There are two reasons I am aware of. The lack of a 6 dB reflection (which is slightly less due to way length), requires a 5 dB lower limit (thus the 35 instead of 40 at 230 MHz).
Why then is this 35 not used at 30 MHz? The fact that a FAR/OATS has a problem in achieving an optimal summation of reflected and direct wave in horizontal polarization by just leveling the antenna (1-4m) annihilates the difference between SAR/OATS and FAR. You can make the calculations yourself with the help of Pythagoras and Excel under consideration that horizontally polarized waves invert in phase upon reflection and vertically polarized waves do not and that EUT is at 0.8 m high and measurement distance is 10 meters. Way direct varies from 10.00-10.49 and reflected from 10.16-11.09 The difference between reflected and direct wave are from 0.16 to 1m approx So only a lambda of 2 m (150 MHz) can be fully compensated for. For lower frequencies simply not enough difference in way length can be created by shifting the antenna up and never a 6 dB summation is achieved. At 30 MHz there is no summation but phase cancelling instead as 1 meter of delta test distance represent only 180/5 (=36) degrees of phase shift from 180 to 216 degrees. This is a serious flaw of a SAR/OATS from a technical point of view. The problem shows up only for point sources and point antenna's. In real life the way lengths may differ due to distribution of receive and transmit points but the principle remains valid (for horizontal polarization). For a SAR at 3 meters the height scan does not behave like that, but the variation of the effective test distance to EUT is too much for consistent measurements: when scanning up to 4 m the test distance for the direct wave changes from 3-4.38m and the reflected wavelength changes from 3.5 to almost 5.66 m. So it's more easy to pass the test on an OATS than in a FAR. Therefore the levels used in a FAR are gradually increased when the frequency lowers, to match those of the OATS (in error !!!!). Gert Gremmen Ce-test -----Oorspronkelijk bericht----- Van: emc-p...@ieee.org [mailto:emc-p...@ieee.org] Namens John Woodgate Verzonden: dinsdag 16 oktober 2012 20:07 Aan: EMC-PSTC@LISTSERV.IEEE.ORG Onderwerp: Re: [PSES] OATS vs FAR Radiated Emissions Limits In message <OFDEF1DD24.6EF50883-ON88257A99.005ECCE0-88257A99.005FC1FD@US.Schneider- E lectric.com>, dated Tue, 16 Oct 2012, ralph.mcdiar...@schneider-electric.com writes: >I've read that, depending on antenna size, 3 metre distance is within >what is called the "near field" and if so, then I assume spectrum of >radio emission wouldn't correlate well to what would be measured at 10m >or 30m. It's not so much antenna size, in principle, as antenna type and wavelength. The 'far field', where the ratio of electric to magnetic field strength is constant at 377 ohms, is established at and beyond about 3 wavelengths from the source antenna, if it's small compared with the wavelength, but it may not be. Nearer the source than 3 wavelengths, the ratio of field strengths is not constant and depends on whether the antenna is electric or magnetic, and how big it is. There is a 'transition region', between about one-sixth of a wavelength and 3 wavelengths; the ratio has a maximum or minimum at one-sixth wavelength. Closer than that is 'near field', and, for a small antenna, the ratio varies with distance from the source but is predictable by simple formulas. I have to say that 'doctors differ' over those numbers. In checking references, I find that at least one 'doctor' considers that the 377 ohms impedance is established at somewhat less than a wavelength from the (small) antenna. I suppose it depends on whether you settle for 337 +/-3.7 or 377 +/- 74. -- OOO - Own Opinions Only. See www.jmwa.demon.co.uk The longer it takes to make a point, the more obtuse it proves to be. 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 <emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. 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...@radiusnorth.net> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com> - ---------------------------------------------------------------- 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.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. 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...@radiusnorth.net> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com>
