David wrote:
The impedance of free space / air is said to be 377 ohms. Impedance is ratio
of E/H. The feedpoint impedance of an antenna is usually 50 or 75 ohms.

Can an antenna ever be regarded as a transducer that transforms a radio wave
from 50 ohms to 377 ohms i.e. provides an impedance transformation?

I'd like to take a somewhat different tack on handling this question.
It is a wave versus photon perspective. It seems to be very easy to
slip into the thought mode where we view radio transmissions as a wave
phenomena in some sort of space media. After all, most of the math we
use for electromagnetic signal propagation work quite well assuming
that. Maxwells math still works wonderfully even today, a century after
Einstien described photons. Maxwell died more than a decade before
Einstien published his seminal papers though.

The real model of the radio operation is for there to be alternating
electrostatic and magnetic fields surounding an antenna when it is
driven by an RF power source. Through some yet poorly understood
physical mechanism, either envolving the acceleration of electrons in
the antenna's conductor or from the alternating E and H fields, photons
are flung off the antenna.

I interpret this to mean that what we call the "near field" around an
antenna is the volume around an antenna where the electrostatic and
magnetic fields are at an energy level significantly above natural
background levels. The "far field" I interpret to mean where RF energy
exists as a photon flux.

To describe an antenna as a transducer is probably correct
symantically. It does convert between electrical energy in the form of
RF current and voltage and photons. This is both for transmitting and
receiving.

The 377 ohm thing though is a function of the releative intensities of
the electrostatic and magnitic fields surrounding an antenna. It is a
constant like pi or e. It is an emperically measured characteristic of
our universe. It does not, however mean that an antenna transforms a
feed impedance to this impedance. It simply tells us what to expect for
when we feed an RF current into an antenna. It is much like knowing
that a 1 foot diameter wheel will travel 3.14159265... feet for every
revolution.

Now, a point worth noting is that while RF current in a conductor
produces photons, photons produce RF current in conductors. That, of
course, is why antennas are able to operate for both receiving and
transmitting. That is also why radio signals bounce. Photons are
absorbed by objects such as wires or dirt and RF currents are produced.
Those currents, in turn, generate new photons. The conductivity and
dielectric constant of the absorbing material determine the amplitude
and phasing of the current and thus the primary direction of emission
of the new photons.

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.

Anyway, that's my take on the subject.

Gary
N0GW

On 10 Sep 2006 14:53:25 -0700, "N0GW" wrote:

Hi Gary,

A number of comments:

I'd like to take a somewhat different tack on handling this question.
It is a wave versus photon perspective.

Photons are no less wave oriented than RF - nor more. Also, RF is no
less "corpuscular" (Einstein's term) than photons.

The real model of the radio operation is for there to be alternating
electrostatic and magnetic fields surounding an antenna when it is
driven by an RF power source. Through some yet poorly understood
physical mechanism, either envolving the acceleration of electrons in
the antenna's conductor or from the alternating E and H fields, photons
are flung off the antenna.

Hardly "flung off" and even less, "poorly understood."

I interpret this to mean that what we call the "near field" around an
antenna is the volume around an antenna where the electrostatic and
magnetic fields are at an energy level significantly above natural
background levels.

The background levels of EM fields are predominantly in the
milli-micro-nano Kelvins of temperature. Nearly everything produces
an energy level significantly above that.

The "far field" I interpret to mean where RF energy
exists as a photon flux.

Photons don't exist in the near field? A flux is simply the bulk of
them; like one electron flowing, or a trillion, is current.

The 377 ohm thing though is a function of the releative intensities of
the electrostatic and magnitic fields surrounding an antenna.

It is a function of permittivity and permeability which exists even if
the fields are not there.

It is a constant like pi or e.

Unless you happen upon something other than a vacuum.

It is an emperically measured characteristic of our universe.

It is empirically measured, but that does not create its value.

Now, a point worth noting is that while RF current in a conductor
produces photons, photons produce RF current in conductors.

How much current is created by your antenna in sunlight?

That, of
course, is why antennas are able to operate for both receiving and
transmitting. That is also why radio signals bounce. Photons are
absorbed by objects such as wires or dirt and RF currents are produced.

An 10 meter doublet of 1mm wire is exposed to 5 - 10 W of power. How
much can you capture and bottle due to this production of current in
conductors?

Those currents, in turn, generate new photons.

Photons are generated on the basis of an electron in an excited state
falling to a lower ground state, not by current flow.

The conductivity and
dielectric constant of the absorbing material determine the amplitude
and phasing of the current and thus the primary direction of emission
of the new photons.

Photons do not follow any particular channel of radiation, not unless
you have a very large lens (few antennas do).

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.

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

73's
Richard Clark, KB7QHC

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

Richard:

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

No... it's not transcendental it's purely algebraic! (grin)

[snip]
Everywhere...
Even in the near field of an antenna.

No. Not even in the near field of an antenna.
[snip]

Where then is Zo = 377 Ohms?

[snip]
That is an antenna itself has no effect on the fundamental u and e of the
media in which it is immersed.

Wrong.
[snip]

Surely uo, eo, Zo and c (velocity of light) are fundamental and invariant
properties of "free space", no?

[snip]
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.
[snip]

No, I'm "contracting" from outer space to near space... using the
contravarient tensor!

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

If a tree falls in the forest and no one is there to hear it, does it make a
sound?

If an antenna radiates somewhere in the Universe and there are no receivers,
does it really radiate?

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

Wrong.
[snip]

Oh, and here I thought that at least tiny remnants of all radiation
eventually passes through every part of space, filling all of space as it
expands throughout the Universe..

[snip]
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.
[snip]

I know that Special Relativity [Maxwell's equations] is not supported in
full by General Relativity, but surely even though space is warped by mass,
superposition must still be supported. The radiation in your neighbourhood
is a superposition of suitably delayed and reduced (by path attenuation) of
all radiation, no?

[snip]
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.
[snip]

And here I thought I was going to be able to sell you a little corner of the
Universe [very near the Brooklyn Bridge] that has any Zo you want. What?

[snip]
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
[snip]

Molly Bloom? How did she get into this... I thought she was still living in
the house on Eccles Street". What?

Now Ulysses, he's my man!

I miss Reg Edwards already :-(

Regards,

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

On Tue, 12 Sep 2006 18:39:25 GMT, "Peter O. Brackett"
wrote:

Where then is Zo = 377 Ohms?

Hi Peter,

To how many places? Your question is rather oblique when we are
discussing near fields and antenna as "transducer" [not a choice of
term I subscribe to].

I seriously doubt that you've unhinged from the origins of that value,
however, it bears only tangentially on the matter.

That is an antenna itself has no effect on the fundamental u and e of the
media in which it is immersed.

Wrong.
[snip]

Surely uo, eo, Zo and c (velocity of light) are fundamental and invariant
properties of "free space", no?

And some toothpaste makes our teeth whiter, no? Your reply does
nothing to answer your error, however.

[snip]
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.
[snip]

No, I'm "contracting" from outer space to near space... using the
contravarient tensor!

Then you have misapplied it, clearly. Arguing does not take the place
of easily demonstrable facts. AH! forgive me, wrong forum, arguing
is classic substitution. However, the entertainment value is rather
poorer this round.

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

Wrong.
[snip]

Oh, and here I thought that at least tiny remnants of all radiation
eventually passes through every part of space, filling all of space as it
expands throughout the Universe..

Are "thoughts" related to "beliefs?" Bloated speculations of
background radiation don't change the basic assertion that in the near
field, there is nothing that remotely approaches the presumed 377 Ohm
specification. You've both (earlier) acknowledged this and (have
since) challenged it with a semantic fog such as:

I know that Special Relativity [Maxwell's equations] is not supported in
full by General Relativity, but surely even though space is warped by mass,
superposition must still be supported. The radiation in your neighbourhood
is a superposition of suitably delayed and reduced (by path attenuation) of
all radiation, no?

EZNEC demonstrates the violation of your "beliefs," yes?

I miss Reg Edwards already :-(

Certainly you're a poor substitute for Punchinello. (and Kelvin is
winding up a pitch to wing a chunk of chalk off your noggin.)

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