[TowerTalk] Near field Far field

[Bob Nielsen]

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