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Far-Field Question

 
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pascualhr
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Joined: 03 Dec 2015
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PostPosted: Wed Feb 03, 2016 1:37 pm    Post subject: Far-Field Question Reply with quote

It is known that the conditions needed to be in the far-field 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 Very Happy ,
Pascual
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admin
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Joined: 03 Jan 2007
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PostPosted: Wed Feb 03, 2016 5:45 pm    Post subject: Reply with quote

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
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PostPosted: Wed Feb 03, 2016 6:27 pm    Post subject: Reply with quote

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! Very Happy
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pascualhr
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PostPosted: Mon Mar 07, 2016 12:35 pm    Post subject: Reply with quote

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
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Joined: 29 Jan 2015
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PostPosted: Wed Mar 09, 2016 3:57 pm    Post subject: Reply with quote

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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