[TowerTalk] Near field Far field

[Bruce Jungwirth]

In the world of EMC, the usual formula for the far feild is anything beyond 
lamda / 2*pi. Some say 1/2 wavelength or more. Take your pick.

Bruce K0SON

----- Original Message ----- 
From: "Steve Hunt" <steve at karinya.net>
To: <towertalk at contesting.com>
Sent: Sunday, September 21, 2008 12:50 PM
Subject: [TowerTalk] Near field Far field


> 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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