Forums

E field impedance

Started by Tom Del Rosso 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?

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. > >
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? >> >> 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.
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.
>> 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.
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.
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