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).
