William E. Sabin wrote:
Roy Lewallen wrote:

In the fourth paragraph, you say that "real power is in the real part
of the impedance", and in the last, that it's "found by integrating
the Poynting vector slightly outside the surface of the antenna". The
impedance is E/H, the Poynting vector E X H. Clearly these aren't
equivalent.

The radiated power is, as you say, the integral of the Poynting vector
over a surface. (And the average, or "real", radiated power is the
average of this.)


Correction "real part of Poynting vector" noted.

The problem remains:

How is the *real* part of the antenna input impedance, regardless of how
it is fed and regardless of what kind of antenna it is, get
"transformed" to the *real* 377 ohms of free space?

I believe (intuitively) that the reactive E and H near-fields
collaborate to create an impedance transformation function, in much the
same way as a lumped-element reactive L and C network. In other words,
energy shuffling between inductive and capacitive fields do the job and
the E and H fields modify to the real values of free space. The details
of this are murky, But I believe the basic idea is correct.

Bill W0IYH


For example, consider an EZNEC solution to an
antenna, say a 50 ohm dipole. The far-field 377
ohm solution provided by the program is precisely
the field that I am thinking about. How does
EZNEC, with its finite-element, method-of-moments
algorithm, transform a 50 ohm dipole input
resistance to 377 ohms in free space?

I don't want the equations, I want a word
description (preferably simple) of how EZNEC
performs this magic.

The far-field E and H fields are different from
the near-field E and H fields. What is going on?

Bill W0IYH