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gelunmak Antenna Theory Regular
Joined: 05 Jun 2009 Posts: 17 Location: Hong Kong

Posted: Mon Aug 24, 2009 9:42 am Post subject: Nearfield vs Farfield radiation pattern 


Hi All
As far as i know, the relationship between NearField Radiation Pattern and FarField Radiation Pattern is bounded by Fourier Transform.
i.e. NearField > (FFT) > FarField
But why FFT?
Thanks 

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admin Site Admin
Joined: 03 Jan 2007 Posts: 197

Posted: Tue Aug 25, 2009 4:32 am Post subject: Fourier Transform 


At a very lowlevel intuitive idea, consider that a very short pulse in time would have a very large frequency span. Similarly, a very large antenna (many wavelengths long) is typically very directive; very short antennas are often more omnidirectional. So at first glance it makes a little bit of sense.
To understand the relationship, think about a simple aperture in an infinite metallic sheet. Let the fields within the aperture (the source fields), be Ea. This, in effect, are the near fields. How do you find the far fields from the aperture fields?
You can integrate across the aperture: suppose you want the fields at some large position R away from the aperture. This can be found from:
integral[ Ea (dot) e^(j*k (dot) R )/R ]
The exponent term comes from the phase propagation, the 1/R is the power falloff of the fields due to distance. This integral is basically a Fourier transform  note that R is fixed. The fourier transform in this case maps the near field domain (the aperture locations) to the far field domain (the angles theta and phi in the far field).
Hence, its just math. If you work out the equations that take the near fields out to the far fields, it resembles a Fourier Transform if you factor it right. 

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kviksand81 AntennaTheory.com Newbie
Joined: 18 Dec 2014 Posts: 3

Posted: Sun Jan 04, 2015 3:52 pm Post subject: Re: Fourier Transform 


admin wrote:  At a very lowlevel intuitive idea, consider that a very short pulse in time would have a very large frequency span. Similarly, a very large antenna (many wavelengths long) is typically very directive; very short antennas are often more omnidirectional. So at first glance it makes a little bit of sense.
To understand the relationship, think about a simple aperture in an infinite metallic sheet. Let the fields within the aperture (the source fields), be Ea. This, in effect, are the near fields. How do you find the far fields from the aperture fields?
You can integrate across the aperture: suppose you want the fields at some large position R away from the aperture. This can be found from:
integral[ Ea (dot) e^(j*k (dot) R )/R ]
The exponent term comes from the phase propagation, the 1/R is the power falloff of the fields due to distance. This integral is basically a Fourier transform  note that R is fixed. The fourier transform in this case maps the near field domain (the aperture locations) to the far field domain (the angles theta and phi in the far field).
Hence, its just math. If you work out the equations that take the near fields out to the far fields, it resembles a Fourier Transform if you factor it right. 
Say, if I have an aperture field from a horn antenna and just want to "brute force" compute the farfield, doing as few approximations as possible, I would just do a double integral across the limits of the aperture multiplied with the complex exponential e^(j*k (dot) R)/R ?
In this case, what would the "k" and "R" in the exponent look like? I guess I would have to do a coordinate transformation at some point along the process, since I have my horn aperture field in rectangular coordinates, would this be before or after the computation of the farfield?
Best Regards, 

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admin Site Admin
Joined: 03 Jan 2007 Posts: 197

Posted: Tue Jan 06, 2015 12:28 am Post subject: 


You can check out some of the math on this page:
http://www.thefouriertransform.com/applications/radiation.php
Basically the math is the same whether you're integrating E or Js (surface current). You can see how the dot product gets factored in, and you basically end up with the Fourier Transform of the aperture distribution. 

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kviksand81 AntennaTheory.com Newbie
Joined: 18 Dec 2014 Posts: 3

Posted: Thu Jan 22, 2015 12:16 pm Post subject: 


My thought exactly!
I've designed a standard pyramidal horn using the design procedure outlined in Balanis Antenna Theory  Analysis and Design, 3rd Ed. and used the dimensions from this design to calculate the aperture field of the horn and stored the field in a matrix by using the expression for the tangential Efield component on each location in the discretized aperture, i.e. the expression shown below:
Then, as per my understanding, it should be possible to propagate the aperture field to the farfield, viewed in spherical coordinates by implementing the following equations, where a1 and b1 are the dimensions of the horn:
The conversion to spherical coordinates is done using
and setting Ex and Ez to zero and the integrals are solved for every point defined by r, theta, phi. I simply let Matlab crunch these equations numerically.
The total Efield in the farfield should then just be given by farfield = sqrt(Er^2+Etheta^2+Ephi^2), right?
Am I getting it right here or not? The reason I ask is that when I compare with a simulation of the farfield for the exact same horn simulated in a EMsolver and monitoring only the Efield, I get a farfield result that differs from the Matlab result. So I'm trying to figure out where I am doing it wrong. And the first very helpful thing would be to know if I at least got the process right?
Thanks for your reply!
Hope to hear from you again!
Best Regards,
Last edited by kviksand81 on Fri Feb 27, 2015 10:33 am; edited 1 time in total 

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admin Site Admin
Joined: 03 Jan 2007 Posts: 197

Posted: Fri Jan 23, 2015 9:53 pm Post subject: 


It looks about right, except:
 why do you need to convert anything from (x,y,z) to (R, theta,phi) coordinates?
 your equation for Ey'....what is pho_2 and pho_1? Usually you assume the dominant mode for Ey which is just the cos() term. I don't recall needing that exponential there 

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