Sorry I am getting in on this late, but I have to comment: Ken Javor <[email protected]> wrote on 08/03/2006 08:09:06 AM:
> As I said in an earlier posting, the far field is equivalent to a plane > wave, which implies a point source radiator. This is not so. Take the case (for example) of a 3/2 wavelength dipole. This antenna is certainly not a point source, but at say 1000 wavelengths away, one is certainly in far field. However, it can be stated that at a distance of 1000 wavelength, the antenna does appear a lot like a point source. > Practically that means that if > the point of observation or measurement is to be in the far field of the > source, that means that the rays traced from the point of observation to any > portion of the source radiating element must be electrically equidistant, > that is, have negligible phase front differences. If one chooses lambda/116 > as a criterion for "negligible" this results in the familiar 2D^2/lambda > criterion. Note that a 3/2 wavelength dipole has a null in its pattern (in far field) at an angle of about 20 degrees off of broadside because at this angle there is a cancellation of the waves from each half of the dipole, due to phase shift caused by the distance differential from the point of observation to the two halves of the dipole. The rays traced back are NOT electrically equidistant. The distance differential exists all the way to infinity. Donald Borowski Schweitzer Engineering Labs Pullman, WA, USA This e-mail may contain SEL confidential or legally privileged information. The opinions expressed are not necessarily those of SEL. Any unauthorized disclosure, distribution or other use is prohibited. If you received this e-mail in error, please notify the sender, permanently delete it, and destroy any printed copies. - 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 ______________________________________________________________________
