[Antennas] Poynting, E, and H vectors

[David Kelley]

> From this, I would say that you and I agree that the EH antenna is magical
>as no one can really explain it.  Using present equations and Poynting
>vector does not.  Then is it wrong?  Or is it right?  We will find out
>later.  If it works then an explaination shall have to be devised.
>
>That poynting vector is magic even though it (a vector) does point.  I have
>not fuly grasped it so it must be magic.

I have been following this discussion a bit and
thought it might be helpful to clarify a few things.

The EH literature, and these discussions, refer
often to the Poynting vector.  The Poynting vector
is merely a mathematical way to describe the
magnitude and direction of the power carried by
an electromagnetic wave.  The Poynting theorem
explains that, in order to calculate the Poynting
vector at a given point in space, you need to
know the magnitude and direction of the electric
and magnetic fields at that point.  It's the
Poynting theorem that tells us that the power
radiated by an antenna propagates directly away
from the antenna.

By the way, "Poynting" is not a misspelling of
the word "pointing," although the pun turns out
to carry a lot of physical significance!  It's in
honor of the late 19th century English physicist
John H. Poynting.  Bet he got teased a lot in
school, huh?

>Why do some call a small loop
>antenna a magnetic loop?  I thought that all
>antennas had both the E and H vectors?

Yes, all antennas do produce an electric (E)
and magnetic (H) field.  In fact, by Maxwell's
equations we know that E and H are intimately
related.  If you know the E field radiated by an
antenna, you can find the H field, and vice versa.
In most antenna analysis, the electric field is
found using some kind of problem solution
technique, and then one of Maxwell's equations
is used to find the magnetic field.  The magnetic
loop gets its name partly because it's easier to
calculate the magnetic field first for that antenna
and then use another of Maxwell's equations to
find the electric field.

The problem with the "theory" offered by the
EH antenna's designer(s) is that they claim
to be able to control the E and H fields
separately.  This is simply impossible in the
case of time-varying fields, which the fields
radiated by an antenna are.  (They vary
sinusoidally with time in the case of CW
transmission.)

I hope these notes help to clarify the issue.

73,
Dave NB4J


--
David F. Kelley, Ph.D.
Assistant Professor
Electrical Engineering Dept.
Bucknell University
Lewisburg, PA 17837
(570) 577-1313
dkelley@bucknell.edu