E field impedance...

T

Tom Del Rosso

Guest
As the story goes, the E field starts with high impedance and it goes
down until it\'s equal to the H field impedance in the far field. It\'s
just so counter-intuitive that impedance would go down as you get
farther from the source. Is there a somewhat intuitive way to look at
that?

On another matter, I\'ve asked before about the disagreement between some
books with diagrams of E and M in phase and some books showing them 90
degrees out of phase. Now I found one source that says they\'re in phase
in the near and 90 degrees in the far.


--
 
On 2020-10-14 16:58, Tom Del Rosso wrote:
As the story goes, the E field starts with high impedance and it goes
down until it\'s equal to the H field impedance in the far field. It\'s
just so counter-intuitive that impedance would go down as you get
farther from the source. Is there a somewhat intuitive way to look at
that?

On another matter, I\'ve asked before about the disagreement between some
books with diagrams of E and M in phase and some books showing them 90
degrees out of phase. Now I found one source that says they\'re in phase
in the near and 90 degrees in the far.
Click to expand...

For a propagating wave in a lossless medium, E and H are in phase. If
the medium is isotropic, they\'re also orthogonal. In the near field it
varies depending on the situation, e.g. between a waveguide horn and a
wire antenna.

The only wave impedance I know about is sqrt(E/H).

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
 
On Wednesday, October 14, 2020 at 4:58:59 PM UTC-4, Tom Del Rosso wrote:
As the story goes, the E field starts with high impedance and it goes
down until it\'s equal to the H field impedance in the far field. It\'s
just so counter-intuitive that impedance would go down as you get
farther from the source. Is there a somewhat intuitive way to look at
that?
Click to expand...
Like the graph on page 9? here,
https://www.itu.int/en/ITU-D/Technology/Documents/Events2013/CI_Training_ARB_Tunis_April13/UIT_EMC_fundamentals.pdf
On another matter, I\'ve asked before about the disagreement between some
books with diagrams of E and M in phase and some books showing them 90
degrees out of phase. Now I found one source that says they\'re in phase
in the near and 90 degrees in the far.
Click to expand...
I have a feeling that near field there\'s a phase difference, with leading
or lagging determined by the type of source.
In the far field E and B are in phase... For a long time I had the wrong
picture of this and imagined that E and B were out of phase in the
far-field. My hand-wavy understanding of this is that E-M waves
travelling in empty space experience no time.... (I don\'t understand
the \'no time\' thing so well either. :^)

George H.

--
Click to expand...
 
Phil Hobbs wrote:
On 2020-10-14 16:58, Tom Del Rosso wrote:
As the story goes, the E field starts with high impedance and it goes
down until it\'s equal to the H field impedance in the far field. It\'s
just so counter-intuitive that impedance would go down as you get
farther from the source. Is there a somewhat intuitive way to look at
that?

On another matter, I\'ve asked before about the disagreement between
some books with diagrams of E and M in phase and some books showing
them 90 degrees out of phase. Now I found one source that says
they\'re in phase in the near and 90 degrees in the far.



For a propagating wave in a lossless medium, E and H are in phase. If
the medium is isotropic, they\'re also orthogonal. In the near field
it varies depending on the situation, e.g. between a waveguide horn
and a wire antenna.
Click to expand...

Thank you.


> The only wave impedance I know about is sqrt(E/H).

What does it even mean for a magnetic field to have impedance? Shouldn\'t
it be reluctance?
 
George Herold wrote:
On Wednesday, October 14, 2020 at 4:58:59 PM UTC-4, Tom Del Rosso
wrote:
As the story goes, the E field starts with high impedance and it goes
down until it\'s equal to the H field impedance in the far field. It\'s
just so counter-intuitive that impedance would go down as you get
farther from the source. Is there a somewhat intuitive way to look at
that?

Like the graph on page 9? here,
https://www.itu.int/en/ITU-D/Technology/Documents/Events2013/CI_Training_ARB_Tunis_April13/UIT_EMC_fundamentals.pdf
Click to expand...

Yes.

On another matter, I\'ve asked before about the disagreement between
some books with diagrams of E and M in phase and some books showing
them 90 degrees out of phase. Now I found one source that says
they\'re in phase in the near and 90 degrees in the far.

I have a feeling that near field there\'s a phase difference, with
leading or lagging determined by the type of source.
In the far field E and B are in phase... For a long time I had the
wrong picture of this and imagined that E and B were out of phase in
the far-field. My hand-wavy understanding of this is that E-M waves
travelling in empty space experience no time.... (I don\'t understand
the \'no time\' thing so well either. :^)
Click to expand...

B? Not H? I still don\'t really get the difference but I understand
some things I didn\'t get a year ago.
 
On 2020-10-17 15:24, Tom Del Rosso wrote:
George Herold wrote:
On Wednesday, October 14, 2020 at 4:58:59 PM UTC-4, Tom Del Rosso
wrote:
As the story goes, the E field starts with high impedance and it goes
down until it\'s equal to the H field impedance in the far field. It\'s
just so counter-intuitive that impedance would go down as you get
farther from the source. Is there a somewhat intuitive way to look at
that?

Like the graph on page 9? here,
https://www.itu.int/en/ITU-D/Technology/Documents/Events2013/CI_Training_ARB_Tunis_April13/UIT_EMC_fundamentals.pdf

Yes.

On another matter, I\'ve asked before about the disagreement between
some books with diagrams of E and M in phase and some books showing
them 90 degrees out of phase. Now I found one source that says
they\'re in phase in the near and 90 degrees in the far.

I have a feeling that near field there\'s a phase difference, with
leading or lagging determined by the type of source.
In the far field E and B are in phase... For a long time I had the
wrong picture of this and imagined that E and B were out of phase in
the far-field. My hand-wavy understanding of this is that E-M waves
travelling in empty space experience no time.... (I don\'t understand
the \'no time\' thing so well either. :^)

B? Not H? I still don\'t really get the difference but I understand
some things I didn\'t get a year ago.
Click to expand...

Physicists (especially those taught out of Purcell\'s electromagnetics
book) tend to concentrate on E and B, because those are the fields that
actually act on matter. D and H are sort of calculating conveniences. ;)

Maxwell\'s equations are simpler in terms of E and H, because the
free-space macroscopic curl equations are

curl E = -1/c dB/dt

curl H = 1/c dD/dt

(I like Gaussian units--sue me.) ;)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
 
Phil Hobbs wrote:
Physicists (especially those taught out of Purcell\'s electromagnetics
book) tend to concentrate on E and B, because those are the fields
that actually act on matter. D and H are sort of calculating
conveniences. ;)
Maxwell\'s equations are simpler in terms of E and H, because the
free-space macroscopic curl equations are

curl E = -1/c dB/dt

curl H = 1/c dD/dt

(I like Gaussian units--sue me.) ;)

Cheers

Phil Hobbs
Click to expand...

You should write a (cheaper) book. :)
 
On 2020-10-17 17:52, Tom Del Rosso wrote:
Phil Hobbs wrote:

Physicists (especially those taught out of Purcell\'s electromagnetics
book) tend to concentrate on E and B, because those are the fields
that actually act on matter. D and H are sort of calculating
conveniences. ;)
Maxwell\'s equations are simpler in terms of E and H, because the
free-space macroscopic curl equations are

curl E = -1/c dB/dt

curl H = 1/c dD/dt

(I like Gaussian units--sue me.) ;)

Cheers

Phil Hobbs

You should write a (cheaper) book. :)
Click to expand...

Well, there are at least an order of magnitude more electronics folks
than electro-optics folks, so amortizing the production cost is a bit
more of an issue. (Notice how cheap programming books are? There\'s a
big difference in the market size there too.)

I\'m finishing up the MS for the third edition of BEOS, and (God willing)
hope to do another one on how to do conceptual design--white boards,
photon budgets, and technical taste. Hopefully that one will go faster,
because I have a lot of archived photon budgets to riff off.

I\'ve been writing BEOS on and off since 1994--it\'s sort of like \"Dear
Diary\". ;)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
 
On Saturday, October 17, 2020 at 3:24:38 PM UTC-4, Tom Del Rosso wrote:
George Herold wrote:
On Wednesday, October 14, 2020 at 4:58:59 PM UTC-4, Tom Del Rosso
wrote:
As the story goes, the E field starts with high impedance and it goes
down until it\'s equal to the H field impedance in the far field. It\'s
just so counter-intuitive that impedance would go down as you get
farther from the source. Is there a somewhat intuitive way to look at
that?

Like the graph on page 9? here,
https://www.itu.int/en/ITU-D/Technology/Documents/Events2013/CI_Training_ARB_Tunis_April13/UIT_EMC_fundamentals.pdf

Yes.

On another matter, I\'ve asked before about the disagreement between
some books with diagrams of E and M in phase and some books showing
them 90 degrees out of phase. Now I found one source that says
they\'re in phase in the near and 90 degrees in the far.

I have a feeling that near field there\'s a phase difference, with
leading or lagging determined by the type of source.
In the far field E and B are in phase... For a long time I had the
wrong picture of this and imagined that E and B were out of phase in
the far-field. My hand-wavy understanding of this is that E-M waves
travelling in empty space experience no time.... (I don\'t understand
the \'no time\' thing so well either. :^)

B? Not H? I still don\'t really get the difference but I understand
some things I didn\'t get a year ago.
Click to expand...

Yeah, as Phil says for the physics types E and B are fundamental.
(E gives the force on a charge and B the torque on a magnetic dipole...
as well as other things.)

Personally I like Feynman\'s definition of the H field.
The Feynman lectures are free and you might enjoy Vol II chap. 36

https://www.feynmanlectures.caltech.edu/II_36.html

George H.
 
On Monday, October 19, 2020 at 10:37:50 AM UTC-4, George Herold wrote:
On Saturday, October 17, 2020 at 3:24:38 PM UTC-4, Tom Del Rosso wrote:
George Herold wrote:
On Wednesday, October 14, 2020 at 4:58:59 PM UTC-4, Tom Del Rosso
wrote:
As the story goes, the E field starts with high impedance and it goes
down until it\'s equal to the H field impedance in the far field. It\'s
just so counter-intuitive that impedance would go down as you get
farther from the source. Is there a somewhat intuitive way to look at
that?

Like the graph on page 9? here,
https://www.itu.int/en/ITU-D/Technology/Documents/Events2013/CI_Training_ARB_Tunis_April13/UIT_EMC_fundamentals.pdf

Yes.

On another matter, I\'ve asked before about the disagreement between
some books with diagrams of E and M in phase and some books showing
them 90 degrees out of phase. Now I found one source that says
they\'re in phase in the near and 90 degrees in the far.

I have a feeling that near field there\'s a phase difference, with
leading or lagging determined by the type of source.
In the far field E and B are in phase... For a long time I had the
wrong picture of this and imagined that E and B were out of phase in
the far-field. My hand-wavy understanding of this is that E-M waves
travelling in empty space experience no time.... (I don\'t understand
the \'no time\' thing so well either. :^)

B? Not H? I still don\'t really get the difference but I understand
some things I didn\'t get a year ago.

Yeah, as Phil says for the physics types E and B are fundamental.
(E gives the force on a charge and B the torque on a magnetic dipole...
as well as other things.)

Personally I like Feynman\'s definition of the H field.
The Feynman lectures are free and you might enjoy Vol II chap. 36

https://www.feynmanlectures.caltech.edu/II_36.html

George H.
Click to expand...

I just wanted to add, (echoing Feynman*) that there is a symmetry
in the electro-static and magneto-static equations when you equate
the E and H fields. Which is the source of the historic B and H
confusion. Read the above for more details.
GH

*and Purcell
 
George Herold wrote:
On Monday, October 19, 2020 at 10:37:50 AM UTC-4, George Herold wrote:
On Saturday, October 17, 2020 at 3:24:38 PM UTC-4, Tom Del Rosso
wrote:
George Herold wrote:
On Wednesday, October 14, 2020 at 4:58:59 PM UTC-4, Tom Del Rosso
wrote:
As the story goes, the E field starts with high impedance and it
goes down until it\'s equal to the H field impedance in the far
field. It\'s just so counter-intuitive that impedance would go
down as you get farther from the source. Is there a somewhat
intuitive way to look at that?

Like the graph on page 9? here,
https://www.itu.int/en/ITU-D/Technology/Documents/Events2013/CI_Training_ARB_Tunis_April13/UIT_EMC_fundamentals.pdf

Yes.

On another matter, I\'ve asked before about the disagreement
between some books with diagrams of E and M in phase and some
books showing them 90 degrees out of phase. Now I found one
source that says they\'re in phase in the near and 90 degrees in
the far.

I have a feeling that near field there\'s a phase difference, with
leading or lagging determined by the type of source.
In the far field E and B are in phase... For a long time I had the
wrong picture of this and imagined that E and B were out of phase
in the far-field. My hand-wavy understanding of this is that E-M
waves travelling in empty space experience no time.... (I don\'t
understand the \'no time\' thing so well either. :^)

B? Not H? I still don\'t really get the difference but I understand
some things I didn\'t get a year ago.

Yeah, as Phil says for the physics types E and B are fundamental.
(E gives the force on a charge and B the torque on a magnetic
dipole... as well as other things.)

Personally I like Feynman\'s definition of the H field.
The Feynman lectures are free and you might enjoy Vol II chap. 36

https://www.feynmanlectures.caltech.edu/II_36.html

George H.

I just wanted to add, (echoing Feynman*) that there is a symmetry
in the electro-static and magneto-static equations when you equate
the E and H fields. Which is the source of the historic B and H
confusion. Read the above for more details.
GH

*and Purcell
Click to expand...

That would be enough to grasp, but there\'s also the fact that one is
dependent on the core material and the other isn\'t.
 
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