On 21 abr, 23:30, "Dave (from the UK)" see-my-signat...@southminster-
branch-line.org.uk wrote:
Wimpie wrote:
Hello,

Your formula for far field distance (Fraunhofer region) assumes a path
difference between the inner and outer antenna with respect to an
observation point of 1/16 lambda.

Someone has said that the formula I gave is not valid for a phased array.

His comment (about the 2 D^2/lambda) is below:

-----
That estimation does not apply in this case. It can be considered to be
valid for aperture antennas which is not the case here. It would only
require to have the transmitting antenna illuminating the pleased array
within its 3dB mainlobe which of course is by far the case at a distance
of 3000m or even more.
------

I'm not to bothered about the odd factor of two. I have seen a
derivation of the formula, but it was based on a rectangular aperture,
not an array of them. It don't know if that may mean the equation is
just not appropriate at all.

Using that forumal puts the far-field distance at about 10 km in my
case. Using someone elses idea, puts it at only a few hundred meters.
There is at least a factor of 10 difference.
--
Dave (from the UK)

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Hi Dave,

Whether or not the formula is applicable, depends on many factors as
mentioned in my previous posting.

For a broadside array, the formula holds with same accuracy as for
continuous aperture antennas. In my antenna courses I use the
broadside array approach to derive the 2B^2/lambda formula.

For an end-fire case, the situation is different. When you are
interested in main lobe gain only (so not the complete radiation
pattern), you can reduce the distance significantly. The reason for
that is that when you come closer to the antenna, the path difference
doesn't change; the amplitude contribution of each array element is of
importance now. However when you need to know the complete pattern
(including broadside directions), you need the large distance.

It is just a matter of change in path length difference amplitude
unbalance when you come too close to the antenna. If you keep this in
mind, you can figure out the measuring distance for you application
(for example with a spread sheet).

I would reserve the term "far field distance" for that distance where
the complete radiation pattern does not change with measuring
distance. In that case, the 2B^2/lambda formula is a good rule of
thumb.


Best regards,

Wim
PA3DJS