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Far field of Spiral Antenna

 
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gmask
Antenna-Theory.com Newbie


Joined: 03 Aug 2010
Posts: 3

PostPosted: Tue Aug 03, 2010 10:27 am    Post subject: Far field of Spiral Antenna Reply with quote

Hi everyone!

I have question that has been bothering me for a while.

I have an Archimedean-like Spiral Antenna (2-18GHz) of 61mm diameter. It is usually assumed that the far field region of an antenna lies approximately at a distance R> 2D^2/lambda, where D is supposed to be "the largest dimension of the antenna". What does this mean in a spiral antenna? I would put the diameter, but the resulting R is 2.5cm@2GHz and 22.2cm@18GHz.

This doesn't sound very nice. In principle, D has to be greater than lambda for the equation to be applicable. Then, what dimension D should I consider in such an antenna?

Thank you all for your time,

Gmask
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Schubert
Antenna Wizard


Joined: 08 Apr 2009
Posts: 161

PostPosted: Wed Aug 04, 2010 12:05 pm    Post subject: Reply with quote

You're calculations are correct and you are using the correct D for the far field equation: R > 2*D^2/lambda (eq 1)

However, the far field also requires that R >> lambda (eq 2). Usually, eq 1 is the limiting equation, so it is used. But in this case you the 2nd equation would be the determining one because D is so small. Hence, at 2 GHz the far field wouldn't begin until R is roughly 10 lambda, or 1.5 meters.

If you look up the fields from a short dipole in a EM textbook or antenna book, you will see the near fields (that die off as 1/R^3 or 1/R^2) are present "close" to the antenna (when R is comparable to lambda). Once R is about 10 lambda away, these fields are negligible to the radiated fields (that die off as 1/R, and the E- and H-fields are orthogonal). The R>10lambda rule gets you out of the near field.
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gmask
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Joined: 03 Aug 2010
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PostPosted: Thu Dec 16, 2010 8:05 pm    Post subject: Reply with quote

Thank you, Schubert.
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