RE(2): Question on 15.31, Near-field
Another aspect to consider is the measurement of magnetic fields using a loop antenna at frequencies much lower than 30 MHz. These measurements are not performed on a typical OATS and a 1-4 meter scan height is not required. Thus we do not need to dig a hole for the rest of the antenna (though sometimes we would like to). Don -- From: Meyerhoff Jerry-G10812[SMTP:g10...@email.mot.com] Reply To: Meyerhoff Jerry-G10812 Sent: Friday, January 21, 2000 11:01 AM To: 'rehel...@mmm.com'; emc-p...@majordomo.ieee.org Subject: RE: Question on 15.31, Near-field Hello group. I substantially agree with the comments of Allen Tudor Don Umbdenstock. My own view is : I believe the 40 dB / decade (of distance) is a questionable approximation for extrapolating the measurement antenna location being reduced from 10 meter to 3 or possibly 1 meter in an effort to see the exceedingly weak emissions from well designed part 15 devices. However, below 30 MHz we're in the near-field of the source. 30 MHz is 10 meter wavelength and a 3 (or 1 meter) meter antenna site spacing violates the accepted one sixth lambda near field to farfield transition zone. Some would argue that 1 wavelength is a safer estimate of the transition zone. Then considering antennas like log periodics below 30 MHz which are huge, even that may not be large enough !! Or a 30 MHz (half-wave) dipole is 5 meters long . (as an aside.. How do you search 1 to 4 meters elevation in vertical polarization ?? (dig a hole for the lower dipole arm to fall into ;) NOT ) My personal rule is that the DUT to antenna spacing must be significantly greater than the antenna structure's max overall dimension to believe the outcome. Additionally , the 40dB approximation may have been derived from an isotropic radiator in free space. I have a serious problem believing practical antennas like dipoles, which have been calibrated defined in the far field , can give proper results when in a near field of any device being tested which may have near fields dominated by either magnetic OR electric radiator effects OR a strange and complex combination of both. I believe the historical basis of the part 15 method was defining VHF/UHF TV's local oscillator unintended emissions (conducted to) its receiving antenna. We've extrapolated a long way from that simple 100 foot far-field antenna to antenna coupling model. These comments are derived from (too) many hours of open field site experience in measuring pt 15 on CB and 2 way VHF radios in the '70's and special CB to TV interference studies with EIA TR-32 committee ~1976 when CB transitioned from 23 to 40 channels receiver emissions limits were reduced from 32 dBuV/m to ~ 10.. Hey this is way too much fun at work. I have to get back to creating value for my customers. Best regards, Jerry Meyerhoff Principal Staff Engineer Motorola AIEG [Automotive Industrial Electronics Group] of IESS [ Integrated Electronic Systems Sector ] 4000 Commercial Ave Northbrook Il. 60062 DISCLAIMER: This report represents the best opinions and analysis of the author based on the information available which may be subject to change without prior notice. The confidentiality of this info is everyone's responsibility. Any opinions implied or expressed in this work are solely those of the author . The suitability of this information for making decisions rests solely with the reader. JDM :-) 1/21/2000 -Original Message- From: rehel...@mmm.com [mailto:rehel...@mmm.com] Sent: Friday, January 21, 2000 5:50 AM To: emc-p...@majordomo.ieee.org Subject: Question on 15.31 I have not heard back fron the FCC on the following question. Does anyone have any insight into the answer? Thanks for your time, Bob Heller = -- Forwarded by Robert E. Heller/US-Corporate/3M/US on 01/21/2000 05:46 AM --- Robert E. Heller 01/19/2000 09:52 AM 3M Product Safety, St. Paul, MN 55107 76-1-01 EMC Laboratory Fax: 651-778-6252 To: rlafo...@fcc.gov cc: Roger D. Kuhn/US-Corporate/3M/US Subject: Question on 15.31 Mr. LaForge, I have a question concerning paragraphs 15.31(f)(1) and 15.31(f)(2). Paragraph 15.31(f)(1) concerns measurements above 30 MHz and at the bottom of the paragraph has a distance extrapolation factor of 20dB/decade. Paragraph 15.31(f)(2) concerns measurements below 30 MHz and at the bottom of the paragraph has a distance extrapolation factor of 40dB/decade. Why are there different extrapolation factors above and below 30 MHz? Thank
Re: RE: Question on 15.31, Near-field
The distance of one sixth lambda comes from empirical data taken from a Hertzian dipole antenna. In his book Introduction to Electromagnetic Compatibility ISBN 0-471-54927-4, Clayton Paul writes: The point where the 1/r3 (cubed) and 1/r2 (squared) terms become insignificant compared with the 1/r terms is referred to the boundary between the near field and far field. This occurs where r = lambda/(2*pi). The reader is cautioned that the boundary between the near and far fields for other antennas is not simply lambda/(2*pi), as is frequently assumed. A more realistic choice for the boundary between the near and far fields will be discussed later, but can be summarized as being the larger of 3*lambda or 2*D*D/lambda, where D is the largest dimension of the antenna. Simply stated, there exists some grey area between near and far field. It depends on how closely your product resembles a Hertzian dipole. Allen Tudor, Compliance Engineer PairGain Technologies tel: (919)875-3382 6531 Meridien Drive fax: (919)876-1817 Raleigh, NC 27616 email: allen_tu...@pairgain.com Meyerhoff Jerry-G10812 g10...@email.mot.com 01/21 11:01 AM Hello group. I substantially agree with the comments of Allen Tudor Don Umbdenstock. My own view is : I believe the 40 dB / decade (of distance) is a questionable approximation for extrapolating the measurement antenna location being reduced from 10 meter to 3 or possibly 1 meter in an effort to see the exceedingly weak emissions from well designed part 15 devices. However, below 30 MHz we're in the near-field of the source. 30 MHz is 10 meter wavelength and a 3 (or 1 meter) meter antenna site spacing violates the accepted one sixth lambda near field to farfield transition zone. Some would argue that 1 wavelength is a safer estimate of the transition zone. Then considering antennas like log periodics below 30 MHz which are huge, even that may not be large enough !! Or a 30 MHz (half-wave) dipole is 5 meters long . (as an aside.. How do you search 1 to 4 meters elevation in vertical polarization ?? (dig a hole for the lower dipole arm to fall into ;) NOT ) My personal rule is that the DUT to antenna spacing must be significantly greater than the antenna structure's max overall dimension to believe the outcome. Additionally , the 40dB approximation may have been derived from an isotropic radiator in free space. I have a serious problem believing practical antennas like dipoles, which have been calibrated defined in the far field , can give proper results when in a near field of any device being tested which may have near fields dominated by either magnetic OR electric radiator effects OR a strange and complex combination of both. I believe the historical basis of the part 15 method was defining VHF/UHF TV's local oscillator unintended emissions (conducted to) its receiving antenna. We've extrapolated a long way from that simple 100 foot far-field antenna to antenna coupling model. These comments are derived from (too) many hours of open field site experience in measuring pt 15 on CB and 2 way VHF radios in the '70's and special CB to TV interference studies with EIA TR-32 committee ~1976 when CB transitioned from 23 to 40 channels receiver emissions limits were reduced from 32 dBuV/m to ~ 10.. Hey this is way too much fun at work. I have to get back to creating value for my customers. Best regards, Jerry Meyerhoff Principal Staff Engineer Motorola AIEG [Automotive Industrial Electronics Group] of IESS [ Integrated Electronic Systems Sector ] 4000 Commercial Ave Northbrook Il. 60062 DISCLAIMER: This report represents the best opinions and analysis of the author based on the information available which may be subject to change without prior notice. The confidentiality of this info is everyone's responsibility. Any opinions implied or expressed in this work are solely those of the author . The suitability of this information for making decisions rests solely with the reader. JDM :-) 1/21/2000 -Original Message- From: rehel...@mmm.com [mailto:rehel...@mmm.com] Sent: Friday, January 21, 2000 5:50 AM To: emc-p...@majordomo.ieee.org Subject: Question on 15.31 I have not heard back fron the FCC on the following question. Does anyone have any insight into the answer? Thanks for your time, Bob Heller = -- Forwarded by Robert E. Heller/US-Corporate/3M/US on 01/21/2000 05:46 AM --- Robert E. Heller 01/19/2000 09:52 AM 3M Product Safety, St. Paul, MN 55107 76-1-01 EMC Laboratory Fax: 651-778-6252 To: rlafo...@fcc.gov cc: Roger D
Re: Question on 15.31
Here is the actual text: (2) At frequencies below 30 MHz, measurements may be performed at a distance closer than that specified in the regulations; however, an attempt should be made to avoid making measurements in the near field. Pending the development of an appropriate measurement procedure for measurements performed below 30 MHz, when performing measurements at a closer distance than specified, the results shall be extrapolated to the specified distance by either making measurements at a minimum of two distances on at least one radial to determine the proper extrapolation factor or by using the square of an inverse linear distance extrapolation factor (40 dB/decade). with the equipment if that length is known. I am going to hazard a guess. Equations for the fields from an infinitesimally small dipole show 1/r E-field variation with r in the far field, but 1/r2 (read one over r squared) in the near field. I am now going to express my opinion that this is an inaccurate assessment. The EUT is not a point source, which the classical equation derivation assumes. The classical derivation also yields the field at a point, and the measurement antenna integrates the field at many points. In fact, I would expect a more gradual roll-off in the near field than in the far field (unless you were close enough to the EUT to be in the quasi-static region). -- From: rehel...@mmm.com To: emc-p...@majordomo.ieee.org Subject: Question on 15.31 Date: Fri, Jan 21, 2000, 3:49 AM I have not heard back fron the FCC on the following question. Does anyone have any insight into the answer? Thanks for your time, Bob Heller = -- Forwarded by Robert E. Heller/US-Corporate/3M/US on 01/21/2000 05:46 AM --- Robert E. Heller 01/19/2000 09:52 AM 3M Product Safety, St. Paul, MN 55107 76-1-01 EMC Laboratory Fax: 651-778-6252 To: rlafo...@fcc.gov cc: Roger D. Kuhn/US-Corporate/3M/US Subject: Question on 15.31 Mr. LaForge, I have a question concerning paragraphs 15.31(f)(1) and 15.31(f)(2). Paragraph 15.31(f)(1) concerns measurements above 30 MHz and at the bottom of the paragraph has a distance extrapolation factor of 20dB/decade. Paragraph 15.31(f)(2) concerns measurements below 30 MHz and at the bottom of the paragraph has a distance extrapolation factor of 40dB/decade. Why are there different extrapolation factors above and below 30 MHz? Thank you, Bob Heller 3M Company 651-778-6336 rehel...@mmm.com - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators). - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
RE: Question on 15.31, Near-field
Hello group. I substantially agree with the comments of Allen Tudor Don Umbdenstock. My own view is : I believe the 40 dB / decade (of distance) is a questionable approximation for extrapolating the measurement antenna location being reduced from 10 meter to 3 or possibly 1 meter in an effort to see the exceedingly weak emissions from well designed part 15 devices. However, below 30 MHz we're in the near-field of the source. 30 MHz is 10 meter wavelength and a 3 (or 1 meter) meter antenna site spacing violates the accepted one sixth lambda near field to farfield transition zone. Some would argue that 1 wavelength is a safer estimate of the transition zone. Then considering antennas like log periodics below 30 MHz which are huge, even that may not be large enough !! Or a 30 MHz (half-wave) dipole is 5 meters long . (as an aside.. How do you search 1 to 4 meters elevation in vertical polarization ?? (dig a hole for the lower dipole arm to fall into ;) NOT ) My personal rule is that the DUT to antenna spacing must be significantly greater than the antenna structure's max overall dimension to believe the outcome. Additionally , the 40dB approximation may have been derived from an isotropic radiator in free space. I have a serious problem believing practical antennas like dipoles, which have been calibrated defined in the far field , can give proper results when in a near field of any device being tested which may have near fields dominated by either magnetic OR electric radiator effects OR a strange and complex combination of both. I believe the historical basis of the part 15 method was defining VHF/UHF TV's local oscillator unintended emissions (conducted to) its receiving antenna. We've extrapolated a long way from that simple 100 foot far-field antenna to antenna coupling model. These comments are derived from (too) many hours of open field site experience in measuring pt 15 on CB and 2 way VHF radios in the '70's and special CB to TV interference studies with EIA TR-32 committee ~1976 when CB transitioned from 23 to 40 channels receiver emissions limits were reduced from 32 dBuV/m to ~ 10.. Hey this is way too much fun at work. I have to get back to creating value for my customers. Best regards, Jerry Meyerhoff Principal Staff Engineer Motorola AIEG [Automotive Industrial Electronics Group] of IESS [ Integrated Electronic Systems Sector ] 4000 Commercial Ave Northbrook Il. 60062 DISCLAIMER: This report represents the best opinions and analysis of the author based on the information available which may be subject to change without prior notice. The confidentiality of this info is everyone's responsibility. Any opinions implied or expressed in this work are solely those of the author . The suitability of this information for making decisions rests solely with the reader. JDM :-) 1/21/2000 -Original Message- From: rehel...@mmm.com [mailto:rehel...@mmm.com] Sent: Friday, January 21, 2000 5:50 AM To: emc-p...@majordomo.ieee.org Subject: Question on 15.31 I have not heard back fron the FCC on the following question. Does anyone have any insight into the answer? Thanks for your time, Bob Heller = -- Forwarded by Robert E. Heller/US-Corporate/3M/US on 01/21/2000 05:46 AM --- Robert E. Heller 01/19/2000 09:52 AM 3M Product Safety, St. Paul, MN 55107 76-1-01 EMC Laboratory Fax: 651-778-6252 To: rlafo...@fcc.gov cc: Roger D. Kuhn/US-Corporate/3M/US Subject: Question on 15.31 Mr. LaForge, I have a question concerning paragraphs 15.31(f)(1) and 15.31(f)(2). Paragraph 15.31(f)(1) concerns measurements above 30 MHz and at the bottom of the paragraph has a distance extrapolation factor of 20dB/decade. Paragraph 15.31(f)(2) concerns measurements below 30 MHz and at the bottom of the paragraph has a distance extrapolation factor of 40dB/decade. Why are there different extrapolation factors above and below 30 MHz? Thank you, Bob Heller 3M Company 651-778-6336 rehel...@mmm.com - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators). - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list
RE: Question on 15.31
The difference is the transition region. Simplified expression of transition region is lambda/2pi. At some number of wavelengths above this point the propagation roll-off is linear, i.e., 20 dB/decade. Well below 30 MHz, near field phenomena are encountered. Also, antenna configurations may influence measurements. Near field effects can make the roll off be as high as 60 dB/decade, and with a phase canceling antenna even higher -- but you have to prove it. So the FCC gives you 40 dB/decade for free. If you want more, you need to present the measurements and calculations substantiating higher roll-offs. This is probably more than you asked for; hope it helped. Don Umbdenstock . -- From: rehel...@mmm.com[SMTP:rehel...@mmm.com] Reply To: rehel...@mmm.com Sent: Friday, January 21, 2000 6:51 AM To: emc-p...@majordomo.ieee.org Subject: Question on 15.31 Tha paragraphs are from FCC Part 15. === -- Forwarded by Robert E. Heller/US-Corporate/3M/US on 01/21/2000 05:49 AM --- Robert E. Heller 01/21/2000 05:49 AM 3M Product Safety, St. Paul, MN 55107 76-1-01 EMC Laboratory Fax: 651-778-6252 To: emc-p...@majordomo.ieee.org cc: Subject: Question on 15.31 I have not heard back fron the FCC on the following question. Does anyone have any insight into the answer? Thanks for your time, Bob Heller = -- Forwarded by Robert E. Heller/US-Corporate/3M/US on 01/21/2000 05:46 AM --- Robert E. Heller 01/19/2000 09:52 AM 3M Product Safety, St. Paul, MN 55107 76-1-01 EMC Laboratory Fax: 651-778-6252 To: rlafo...@fcc.gov cc: Roger D. Kuhn/US-Corporate/3M/US Subject: Question on 15.31 Mr. LaForge, I have a question concerning paragraphs 15.31(f)(1) and 15.31(f)(2). Paragraph 15.31(f)(1) concerns measurements above 30 MHz and at the bottom of the paragraph has a distance extrapolation factor of 20dB/decade. Paragraph 15.31(f)(2) concerns measurements below 30 MHz and at the bottom of the paragraph has a distance extrapolation factor of 40dB/decade. Why are there different extrapolation factors above and below 30 MHz? Thank you, Bob Heller 3M Company 651-778-6336 rehel...@mmm.com - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators). - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
Re: Question on 15.31
Sounds to me as if they're being doubly cautious here. First they warn you not to make measurements in the near field. Then they use the 40dB per decade extrapolation factor to account for measurements taken in the near field (or perhaps the cross-over point between the near and far fields). If you look in a fields book, the equations for the electric field strength indicate that in the extreme far field, the electric field decays at a rate of 1/r, where r is the distance. This is where the 20dB per decade comes from by the relation of 20log(r1/r2). Since a decade is defined as the ratio of 10:1, 20log(10) is 20dB per decade. Getting back to the equation for electric field strength, the electric field decays at the rate of 1/(r*r) in the midrange region (somewhere between near and far field). This is where 40 db per decade comes from. The squared term in the logarithm expression comes out front as 2*20(r1/r2). Furthermore, in the extreme near field, the electric fi! eld decays at a rate of 1/(r*r*r). This equates to 60 dB per decade. In this case, the cubed term comes out front as 3*20log(r1/r2). If you look at Figure 3-1 and 3-2 in GR-1089, they switch from 20dB per decade to 60dB per decade at 15.9 MHZ, which is the crossover point from near to far field at 3 meters if you assume the crossover point to be lambda/pi. rehel...@mmm.com 01/21 6:51 AM Tha paragraphs are from FCC Part 15. === -- Forwarded by Robert E. Heller/US-Corporate/3M/US on 01/21/2000 05:49 AM --- Robert E. Heller 01/21/2000 05:49 AM 3M Product Safety, St. Paul, MN 55107 76-1-01 EMC Laboratory Fax: 651-778-6252 To: emc-p...@majordomo.ieee.org cc: Subject: Question on 15.31 I have not heard back fron the FCC on the following question. Does anyone have any insight into the answer? Thanks for your time, Bob Heller = -- Forwarded by Robert E. Heller/US-Corporate/3M/US on 01/21/2000 05:46 AM --- Robert E. Heller 01/19/2000 09:52 AM 3M Product Safety, St. Paul, MN 55107 76-1-01 EMC Laboratory Fax: 651-778-6252 To: rlafo...@fcc.gov cc: Roger D. Kuhn/US-Corporate/3M/US Subject: Question on 15.31 Mr. LaForge, I have a question concerning paragraphs 15.31(f)(1) and 15.31(f)(2). Paragraph 15.31(f)(1) concerns measurements above 30 MHz and at the bottom of the paragraph has a distance extrapolation factor of 20dB/decade. Paragraph 15.31(f)(2) concerns measurements below 30 MHz and at the bottom of the paragraph has a distance extrapolation factor of 40dB/decade. Why are there different extrapolation factors above and below 30 MHz? Thank you, Bob Heller 3M Company 651-778-6336 rehel...@mmm.com - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators). Allen Tudor, Compliance Engineer PairGain Technologies tel: (919)875-3382 6531 Meridien Drive fax: (919)876-1817 Raleigh, NC 27616 email: allen_tu...@pairgain.com - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
Question on 15.31
Tha paragraphs are from FCC Part 15. === -- Forwarded by Robert E. Heller/US-Corporate/3M/US on 01/21/2000 05:49 AM --- Robert E. Heller 01/21/2000 05:49 AM 3M Product Safety, St. Paul, MN 55107 76-1-01 EMC Laboratory Fax: 651-778-6252 To: emc-p...@majordomo.ieee.org cc: Subject: Question on 15.31 I have not heard back fron the FCC on the following question. Does anyone have any insight into the answer? Thanks for your time, Bob Heller = -- Forwarded by Robert E. Heller/US-Corporate/3M/US on 01/21/2000 05:46 AM --- Robert E. Heller 01/19/2000 09:52 AM 3M Product Safety, St. Paul, MN 55107 76-1-01 EMC Laboratory Fax: 651-778-6252 To: rlafo...@fcc.gov cc: Roger D. Kuhn/US-Corporate/3M/US Subject: Question on 15.31 Mr. LaForge, I have a question concerning paragraphs 15.31(f)(1) and 15.31(f)(2). Paragraph 15.31(f)(1) concerns measurements above 30 MHz and at the bottom of the paragraph has a distance extrapolation factor of 20dB/decade. Paragraph 15.31(f)(2) concerns measurements below 30 MHz and at the bottom of the paragraph has a distance extrapolation factor of 40dB/decade. Why are there different extrapolation factors above and below 30 MHz? Thank you, Bob Heller 3M Company 651-778-6336 rehel...@mmm.com - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
