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Why E & B fields 90 degrees?


Marvin Lukis

Jan 1, 1970
Can someone please explain why the E and B fields from the same
emitter have a 90 degree phase difference?

How can this be best visualized?



Bob Myers

Jan 1, 1970
Marvin Lukis said:
Can someone please explain why the E and B fields from the same
emitter have a 90 degree phase difference?

This may not be easy, but we'll give it a go.

I'm assuming that you mean with respect to an electromagnetic (or "EM")
wave, and that by "phase difference" you mean the polarization
of the electric and magnetic field components; if this isn't
the case, then I'm not sure what you're talking about, because
the the *phase* of these components (in terms of their
amplitude vs. time) is NOT 90 deg. apart (instead, they
are perfectly in phase).

Now, to figure out all of this through a rigorous
mathematical treatment takes some doing, and is typically
the subject of the better part of an introductory course in
electromagnetics. So we're not going to do that here. But
to visualize the situation, consider the plain ol' dipole antenna
- overall, a half-wavelength long, with the feedpoint in the
middle (which splits the antenna into two quarter-wave
elements which lie along the same line).

Such an antenna can be viewed as an open-ended
quarter-wave section of transmission line, "opened up"
(i.e., the two conductors of the line separated, forming the
two quarter-wave elements). At the feedpoint (one-quarter
wave back from the "open" end - or an "infinite impedance
load" - in the original transmission line section), we have
a very low-impedance point; this is, on that same original
line, a point of current maximum and voltage minimum.
The open end of the line - which, when "opened up" into the
dipole antenna - is a voltage maximum and current minimum
(it better be, since the load impedance there is really high,

Now visualize what this means in terms of the "standing wave"
pattern on the antenna. You have a current maxmimum point
right in the middle (at the feedpoint), and the current is moving
along the elements, right? You also have a voltage maximum
at the end points - the tips of the elements - and since the
two ends are opposing one another, what this means is a large
potential (voltage) ACROSS the antenna, end-to-end. Got it?

So think about the fields that result from this. The electric field
generated by the big potential across the antenna is aligned
with the elements of the antenna - i.e., the "field lines" would
be visualized as connecting the tips of the elements. On the
other hand, you know that magnetic fields which result from
current in a conductor are visualized SURROUNDING
that conductor - so view the magnetic field as encircling the
antenna elements.

And voila, there you are - crossed electric and magnetic
fields! Which, in this case, are being produced in such a
way that the result is an EM wave being launched from the
antenna, which was the whole idea in the first place.

Bob M.