E field impedance

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

Like the graph on page 9? here,

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.

Thank you.

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

Yes.

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

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

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

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

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.