[TowerTalk] Near field Far field
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?
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