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: [email protected] >>> Meyerhoff Jerry-G10812 <[email protected]> 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: [email protected] [mailto:[email protected]] Sent: Friday, January 21, 2000 5:50 AM To: [email protected] 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: [email protected] 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 [email protected] --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators). --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators). --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators).
