This is a reply to the original post, not to Mr. Woodgate's reply. I missed the original post. The derivation of 2D^2/lambda is straightforward. The far field occurs when a plane wave is emitted. That in turn implies a point-source radiator. Imagine a dipole antenna. The far field is achieved when the point of observation in the far field is the same distance from all points on the dipole. Clearly that occurs at infinity. If we agree that a phase front difference of lambda/16 between the longest and shortest ray is a sufficiently small difference to qualify as "far field", then the distance to that point of observation will be 2 D^2/lambda, where D is the length of the dipole. This involves nothing more complex than the Pythagorean theorem.
> From: John Woodgate <[email protected]> > Date: Wed, 2 Aug 2006 08:59:12 +0100 > To: [email protected] > Subject: Re: Near field/Far field what is D. > > In message <001c01c6b604$c0ed2810$6401a8c0@homepc>, dated Wed, 2 Aug > 2006, Iain Summers <[email protected]> writes >> There are various references and opinions as to what constitutes the >> near field far field boundary. The common one from antenna theory is >> 2D^2/lambda where D is defined as the largest aperture of the antenna. > > There isn't a sharp boundary. What you take as the boundary depends on > why you want to know where it is, and you don't tell us that. > > For example, you might decide that you want to know where the phase > angle between E and H is only 1 degree. Or is 45 degrees. Or more. >> >> Has anyone researched what the limitations of this formula are? I am >> interested in the far field boundary between 1-30MHz for large loops of >> wire. > > Large compared with the wavelength at 1 MHz? That's large indeed! > >> In terms of a loop if this formula is applicable is D the circumference >> or diameter of the loop. > > Or maybe the area? > -- > OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk > 2006 is YMMVI- Your mileage may vary immensely. > > John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK > > - > ---------------------------------------------------------------- > This message is from the IEEE Product Safety Engineering Society > emc-pstc discussion list. Website: http://www.ieee-pses.org/ > > To post a message to the list, send your e-mail to [email protected] > > Instructions: http://listserv.ieee.org/request/user-guide.html > > List rules: http://www.ieee-pses.org/listrules.html > > For help, send mail to the list administrators: > > Scott Douglas [email protected] > Mike Cantwell [email protected] > > For policy questions, send mail to: > > Jim Bacher: [email protected] > David Heald: [email protected] > > All emc-pstc postings are archived and searchable on the web at: > > http://www.ieeecommunities.org/emc-pstc > - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________
