Re: Is antenna a transducer to 377 ohms?
- From: Roy Lewallen <w7el@xxxxxxxxx>
- Date: Tue, 12 Sep 2006 16:26:43 -0700
Peter O. Brackett wrote:
Correct, but don't we all believe that the wave impedance of "free space" is approximately 377 Ohms...>
Even in the near field of an antenna.
That is an antenna itself has no effect on the fundamental u and e of the media in which it is immersed. u and e are defined only in terms of and as affecting "plane wave" [TEM mode?] propagation, and...
After all the antenna is very small, and free space is very large (grin), and so a tiny antenna cannot change u and e everywhere!
The fields E and H in the "near region" of an antenna where the waves are not "plane" on the other hand may not be related by 377 Ohms, simply because the waves emanating from the "near" antenna are not plane, but...
There might just also be plane waves passing through identically the same region of space, say emanating from a more distant antenna. The ratio for those plane E and H fields will indeed be 377 Ohms over the exact same region of space where Zo is different because of simultaneous but non-planar waves.
So in fact... the wave impedance of free space can have many values simultaneously, one [universal?] constant value of ~377 Ohms for plane waves, while it may have many other [arbitrary] values for waves passing through the same region of space that are not plane.
I don't believe I've ever encountered the term "wave impedance of free space", and its use is certain to cause confusion, as I sense here.
The *intrinsic* impedance of free space is 377 ohms. The *wave* impedance of an EM wave in that medium is 377 ohms if it's a plane wave in the far field of a radiator, and some other value if it's close to an antenna or other conductor or dielectric. The *intrinsic* impedance of free space is determined only by the conductivity, permittivity, and permeability of the medium; the impedance of a wave is governed not only by the intrinsic impedance of the medium but also other factors.
If you have a reference that defines and uses the term "wave impedance of free space", I'd like to look it up to see how the author deals with this potentially confusing combination of terms. If it does indeed "have many values simultaneously", it's pretty useless in my opinion.
Roy Lewallen, W7EL
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