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 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). > > --------- 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).
