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
> 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
> 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
> 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
> 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.
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