[TowerTalk] Near field Far field

[jimlux]

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".

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.