On Sep 21, 2008, at 3:00 PM, jimlux wrote:
> Bob Nielsen wrote:
>> That is the generally-used formula. As you said it isn't an
>> abrupt transition, but the point where the 1/r^2 and 1/r^3 terms
>> in the field strength equations become small enough to be
>> ignored. The relevant math can be found in "Antennas" by Kraus
>> (W8JK).
>> 73,
>> Bob, N7XY
>> On Sep 21, 2008, at 10:50 AM, Steve Hunt wrote:
>>> When I'm making Far Field measurements on an HF antenna - for
>>> example
>>> plotting its azimuth pattern by rotating it whilst measuring
>>> relative
>>> field strength at a remote point - how far away do I need to be to
>>> ensure I'm in the Far Field?
>>>
>
>
> The equations that Kraus (and others) have are related to the
> reactive near field.. the area where more energy is stored in the
> field than is radiated away. The notional boundary is where an
> equal amount of energy is stored and radiated. As a conceptual
> thing, the "near field" is that area where if you put something
> with conductivity and or dielectric constant, it changes the pattern.
>
>
> That's really, really different from the "far field" in antenna
> range terms, which is where you are far enough away that the
> difference in the measurement from a true "infinitely far source
> with a plane wave" and the measurement you're making (with a
> spherical wavefront) is "small".
Different yes, but if I recall correctly (it's been many years since
I have worked on an antenna range) the 2D^2/lambda criterion applies
to both cases and is where the deviation from a plane wave is <1/8
wavelength at the edges of the aperture.
>
> If you have something like a compact range, there's a big reflector
> that turns the spherical wavefront from the test feed into a plane
> wave incident on the Antenna Under Test (AUT). Obviously, the big
> reflector has to be bigger than the AUT for this to work, and it's
> got to be in a anechoic chamber.
True. The "virtual path length" is much greater than the physical
distance. The same effect can be achieved with a lens (much easier
to realize in optics than at RF). I retired just before a compact
range was installed at my workplace so I never had the opportunity to
get my hands dirty with that method.
Bob, N7XY
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