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: [email protected] >To: [email protected] >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: [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).
