I agree with John - it is necessary to know what purpose the question is asked, or the purpose to which the answer is to be put!
In EMC I mostly want to know that the emission wave front is close to 377 Ohms - as this is (supposed to be) the impedance at which the "antenna" calibration is made. My Don White gives the distance to this "impedance" boundary as lamda/(2.Pi) - this is where the wave impedance is within 10% or so of 377 Ohms. There is a constraint, in that the above equation is for an isotropic radiator - that is unity gain. If lambda is multiplied by the (numeric) gain of the antenna and then divided by 2Pi this gives a better approximation to the impedance boundary for antennas with gain. I hope that this helps. Tim From: [email protected] [mailto:[email protected]] On Behalf Of John Woodgate Sent: 02 August 2006 08:59 To: [email protected] Subject: Re: Near field/Far field what is D. *** WARNING *** This mail has originated outside your organization, either from an external partner or the Global Internet. Keep this in mind if you answer this message. 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 email and any attachments are confidential to the intended recipient and may also be privileged. If you are not the intended recipient please delete it from your system and notify the sender. You should not copy it or use it for any purpose nor disclose or distribute its contents to any other person. ******************************************************************** - 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 ______________________________________________________________________
