Re: Near field vs Far field measurements at 2M
- From: Steve Reinhardt <stevereinhardt@xxxxxxxxxxxxxxxxxxx>
- Date: Tue, 31 Jul 2007 20:39:15 -0400
Jim Lux wrote:
When your "Antennas Under Test" are moderate gain devices, I would go
for several wavelengths. For low to moderate gain (up to 10 dBi), you
are in the far field within about 4 WL.
The reason for the short distance is that the direct signal is strong,
hence influence of reflections is less. You can reduce the effect of
reflections by taking a receive antenna with some directivity.
You can be sure that you are in the far field distance when
D > 2*B^2/lambda, where B = overall size of the antenna (from one
extremity to another). For several antenna types (like yagis), you can
halve this distance when you are interested in main beam gain only.
This formula is actually an embodiment of the venerable Rayleigh limit,
It actually says that wavefront is flat to within a fraction of a wavelength (about 1/13th or 22 degrees). The implications for gain measurement is that the gain you measure at 2*b^2/lambda distance will be the same as you'd measure if you were truly in the far field, to within about a reasonable degree of accuracy (1% or there abouts).
The derivation is this:
distance to center of antenna = D
distance to edge of antenna Dedge = sqrt(D^2+(B/2)^2) {Pythagorean formula}
phase error = (D-Dedge)/lambda {wavelengths}
etc.
if you start getting lambda close to B, then the relative path length difference gets quite large, and you have to start worrying about the current distribution or illumination non-uniformity.
This is a big help! The equations I read did not help me understand the problem. (Though when I read 'Rayleigh', thoughts of optical flats and oversized college physics texts popped into my head.)
So, if I have a 4 element collinear, measuring 2 wl, or about 4 meters, and the frequency of interest is about 2 meters, then I'm effectively far field when I reach a distance > 16 wl.
Cool. The neat part about the football field is that the nearest reflection is well over 1.4 times the distance between source and measurement antennae. It's flat with no RF hard surfaces around the perimeter. That's not to say there are no other sources of measurement error, just that I think their contribution will be small.
I'll report back if I can get it done before school starts, and they want my RF range back for their sports activities :-)
73,
Steve
W1KF
.
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