actually i would expect that a change in E/H would change the driving point
impedance and also the performance of the antenna. some possible examples
that show this effect are the changes in element sizes when modeling an
antenna printed on a dielectric circuit board material or sandwiched in a
dielectric media. the change in wire length due to insulation is another
example, the dielectric properties of the insulation change the E/H ratio
near the wire. some examples may be found in many electromagnetics texts,
look at things like dielectric waveguides, or dielectrics in waveguides,
wires in dielectric media. even the detailed calculation of fields within a
dielectric filled coaxial cable should show this effect, change the
dielectric and you change the characteristic impedance... a measurable
effect from changing the 'space impedence' between the wires.

"Roy Lewallen" wrote in message
...
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.) The integral doesn't need to be taken slightly outside
the surface of the antenna, but can be any closed surface enclosing the
antenna. There's no necessity for E/H, or the real part of E/H, to be
constant in order to have the integral of E X H be constant.

The driving point impedance of the antenna depends on where you drive
it, and it bears no relationship I know of to the wave impedance (which
is, I assume, what you mean by "resistive space impedance") close to the
antenna. If you find any published, modeled, measured, or calculated
support for that contention, I'd be very interested in it.

Roy Lewallen, W7EL

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