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pascualhr AntennaTheory.com Newbie
Joined: 03 Dec 2015 Posts: 6

Posted: Wed Feb 03, 2016 1:37 pm Post subject: FarField Question 


It is known that the conditions needed to be in the farfield region are:
d > 2D^2/lambda
d > 5D
d > 1.6*lambda
Being D = the largest physical linear dimension of the antenna.
If in my case I am working with a planar array of elements, my question is: D would be the length of my element or the length of the whole array?
Many thanks in advance ,
Pascual 

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admin Site Admin
Joined: 03 Jan 2007 Posts: 199

Posted: Wed Feb 03, 2016 5:45 pm Post subject: 


If you use the size of each element (D), then at that distance you would be in the far field for each element individually. WHat I mean is, if you are at a point x, where the minimum distance to the nearest element is larger than d (as defined by the equations below), then you would technically be in the far field at point x, for all antennas, assuming you evaluated your functions d with the individual element size D.
However, if you are making a phased array and applying array factor equations, then you will need to use the total size of the array. This ensures the rays from each antenna are approximately parallel. 

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pascualhr AntennaTheory.com Newbie
Joined: 03 Dec 2015 Posts: 6

Posted: Wed Feb 03, 2016 6:27 pm Post subject: 


admin wrote:  If you use the size of each element (D), then at that distance you would be in the far field for each element individually. WHat I mean is, if you are at a point x, where the minimum distance to the nearest element is larger than d (as defined by the equations below), then you would technically be in the far field at point x, for all antennas, assuming you evaluated your functions d with the individual element size D.
However, if you are making a phased array and applying array factor equations, then you will need to use the total size of the array. This ensures the rays from each antenna are approximately parallel. 
Thank you so much for the clarification! 

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pascualhr AntennaTheory.com Newbie
Joined: 03 Dec 2015 Posts: 6

Posted: Mon Mar 07, 2016 12:35 pm Post subject: 


Hi,
I am calculating the field regions and for that purpose I need D (the maximum distance of my antenna). In my case I have this array:
My question is: D should be d1 or d2?
Many thanks in advance! 

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helix Antenna Theory Regular
Joined: 29 Jan 2015 Posts: 64

Posted: Wed Mar 09, 2016 3:57 pm Post subject: 


Nothing "magic" happens suddenly when you cross d^2/lambda. Look up how it's derived... It has to do with phase front curvature (deviation from plane wave) from an aperture (which you don't have) being a certain (arbitrary) amount. Whether you use d1 or d2 in your calc is going to be insignificant as far as any practical antenna application is concerned. 

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