[TowerTalk] Near field Far field
jimlux
jimlux at earthlink.net
Sun Sep 21 17:53:53 EDT 2008
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?
>
> Clearly there isn't an abrupt transition from Near Field to far Field,
> but some references seem to quote [2* D*D/Wavelength] as a transition
> point, where D is the maximum dimension of the antenna.
That's the distance at which the phase error from considering a
spherical wavefront is the same as plane is "small" (in terms of a
wavelength). It's comparable to the "Rayleigh limit" when looking at
telescopes (i.e. where the physical aperture is diffraction limited).
If you're making boresight gain measurements this is the usual minimum
distance on an antenna range (with anechoic environment), because the
gain error due to non plane wavefront is less than the typical
measurement uncertainty.
It has NOTHING to do with the "near field" as far as stored energy in
the field around the antenna goes.
For the antennas where this guideline is used (i.e. parabolic dishes and
horns), the 2d^2/lamdba is a lot farther than the reactive near field
distance, because these antennas are typically many wavelengths in
dimensions (i.e. d>>lambda).
I wouldn't use it for dipoles, etc.
So, for a point
> source the answer is zero, which sounds right. But for a 20m half-wave
> dipole the answer would be about 32ft which sounds a bit close in. For a
> mini-beam such as the MA5B the prediction would be even closer - about 9ft.
>
> Does this sound right? If not, what is the right formula?
>
> 73,
> Steve G3TXQ
>
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