[TowerTalk] Near field Far field

Steve Hunt steve at karinya.net
Sun Sep 21 13:50:57 EDT 2008

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?

Steve G3TXQ

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