G8WRB AntennaTheory.com Newbie
Joined: 26 Oct 2011 Posts: 2

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


The usual formula for computing the distance to the farfield 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 YagiUda 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 YagiUda array, the longest elements is about half a wavelength?
There was some complete junk on the Wikipedia page on farfield, 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 
