On Sun, 10 Sep 2006 16:01:34 -0700, Richard Clark
wrote:

In fact, in the near field of an antenna, there is nothing that
resembles 377 Ohms of Z.

The page at:
http://home.comcast.net/~kb7qhc/ante...pole/index.htm
dramatically reveals that the near fields fluctuate wildly from 377
Ohms, and I have restricted my analysis to those values falling at
roughly 100 Ohms or 1000 Ohms (the hot spots marking the feed point
region and the tips of the dipole).

Other antenna design's modification of the 377 near field around them
can be observed at:
http://home.comcast.net/~kb7qhc/ante...elds/index.htm

73's
Richard Clark, KB7QHC

Richard:

[snip]
"Richard Clark" wrote in message
...
On Sun, 10 Sep 2006 16:01:34 -0700, Richard Clark
wrote:

In fact, in the near field of an antenna, there is nothing that
resembles 377 Ohms of Z.
[snip]

Correct, but don't we all believe that the wave impedance of "free space" is
approximately 377 Ohms...

Everywhere...

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.

Thoughts, comments?

--
Pete K1PO
Indialantic By-the-Sea, FL

The page at:
http://home.comcast.net/~kb7qhc/ante...pole/index.htm
dramatically reveals that the near fields fluctuate wildly from 377
Ohms, and I have restricted my analysis to those values falling at
roughly 100 Ohms or 1000 Ohms (the hot spots marking the feed point
region and the tips of the dipole).

Other antenna design's modification of the 377 near field around them
can be observed at:
http://home.comcast.net/~kb7qhc/ante...elds/index.htm

73's
Richard Clark, KB7QHC

On Tue, 12 Sep 2006 16:00:24 GMT, "Peter O. Brackett"
wrote:

In fact, in the near field of an antenna, there is nothing that
resembles 377 Ohms of Z.
[snip]

Correct, but don't we all believe that the wave impedance of "free space" is
approximately 377 Ohms...

Hi Peter,

Beliefs. -sigh- Is this one of those transcendental statements about
navel gazing?

Everywhere...

Even in the near field of an antenna.

No. Not 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.

Wrong.

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!

Abstracting from near space to everywhere is the source of your error.

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...

The waves are not plane where the waves are not plane, but... Is this
a Zen "but?"

There might just also be plane waves passing through identically the same
region of space, say emanating from a more distant antenna.

Wrong.

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.

Wrong.

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.

Thoughts, comments?

Wrong.

Peter, are you trying to bust loose a seized bearing? Most of this
reads like the Molly Bloom citation from a technical translation of
"Ulysses."

73's
Richard Clark, KB7QHC

Peter O. Brackett wrote:

Correct, but don't we all believe that the wave impedance of "free space" is
approximately 377 Ohms...

Everywhere...

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.

Thoughts, comments?

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

N0GW wrote:
. . .
So... Yes the antenna is a transducer. No, it does not transform 50
ohms into 377 ohms. 377 ohms refers to the eletrostatic and magnentic
fields as they exist in the near field of an antenna or conductor. It
does not refer to what is going on electrically in the antenna
conductors.
. . .

377 ohms does not describe the E and H fields in the near field. 377
ohms is the ratio of E to H in the *far field* when the medium is free
space or, for practical purposes, air. In the near field, the ratio of E
to H can be not only far from 377 ohms, but it's commonly also complex
(that is, E and H not in time phase). For an illustration, model a short
dipole or small loop with EZNEC or NEC-2, and use the near field
analysis to find E and H at some point close to the antenna (within a
fraction of a wavelength). When you divide E by H, you'll get a wide
variety of results(*) depending on the type of antenna and the
observation point. But as you get farther and farther from *any*
antenna, you'll find that the ratio always converges to 377 ohms, purely
real (that is, the E and H fields in time phase).

(*) The ratio of E to H is called the "wave impedance". In the far
field, and only in the far field, it equals the intrinsic impedance of
the medium. And, as Gary and others have said, this shouldn't be
confused with the ratio of voltage to current at an antenna feedpoint.
They are not at all the same thing, in spite of having the same units of
ohms.

Roy Lewallen, W7EL

Roy Lewallen wrote:

377 ohms does not describe the E and H fields in the near field. 377
ohms is the ratio of E to H in the *far field* when the medium is free
space or, for practical purposes, air. In the near field, the ratio of E
to H can be not only far from 377 ohms, but it's commonly also complex
(that is, E and H not in time phase). For an illustration, model a short
dipole or small loop with EZNEC or NEC-2, and use the near field
analysis to find E and H at some point close to the antenna (within a
fraction of a wavelength). When you divide E by H, you'll get a wide
variety of results(*) depending on the type of antenna and the
observation point. But as you get farther and farther from *any*
antenna, you'll find that the ratio always converges to 377 ohms, purely
real (that is, the E and H fields in time phase).

Yes, I agree with that completely Roy. I apologize for simplifying my
response so much as to not mention this. I was trying to answer the
question at the same level as was asked. I did not mean to offend the
more mathematically astute members of this group.

I will stand by my comment that radiation from antennas, no matter how
well predicted mathematically, is not well understood at a subatomic
level. I personally prefer a model that assumes photons result from
electron acceleration (or deceleration or energy level decrease).
There are obviously competing models.

Gary
N0GW

N0GW wrote:
Roy Lewallen wrote:
377 ohms does not describe the E and H fields in the near field. 377
ohms is the ratio of E to H in the *far field* when the medium is free
space or, for practical purposes, air. In the near field, the ratio of E
to H can be not only far from 377 ohms, but it's commonly also complex
(that is, E and H not in time phase). For an illustration, model a short
dipole or small loop with EZNEC or NEC-2, and use the near field
analysis to find E and H at some point close to the antenna (within a
fraction of a wavelength). When you divide E by H, you'll get a wide
variety of results(*) depending on the type of antenna and the
observation point. But as you get farther and farther from *any*
antenna, you'll find that the ratio always converges to 377 ohms, purely
real (that is, the E and H fields in time phase).

Yes, I agree with that completely Roy. I apologize for simplifying my
response so much as to not mention this. I was trying to answer the
question at the same level as was asked. I did not mean to offend the
more mathematically astute members of this group.

I will stand by my comment that radiation from antennas, no matter how
well predicted mathematically, is not well understood at a subatomic
level. I personally prefer a model that assumes photons result from
electron acceleration (or deceleration or energy level decrease).
There are obviously competing models.

I'm not the least bit offended; I just corrected a statement which
wasn't true.

Intelligent discussion of the subatomic and quantum physical aspects of
electromagnetic radiation are for people mathematically much more astute
than I, so I'll leave that for you.

Roy Lewallen, W7EL