I haven't read many of the posts on this subject, so if this is redundant, I apologize.
Mr. Woodgate cited the need for the cable-under-test (CUT) to be a 150 Ohm transmission line. That is also my recollection of the requirement. But it is problematical. The equation for the characteristic impedance of a wire above a ground plane is (assuming air dielectric): Z = 138 log (4h/d), where, h = the height above ground, and d = the cable diameter. If you plot that impedance vs. cable diameter, at 10 cm height the diameter needs to be 32-33 mm. But the slope of the curve is very steep at 32 mm. If the cable is smaller than 26 cm or larger than 39 cm, you will be off by more than 10% in transmission line characteristic impedance. That corresponds to a return loss about 26 dB down, which is still very good, in my opinion. If instead, you hold the cable diameter steady at 32.5 mm, and change the height above ground by 10%, you get the same type of minimal mismatch. Given that cable diameter is not controlled, nor are there any instructions to adjust the cable height to account for cable diameter, the idea that 10 cm needs to be tightly controlled is at best a "tempest in a teapot." If anyone is really serious about this, the fix is to adjust cable height above ground such that the resultant ratio of cable diameter and cable height yields a constant 150 Ohms. Given that the EUT end of the cable is a completely uncontrolled impedance, there doesn't seem to be much need to tightly control the transmission line impedance. In my opinion. Ken Javor Phone: (256) 650-5261 > From: Ralph McDiarmid <[email protected]> > Date: Wed, 1 Apr 2009 11:08:24 -0700 > To: <[email protected]> > Conversation: [PSES] 10cm above GRP requirement for 61000-4-6 > Subject: RE: [PSES] 10cm above GRP requirement for 61000-4-6 > > The conventions regarding significant digits and rounding are widely > misunderstood in our general industry, despite adequate coverage in > undergraduate courses at colleges and universities. > > This misnomers 'round-up' and 'round-down' are just two that bug me a > little. > > > Ralph McDiarmid, AScT > Compliance Engineering Group > Xantrex Technology Inc > > > -----Original Message----- > From: [email protected] [mailto:[email protected]] On Behalf Of John > Woodgate > Sent: Tuesday, March 31, 2009 10:55 PM > To: [email protected] > Subject: Re: [PSES] 10cm above GRP requirement for 61000-4-6 > > In message <[email protected]>, > dated Wed, 1 Apr 2009, "ce-test, qualified testing bv - Gert Gremmen" > <[email protected]> writes: > >> At school I learned: >> >> " When no tolerance is given, +/- half of the number of significant >> digits is assumed. " >> >> When writing down 10 there is only one significant digit, so +/- 5 cm >> will do. >> >> Note : 10. (10dot) has 2 significant digits >> >> http://en.wikibooks.org/wiki/FHSST_Physics_Units:Scientific_Notation,_S > i >> gnificant_Figures_and_Rounding > > That notation isn't used by IEC/CISPR, where the standard came from. > > Don't just discuss it, take it up with the TAG for IEC SC77B and get it > changed. > -- > OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk > Things can always get better. But that's not the only option. > John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK > > - > ---------------------------------------------------------------- > 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 <[email protected]> > > 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 <[email protected]> > Mike Cantwell <[email protected]> > > For policy questions, send mail to: > Jim Bacher: <[email protected]> > David Heald: <[email protected]> > > - > ---------------------------------------------------------------- > 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 > <[email protected]> > > 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 <[email protected]> > Mike Cantwell <[email protected]> > > For policy questions, send mail to: > Jim Bacher: <[email protected]> > David Heald: <[email protected]> - 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 <[email protected]> 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 <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
