[Antennaware] Testing the theory..

Bob Nielsen nielsen at oz.net
Thu Mar 29 19:02:39 EST 2007

On Mar 29, 2007, at 3:28 PM, Andrew Ingraham wrote:

>> I recall reading somewhere that you should be at least 10 wavelengths
>> distant from the antenna. This is to ensure you are out of the near
>> field when taking the measurement.
> I also recall near-field/far-field expressed in terms of wavelengths.
> But I believe it depends on both wavelengths and antenna size.  So,  
> it seems
> appropriate to recommend the larger of (say) 10 times the  
> wavelength, or 10
> times the antenna size.
> Virtually all antenna modeling programs and formulas (as well as  
> radiation
> safety estimators) estimate ONLY far field emissions, and say  
> nothing about
> what happens in the near field.
> In the far field, the ratio of electric to magnetic fields equals  
> 377 ohms;
> but not (necessarily) in the near field.  Since many field strength  
> meters
> respond primarily to the electric, or the magnetic, field, you  
> could get
> incorrect measurements in the near field that don't apply to the  
> far field.
> Another case where near field measurements fall down, is if you are  
> in the
> direction of a null in the antenna pattern.  In the near field that  
> null
> might not exist at all, because you are too close to some parts of the
> antenna than others.  While that is an extreme case with extremely
> inaccurate results, it illustrates why you really need to be in the  
> far
> field before the measurements can be considered meaningful.

I haven't done any serious antenna design work in 25 years or so, but  
the rule of thumb I remember using was 2d^2/wavelength, where d was  
the largest dimension of the antenna.  While not an abrupt  
transition, this is about where the 1/r dependence of the field  
strength dominates over higher-order terms.

Bob, N7XY

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