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?
>
> 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. 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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