[TowerTalk] Near field Far field

[jimlux]

Bob Nielsen wrote:
> 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.


That's basically it.... but where the deviation from plane wave happens 
to be 1/8 wave doesn't have anything to do with where the reactive near 
field ends.. defining the "near field" as everything inside the boundary 
where more energy flows back and forth from field to antenna than 
radiates away.

It is true that for some simple antennas, the two distances are close.

> 
>> 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.


And now-a-days, with computer power being cheap, you can "simulate" any 
wavefront you want with a sufficiently dense sampling in the near field 
(e.g. a near field range).  It's not trouble free: the probe's not 
infinitely small, and needs to be characterized. And, you need to 
measure both amplitude and phase pretty accurately, in both 
polarizations.  But, near field (or quasi near field) techniques have 
come a long ways.  I was working on a 7.15 GHz and 32 GHz antenna system 
about 3 meters across, where we needed measurements of the pattern 
within milliradians of boresight; implying that for the accuracy, our 
source would have had to be 20 some km away, clearly impractice (the 
atmosphere would destroy the planarity of the wavefront).  Doing quasi 
near field measurements in an indoor range was the only practical solution.

Jim, W6RMK