Mr. Stults is right, my analogy did not directly address his concern. The fact that I thought it did shows how thoroughly I see the two issues as identical.
I can provide the derivation of the 2D**2/wavelength to those who are interested. It has to be an attachment. presently I am sending it only to Mr. Stults. ---------- >From: George Stults <[email protected]> >To: "'Ken Javor'" <[email protected]>, "'[email protected]'" <[email protected]> >Subject: RE: rayleigh criterion and farfield >Date: Thu, Nov 21, 2002, 2:37 PM > > As I understand it, your analogy agrees nicely with the prediction that as > the dimensions increase, the far field distance increases. I think my > question is a little different. By the same analogy and considering a fixed > dimension lense versus the length at which various frequencies could be > focused; it seems like the formula predicts that higher frequencies of light > would focus further out. Does that happen? > > -----Original Message----- > From: Ken Javor [mailto:[email protected]] > Sent: Thursday, November 21, 2002 12:12 PM > To: George Stults; '[email protected]' > Subject: Re: rayleigh criterion and farfield > > > The fact that an aperture antenna's (horn/dish) gain increases with > increasing frequency DOES seem intuitively obvious to me. Consider an > optical analogy. Lenses. If you are familiar with 35 mm photography, you > will recognize that a short lens like a 28 mm will focus from a couple > inches from the lens to infinity. Whereas a long lens like 200 mm won't > focus closer than about 6 feet. Minimum focusing distance is the same as > far field. The higher the gain, the further out from the antenna you have > to be before achieving rated gain. > > ---------- >>From: George Stults <[email protected]> >>To: "'[email protected]'" <[email protected]> >>Subject: rayleigh criterion and farfield >>Date: Thu, Nov 21, 2002, 11:59 AM >> > >> >> Hello Group, >> >> A book I've been reading gives the Rayleigh criterion for farfield >> conditions based on antenna (or EUT max dimension) size as >> >> dist for farfield conditions > 2*(max antenna dimension)^2/lambda >> >> When I look at this, it says that the required distance for far field >> conditions increases as the square of the dimensions of the antenna, which >> seems intuitive. >> >> What I found strange is that if you hold the antenna dimension constant, > (ie >> for a given fixed antenna dimension) it predicts that the distance for >> farfield conditions will increase linearly with the frequency. That does >> not seem intuitive. >> >> Does anyone have a thought about how this works? >> >> Regards, >> >> George Stults >> WatchGuard Technologies Inc >> >> >> >> ------------------------------------------- >> This message is from the IEEE EMC Society Product Safety >> Technical Committee emc-pstc discussion list. >> >> Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ >> >> To cancel your subscription, send mail to: >> [email protected] >> with the single line: >> unsubscribe emc-pstc >> >> For help, send mail to the list administrators: >> Ron Pickard: [email protected] >> Dave Heald: [email protected] >> >> For policy questions, send mail to: >> Richard Nute: [email protected] >> Jim Bacher: [email protected] >> >> All emc-pstc postings are archived and searchable on the web at: >> http://ieeepstc.mindcruiser.com/ >> Click on "browse" and then "emc-pstc mailing list" >> > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
