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


William E. Sabin wrote:


There seems to more explanation needed.

If a lossless dipole is loaded with 100 W of *real* power, that is the
real power in the far field, and it is also the real power very close
to the antenna, regardless of the type of antenna.

The value of real power is the same everywhere.

Since real power is in the real part of the impedance, then how does
the value of real impedance (not the magnitude of impedance) vary with
distance from the antenna?

It seems that very close to (but slightly removed from) the antenna
the real part of the resistive space impedance is nearly the same as
the real part of the driving point impedance of the antenna. This real
part is then transformed to 377 ohms (real) within the near field,
suggesting that the open space adjacent to the antenna performs an
impedance transformation. The near-field reactive fields perform this
function in some manner.

The real power radiated is found by integrating the Poynting vector
slightly outside the surface of the antenna, and is equal to the real
power into the (lossless) antenna. This value is constant everywhere
beyond the antenna.

Bill W0IYH