[PSES] OATS vs FAR Radiated Emissions Limits
We've just received EN 61000-6-3:2007 + A1:2011 which has added the ability to test radiated emissions in either an OATS, Fully Anechoic Room (FAR) or TEM waveguide. What puzzled me is the limits for the FAR @ 3m: 30MHz ~ 230MHz: 42 to 35dBuV decreasing linearly with the log of the frequency 230MHz ~ 1GHz: 42dBuV Checking a draft version of CISPR 32 also shows these same limits for an FAR. These limits are lower than those for an OATS at 3m. Does anyone know why the limits are lower and of a different characteristic? I'm guessing something to do with the lack of reflective ground plane in the FAR. Does anyone have any further information? Many thanks James ~ James Pawson Leading Hardware Engineer - EMC EchoStar Europe - 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
Re: [PSES] OATS vs FAR Radiated Emissions Limits
James It is to account for the lack of ground plane with its potential +6dB effect due to reflection found during height scanning The FCC have already considered this in KDB558074 when allowing conducted spurious emissions measurements to be performed in lieu of radiated ones for some unlicensed transmitters, however, the FCC requirements are: 3. Add appropriate factor to model worst-case ground reflections For emissions 30 MHz, add a factor of 6.0 dB; For emissions 30 MHz and 1000 MHz, add a factor of 4.7 dB. Regards Charlie From: Pawson, James [mailto:james.paw...@echostar.com] Sent: 16 October 2012 15:27 To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: [PSES] OATS vs FAR Radiated Emissions Limits We've just received EN 61000-6-3:2007 + A1:2011 which has added the ability to test radiated emissions in either an OATS, Fully Anechoic Room (FAR) or TEM waveguide. What puzzled me is the limits for the FAR @ 3m: 30MHz ~ 230MHz: 42 to 35dBuV decreasing linearly with the log of the frequency 230MHz ~ 1GHz: 42dBuV Checking a draft version of CISPR 32 also shows these same limits for an FAR. These limits are lower than those for an OATS at 3m. Does anyone know why the limits are lower and of a different characteristic? I'm guessing something to do with the lack of reflective ground plane in the FAR. Does anyone have any further information? Many thanks James ~ James Pawson Leading Hardware Engineer - EMC EchoStar Europe - 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.orgmailto: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.netmailto:emcp...@radiusnorth.net Mike Cantwell mcantw...@ieee.orgmailto:mcantw...@ieee.org For policy questions, send mail to: Jim Bacher j.bac...@ieee.orgmailto:j.bac...@ieee.org David Heald dhe...@gmail.commailto: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
Re: [PSES] OATS vs FAR Radiated Emissions Limits
In message a60ff8a65589d24a98d82fafc6f0ac56062eb8c...@euroexcc1.sats.corp, dated Tue, 16 Oct 2012, Pawson, James james.paw...@echostar.com writes: These limits are lower than those for an OATS at 3m. Does anyone know why the limits are lower and of a different characteristic? I'm guessing something to do with the lack of reflective ground plane in the FAR. In principle, an attempt has been made to reconcile the limits applicable to the different measurement environments so that products with given emission levels produce at least similar test results (dB above or, preferably, below the limit) in each environment. The deliberations that resulted in these limits were long and very complex. -- 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
Re: [PSES] OATS vs FAR Radiated Emissions Limits
Certainly radio frequency emissions measurement is far from an exact science, but that's probably just fine for what it is intended to do. Pass at one lab, fail at another seems to be a common theme, so there must be many variables at play. 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. ___ Ralph McDiarmid | Schneider Electric | Solar Business | CANADA | Regulatory Compliance Engineering From: John Woodgate j...@jmwa.demon.co.uk To: EMC-PSTC@LISTSERV.IEEE.ORG Date: 10/16/2012 09:40 AM Subject: Re: [PSES] OATS vs FAR Radiated Emissions Limits In message a60ff8a65589d24a98d82fafc6f0ac56062eb8c...@euroexcc1.sats.corp, dated Tue, 16 Oct 2012, Pawson, James james.paw...@echostar.com writes: These limits are lower than those for an OATS at 3m. Does anyone know why the limits are lower and of a different characteristic? I'm guessing something to do with the lack of reflective ground plane in the FAR. In principle, an attempt has been made to reconcile the limits applicable to the different measurement environments so that products with given emission levels produce at least similar test results (dB above or, preferably, below the limit) in each environment. The deliberations that resulted in these limits were long and very complex. -- 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 email has been scanned by the Symantec Email Security.cloud service. __ - 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
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
Re: [PSES] OATS vs FAR Radiated Emissions Limits
Several years ago I was using a 3m FAR for pre-compliance measurements. They were using a giant BiLog antenna for the measurements. A constant factor was added to the measured values to arrive at the OATS value. The reason I stopped using their facility was because the data didn't correlate well with the 10m semi-anechoic chamber. Regards Ravinder Ajmani HGST, a Western Digital company ravinder.ajm...@hgst.com ralph.mcdiar...@schneider-electric.com Sent by: emc-p...@ieee.org 10/16/2012 10:25 AM To EMC-PSTC@LISTSERV.IEEE.ORG cc Subject Re: [PSES] OATS vs FAR Radiated Emissions Limits Certainly radio frequency emissions measurement is far from an exact science, but that's probably just fine for what it is intended to do. Pass at one lab, fail at another seems to be a common theme, so there must be many variables at play. 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. ___ Ralph McDiarmid | Schneider Electric | Solar Business | CANADA | Regulatory Compliance Engineering From: John Woodgate j...@jmwa.demon.co.uk To: EMC-PSTC@LISTSERV.IEEE.ORG Date: 10/16/2012 09:40 AM Subject: Re: [PSES] OATS vs FAR Radiated Emissions Limits In message a60ff8a65589d24a98d82fafc6f0ac56062eb8c...@euroexcc1.sats.corp, dated Tue, 16 Oct 2012, Pawson, James james.paw...@echostar.com writes: These limits are lower than those for an OATS at 3m. Does anyone know why the limits are lower and of a different characteristic? I'm guessing something to do with the lack of reflective ground plane in the FAR. In principle, an attempt has been made to reconcile the limits applicable to the different measurement environments so that products with given emission levels produce at least similar test results (dB above or, preferably, below the limit) in each environment. The deliberations that resulted in these limits were long and very complex. -- 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 email has been scanned by the Symantec Email Security.cloud service. __ - 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:
Re: [PSES] OATS vs FAR Radiated Emissions Limits
In message of1052898f.98b070cf-on88257a99.006668eb-88257a99.0066f...@hgst.com, dated Tue, 16 Oct 2012, ravinder.ajm...@hgst.com writes: Several years ago I was using a 3m FAR for pre-compliance measurements. They were using a giant BiLog antenna for the measurements. A constant factor was added to the measured values to arrive at the OATS value. The reason I stopped using their facility was because the data didn't correlate well with the 10m semi-anechoic chamber. This sort of thing will continue, but we may hope that it doesn't create an insuperable problem, because the correct technical procedure would be to establish an empirical correlation (possibly very complex) between SAR, FAR and OATS by means of a huge 'round-robin' test of hundreds of different product samples in each environment. Clearly, no-one will pay for this. The standards-makers have done their best within the realms of the possible. -- 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
Re: [PSES] OATS vs FAR Radiated Emissions Limits
Ravinder Unfortunate, but predictable (and I have come across smaller ones!) - very few small test labs can afford a properly-built and fitted out 10m chamber. As you realised, the dodgy area is the lower end of the frequency spectrum where you are more likely to be in the near field than in a bigger chamber at the same frequencies. As for the results, it has to be said that to a great extent you get what (you can afford to!) pay for - both in terms of facilities/equipment and of the skill and experience of the personnel performing the testing. I suppose the only way to really reconcile the use of results from a small chamber is when they show you are so far below the relevant emissions limits that even major uncertainties factors would keep you inside them! OTOH, from a compliance perspective, all that a lot of companies want (and actually believe that they need for their compliance files) is to be able to demonstrate that their products are (or appear to be) within the relevant limits when tested in a compliant chamber by a nationally accredited test house - anything more is icing on the cake as far as they are concerned. John Allen Compliance With Experience Ltd. W. London, UK From: emc-p...@ieee.org [mailto:emc-p...@ieee.org] On Behalf Of ravinder.ajm...@hgst.com Sent: 16 October 2012 19:44 To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] OATS vs FAR Radiated Emissions Limits Several years ago I was using a 3m FAR for pre-compliance measurements. They were using a giant BiLog antenna for the measurements. A constant factor was added to the measured values to arrive at the OATS value. The reason I stopped using their facility was because the data didn't correlate well with the 10m semi-anechoic chamber. Regards Ravinder Ajmani HGST, a Western Digital company ravinder.ajm...@hgst.com ralph.mcdiar...@schneider-electric.com Sent by: emc-p...@ieee.org 10/16/2012 10:25 AM To EMC-PSTC@LISTSERV.IEEE.ORG cc Subject Re: [PSES] OATS vs FAR Radiated Emissions Limits Certainly radio frequency emissions measurement is far from an exact science, but that's probably just fine for what it is intended to do. Pass at one lab, fail at another seems to be a common theme, so there must be many variables at play. 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. ___ Ralph McDiarmid | Schneider Electric | Solar Business | CANADA | Regulatory Compliance Engineering From: John Woodgate j...@jmwa.demon.co.uk To: EMC-PSTC@LISTSERV.IEEE.ORG Date: 10/16/2012 09:40 AM Subject: Re: [PSES] OATS vs FAR Radiated Emissions Limits _ In message a60ff8a65589d24a98d82fafc6f0ac56062eb8c...@euroexcc1.sats.corp, dated Tue, 16 Oct 2012, Pawson, James james.paw...@echostar.com writes: These limits are lower than those for an OATS at 3m. Does anyone know why the limits are lower and of a different characteristic? I'm guessing something to do with the lack of reflective ground plane in the FAR. In principle, an attempt has been made to reconcile the limits applicable to the different measurement environments so that products with given emission levels produce at least similar test results (dB above or, preferably, below the limit) in each environment. The deliberations that resulted in these limits were long and very complex. -- 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 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/ http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html 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
[PSES] Need RFID consultant
Need an RFID expert consultant who can do system design and programming (not for me, but for another party). Please send resume and references for completed jobs. I will forward to the other party. Richard Nute Bend, Oregon ri...@ieee.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.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
Re: [PSES] OATS vs FAR Radiated Emissions Limits
Around 1984 we were using a too-low 3 semi-anechoic chamber at Wang Labs to audit out outgoing shipments. Not far away, Glenn Dash was arguing that this was not nearly accurate enough. We transported a specimen equipment to Dash, Strauss and Goodhue in Boxboro, and had his people measure it, then back to our chamber and had them repeat the test. Then we used correction factors to adjust the worst case in the chamber to what was seen outside. We never got one back failing. However, that was just for audit. I DID find a few things that had been done wrongly by manufacturing, domestically and in China. Got one memorable call in the middle of *his* night from a VERY p*ssed off Chinese plant manager. Heh. Old times. Who audits now? Cortland Richmond (ka5s) On 10/16/2012 1503, John Woodgate wrote: In message of1052898f.98b070cf-on88257a99.006668eb-88257a99.0066f...@hgst.com, dated Tue, 16 Oct 2012, ravinder.ajm...@hgst.com writes: Several years ago I was using a 3m FAR for pre-compliance measurements. They were using a giant BiLog antenna for the measurements. A constant factor was added to the measured values to arrive at the OATS value. The reason I stopped using their facility was because the data didn't correlate well with the 10m semi-anechoic chamber. This sort of thing will continue, but we may hope that it doesn't create an insuperable problem, because the correct technical procedure would be to establish an empirical correlation (possibly very complex) between SAR, FAR and OATS by means of a huge 'round-robin' test of hundreds of different product samples in each environment. Clearly, no-one will pay for this. The standards-makers have done their best within the realms of the possible. - 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
Re: [PSES] OATS vs FAR Radiated Emissions Limits
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
