RE: rayleigh criterion and farfield
Thanks to one and all. A summary of what I've understood by this and other replies is: The formula for the Rayleigh criterion is based on solving a right triangle for the distance to the center of the antenna element as described below by Dr Turnbull and neglecting a small lambda term in the result, thus the formula dist > 2*(Ant dimension)^2/lambda is a slight over estimate for a given wavelength. Ken Javor's derivation gave Lamda/16 as "an accepted path length difference at the far field..." That is, the difference in distance between the line (from the center of the antenna) to a point and a line from that same point (hypotenuse) to the edge of the antenna element. The phenomena I was curious about, what happens when you hold the antenna dimension constant and vary the frequency does work the way I thought it did. It just seemed odd because it goes against my intuition that shorter wavelengths should have a shorter focal length for a given aperture or antenna. But the right triangle geometry says otherwise. One last thought, there is another far field distance criterion based on wave impedance (Maxwell's) dist > Lambda /(2*PI ) For a given antenna size and wave length Maxwell's criterion could give a longer or shorter required distance, so it looks like one should check both and choose the greater. Best, George Stults -Original Message- From: Luke Turnbull [mailto:luke.turnb...@trw.com] Sent: Friday, November 22, 2002 1:53 AM To: 'emc-p...@majordomo.ieee.org'; George Stults Subject: Re: rayleigh criterion and farfield The derivation of this formula involves considering a position in front of the antenna, on the line in the direction of radiation. The distance from this point to the edge of the antenna will be slightly more than the distance to the centre. If this distance is a significant fraction of a wavelength, the the far field pattern is not realised because of phase error / cancellation. Hence at higher frequencies (shorter wavelengths), the same position in front of the antenna will have phase errors where there were no problems at lower frequencies. Hope this helps, Dr Luke Turnbull Principal EMC Engineer TRW Conekt Statford Road Solihull B90 4GW Tel: +44 (0)121.627.3966 Fax:+44 (0)121.627.4353 email: luke.turnb...@trw.com web: www.trw.com/conekt/ >>> George Stults 11/21/02 05:59pm >>> 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org 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"
Re: rayleigh criterion and farfield
The derivation of this formula involves considering a position in front of the antenna, on the line in the direction of radiation. The distance from this point to the edge of the antenna will be slightly more than the distance to the centre. If this distance is a significant fraction of a wavelength, the the far field pattern is not realised because of phase error / cancellation. Hence at higher frequencies (shorter wavelengths), the same position in front of the antenna will have phase errors where there were no problems at lower frequencies. Hope this helps, Dr Luke Turnbull Principal EMC Engineer TRW Conekt Statford Road Solihull B90 4GW Tel: +44 (0)121.627.3966 Fax:+44 (0)121.627.4353 email: luke.turnb...@trw.com web: www.trw.com/conekt/ >>> George Stults 11/21/02 05:59pm >>> 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org 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"
Re: rayleigh criterion and farfield
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 >To: "'Ken Javor'" , "'emc-p...@majordomo.ieee.org'" >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:ken.ja...@emccompliance.com] > Sent: Thursday, November 21, 2002 12:12 PM > To: George Stults; 'emc-p...@majordomo.ieee.org' > 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 >>To: "'emc-p...@majordomo.ieee.org'" >>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: >> majord...@ieee.org >> with the single line: >> unsubscribe emc-pstc >> >> For help, send mail to the list administrators: >> Ron Pickard: emc-p...@hypercom.com >> Dave Heald: davehe...@attbi.com >> >> For policy questions, send mail to: >> Richard Nute: ri...@ieee.org >> Jim Bacher: j.bac...@ieee.org >> >> 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org 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"
RE: rayleigh criterion and farfield
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:ken.ja...@emccompliance.com] Sent: Thursday, November 21, 2002 12:12 PM To: George Stults; 'emc-p...@majordomo.ieee.org' 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 >To: "'emc-p...@majordomo.ieee.org'" >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: > majord...@ieee.org > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Ron Pickard: emc-p...@hypercom.com > Dave Heald: davehe...@attbi.com > > For policy questions, send mail to: > Richard Nute: ri...@ieee.org > Jim Bacher: j.bac...@ieee.org > > 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org 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"
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 >To: "'emc-p...@majordomo.ieee.org'" >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: > majord...@ieee.org > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Ron Pickard: emc-p...@hypercom.com > Dave Heald: davehe...@attbi.com > > For policy questions, send mail to: > Richard Nute: ri...@ieee.org > Jim Bacher: j.bac...@ieee.org > > 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org 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"
