Purely an idle speculation, because I don't know what the 1995 source was, but if it had been something like a site source with a fast oscillator in it then the box top could have been an effective ground plane for the stub radiator, in which case the radiation pattern would have had a null towards the chamber floor, as well as the usual vertical null. This would explain a lot in terms of a lack of interference patterns from ground reflections as well as no direct rays above a certain elevation due to the stub toroidal radiation pattern.
> From: "Eric Lifsey" <[email protected]> > Reply-To: [email protected] > Date: Sat, 1 Oct 2005 13:28:58 -0600 (MDT) > To: [email protected] > Cc: [email protected] > Subject: Re: Above 1 GHz EMI testing with horn antenna > > Circa 1995 I saw plotted emission data taken at a semi-anechoic > chamber in Austin, Texas, that justified reducing the scan > height to a maximum of around 1.4 meters (IIRC) for emissions in > the GHz range. > > However, when I ran the first 1-2 GHz emissions measurements > where I worked I found maximums at 2.5 meter heights. (Using an > octave horn.) I suspect the other lab tested a physically small > source which would hide the effect of a real EUT of random > dimensions and apetures. > > I mounted the horn with a slight downward tilt to help insure > that the EUT emissions would be collected at the higher antenna > elevation and as frequency approached 2 GHz. The measurement > distance was 3 meters within a compact shielded room with cone > absorbers. This chamber was used primarily to pre-scan > emissions below 1 GHz at the time. > > IMO the best approach as suggested by others is to scan per the > written requirements. I'd add that I would have preferred to > collect emissions from greater than 3 meters to avoid the > concerns of the narrow field of illumination/sensitivity in the > octave horn. > > Best Regards, > Eric Lifsey > > [email protected] said: >> Hello, >> >> A discussion has come up about the need to raise the horn >> antenna up to 4 >> meters when doing an EMI test above 1 GHz. ANSI states that the >> antenna height >> should be adjusted between 1-4 meters to maximize the emissions. >> Some labs >> are only doing this test at 1 meter or maybe 2 meters high, >> (probably to >> reduce cable loss issues). Normally, the emissions are worst >> case below 2 >> meters, but how can one be absolutely sure, without checking? >> What is the legal >> requirement for the antenna height? These questions apply to >> unintentional >> radiators, where measurement above 1 GHz is required. >> >> Regards, >> Tim Pierce >> >> - >> 2005 IEEE Symposium on Product Safety Engineering >> 3-4 October Schaumburg, IL >> http://www.ieee-pses.org/symposium >> >> ---------------------------------------------------------------- >> 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/listserv/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: >> >> Richard Nute: [email protected] >> Jim Bacher: [email protected] >> >> All emc-pstc postings are archived and searchable on the web at: >> >> http://www.ieeecommunities.org/emc-pstc >> > > > --- > http://www.relia.net/~elifsey/ > > - > 2005 IEEE Symposium on Product Safety Engineering > 3-4 October Schaumburg, IL > http://www.ieee-pses.org/symposium > > ---------------------------------------------------------------- > 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/listserv/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 > - 2005 IEEE Symposium on Product Safety Engineering 3-4 October Schaumburg, IL http://www.ieee-pses.org/symposium 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/listserv/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
