 3 Aug 2010, Elliott Mac-FME001 <fme...@motorola.com> writes:
>I may be able to fit one in my building but may not be able to >electrically isolate the chamber from the parent building structure >[will be touching the ceiling at points] Only metal-to-metal contact matters at all. Can you not introduce even a thin but robust layer of insulation? An intermittent contact may produce random electrical noise inside the box. > >If we put 1 very good ground connection the thought is that any surface >currents introduced to the shield will follow the path of least >impedance and go to ground there and not cause standing waves in the >chamber. In principle, there is no such thing as a 'very good ground connection' unless your chamber stands in a swamp! Any wired connection has enough inductance at radio-frequencies to represent a significant impedance. What do you expect the ground connection to do? An *isolated* metal box is not likely to have any current flowing on its inside surface due to external sources at frequencies where the skin depth is less than the thickness of the metal. Grounding the outside surface may *increase* currents in it rather than decrease them. Think of the box as one plate of a capacitor, the other being the Earth. It acts as an antenna, so it has a voltage to Earth over the radio-frequency band. This has no effect inside the box, which is a Faraday shield. But if you connect it to Earth, current flows in the outside surface and some leaks inside at low radio frequencies where the skin effect does not prevent it. -- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK If at first you don't succeed, delegate. But I support unbloated email http://www.asciiribbon.org/ - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ 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 <emcp...@socal.rr.com> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com>
