# E field impedance

Started by October 14, 2020
```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?

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.
>
>

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?
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.
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.

>
>
> --

```
```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?
>>
>> 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.

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

Yes.

>> 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.

```
```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.
>
>>> 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.

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

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. :)

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.

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.

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
```