# E field impedance

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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.

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Re: E field impedance
On 2020-10-14 16:58, Tom Del Rosso wrote:

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

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Dr Philip C D Hobbs
Principal Consultant
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Re: E field impedance
Phil Hobbs wrote:

Thank you.

What does it even mean for a magnetic field to have impedance? Shouldn't
it be reluctance?

Re: E field impedance
On Wednesday, October 14, 2020 at 4:58:59 PM UTC-4, Tom Del Rosso wrote:

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

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.

Re: E field impedance
George Herold wrote:

Yes.

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

Re: E field impedance
On 2020-10-17 15:24, Tom Del Rosso 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

--
Dr Philip C D Hobbs
Principal Consultant
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Re: E field impedance
Phil Hobbs wrote:

You should write a (cheaper) book. :)

Re: E field impedance
On 2020-10-17 17:52, Tom Del Rosso wrote:

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
We've slightly trimmed the long signature. Click to see the full one.
Re: E field impedance
On Saturday, October 17, 2020 at 3:24:38 PM UTC-4, Tom Del Rosso wrote:

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.

Re: E field impedance
On Monday, October 19, 2020 at 10:37:50 AM UTC-4, George Herold wrote:

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

Re: E field impedance
George Herold wrote:

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.