Mr. Borowski's first objection is incorrect. He says so himself when he removes himself to 1000 wavelengths. The fact that an antenna is not a point source radiator adjacent to the antenna itself is self-evident. The question of importance is, at one point can it be considered a point source radiator, which was the question to which I responded. I provided a more useful criterion than 1000 wavelengths, and explained the rationale behind the determination of a useful far-field criterion, and a way to calculate it for yourself if the criterion I used (lambda/16 phase difference) was not the proper criterion. 'Nuff said on that account.
His second objection is also incorrect, and is also inconsistent with his 1000 wavelength observation. As you move further from the antenna, the difference between max and min separation distances from the point of observation approaches zero. At one mile away from a two meter long dipole, the max minus min distance difference is 0.03 centimeters. Clearly it is not this separation that causes the pattern for an antenna cut to operate at 75 MHz as a half wave or 225 MHz as a 3/2 wavelength dipole. The far field antenna pattern is set up by the physical size of the antenna elements relative to a wavelength and the retarded potentials and the flow of resonant rf currents thus induced. Kraus' classic Antennas has good mathematical treatment of the physics of antennas. Don White's EMC encyclopedia gives a good non-mathematical summary (some math, but algebra only) - and gives a very good feel for how antennas work without the vector calculus. > From: [email protected] > Date: Mon, 7 Aug 2006 07:31:15 -0700 > To: <[email protected]> > Subject: Re: Near field/Far field what is D. > > 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 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 ______________________________________________________________________
