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"
