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Far field - when do equations other than 2 D^2/lambda apply?

 
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G8WRB
Antenna-Theory.com Newbie


Joined: 26 Oct 2011
Posts: 2

PostPosted: Fri Dec 02, 2011 9:39 am    Post subject: Far field - when do equations other than 2 D^2/lambda apply? Reply with quote

The usual formula for computing the distance to the far-field of an antenna is 2 D^2 / lambda, but I've seen several references to occasions when this does not apply.

Are there exceptions if the antenna is very small compared to a wavelength?

On a related topic, how far away should one be from an antenna when making measurements? I've seen mention of at least 10 wavelengths, but certanily that would not agree with the 2 D^2/lambda, unless D was 2.2 wavelenghts or more, which for many antennas it is not so.

If one is only interested in the peak gain of an antenna such as a Yagi-Uda array, and therefore making measurements along the axis, is there any need to consider D as the largest dimension of the antenna, or could one consider D as being lambda/2, since for a Yagi-Uda array, the longest elements is about half a wavelength?

There was some complete junk on the Wikipedia page on far-field, which I've removed, but my knowledge of the subject is not great. I've marked the page with the "expert" tag, so if anyone really is an expert in this area, perhaps they could edit the Wikipedia page.

If you want to test antenna gain by directing two identical antennas together, does the distance need to be 2 D^2/lambda, or would it become 4 D^2 / lambda to ensure that the radiation is plane at a point where both antennas are in the far field.


Dave
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qcu89
Antenna Theory Regular


Joined: 01 Nov 2011
Posts: 13

PostPosted: Fri Dec 02, 2011 2:42 pm    Post subject: Reply with quote

Hi!
I was measuring once horn antenna and I has to keep farfield in 2d^2/lambda.
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bigSteve
Antenna Wizard


Joined: 14 Mar 2009
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PostPosted: Fri Dec 02, 2011 6:15 pm    Post subject: Reply with quote

G8WRB - you state that "distance to the far-field of an antenna is 2 D^2 / lambda, but I've seen several references to occasions when this does not apply. ".

Can you give examples of when this doesn't apply? It is a requirement.

Then you state: "Are there exceptions if the antenna is very small compared to a wavelength? "

You will need to look at the other two far field conditions, namely R>>lambda in that case.

Take a look at this page and note that all 3 equations need to be valid to be in the far field:
http://www.antenna-theory.com/basics/fieldRegions.php
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