On 1 ago, 02:39, Steve Reinhardt
wrote:
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

Hello Steve,

The result of the 2*B^2/lambda formula is in meters. For your antenna
with 4m size, you can be sure to be in the far field at 16m (AKA
Fraunhofer region). The minimum distance must be the sum of the far
field distance for both AUT and reference antenna.

As Roy also mentioned, real measurements are difficult. To play your
own devil's advocate, you could run gain measurements for different
(many) distances. From the Gain versus Distance graph you can get
an impression of the accuracy. The same you can do for various heights
to get an impression of the influence of ground reflection.

A complete other approach is to include the ground reflection in the
measurement. That might no be a bad option. The requirement for such a
measurement setup is that for both receive and transmit antennas, the
ground reflection must fall well within the main beam. Horizontal
polarization is preferred. VSWR of both antennas must not be affected
by ground reflection.

Best regards,

Wim
PA3DJS
www.tetech.nl