On 9/5/21 6:31 AM, Joe Subich, W4TV wrote:
On 2021-09-04 11:19 PM, Lux, Jim wrote:
D =30m (across the 4 square diagonally) is almost certainly bigger
than the height of the 80m elements (20m?)
Even the distance across the diagonal doesn't include the capture
area of the individual verticals. If one looks at the trends for
antenna stacking, the capture area of a single vertical/dipole
would be in the neighborhood of a half wave perpendicular to the
element and on the order of a full wave in the plane of a dipole,
half wave in the plane of a vertical.
Effective area (or capture area, as you describe it) is not really the
relevant thing here - that relates more to the radiation resistance, and
in any case, the effective aperture of an infinitely small dipole is
3/(8 pi) * lambda^2.
But here, you're more concerned about whether the wavefronts from
element 1 and element 2 are "sufficiently close" that the represent the
far field (also, often given as Fraunhofer condition or where a
spherical wavefront is the same as "plane" for the antenna under test).
I'd always understood the requirement for an antenna range to be
something like five or 10 wavelengths so that the measurements
were beyond the near field.
Near field has multiple meanings - one is "inside the line where the
energy stored in the magnetic and electric fields is equal to the energy
being radiated away" - the other is "where the illumination is
effectively planar, so a gain measurement is accurate"
The 2 d^2/lambda is the latter, and I think you hit on the other
constraint - you don't want to be so close to the antenna(s) under test
that you are in the reactive near field. The magnetic field drops off
as 1/r^3, the electric field as 1/r^2, and the radiated field (what you
want to measure) as 1/r, so you need to be far enough away that the
inverse r cubed and inverse r squared terms are "small" relative to the
radiated field.
https://www.antenna-theory.com/basics/fieldRegions.php has a nice
discussion
If one takes a lesson from the FCC/broadcast "proof of performance"
measurements, they measure field strength at one kilometer (one
mile?). That (1 km) works out to be 2 wavelengths at the lowest
frequency in the broadcast band and 6 wavelengths at what used to
be the top of the band.
1 mile, and that's chosen for historical reasons, probably (i.e. a round
number), rather than a detailed consideration of the fields.
73,
... Joe, W4TV
On 2021-09-04 11:19 PM, Lux, Jim wrote:
On 9/4/21 8:05 PM, Wes wrote:
For an illustration try this:
http://www.cuminglehman.com/wp-content/uploads/Introduction_to_Antenna_Test_Ranges_Measurements_Instrumentation.pdf
and look at the figure on page 4. This shows the usual antenna
range situation where the test antenna is receiving a signal from a
point, or small aperture source. This is how I would run this
comparison. I think, but do not know for sure, that I would use the
larger dimension of the vertical(s) as the "D" in the equation. The
idea is to have a plane, or near plane, wave over the whole aperture
of the test antenna in both directions. Note that some antennas,
Yagis for instance, can have an effective aperture larger that the
physical aperture.
Wes N7WS
But that's the 2D^2/lambda - and that comes out strangely small.
And it's not effective aperture (that's more about voltage/power at
the feed) - this is about the physical optics.
D =30m (across the 4 square diagonally) is almost certainly bigger
than the height of the 80m elements (20m?)
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