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

On Fri, 15 Aug 2003 14:57:46 -0500, "William E. Sabin"
sabinw@mwci-news wrote:


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


Hi Bill,

The continuum of the structure presents a delay (by "moments" to use
the vernacular of MOM) that combines with all "moments" of the
previously existing and "near" separated field(s) to cause local
free-space media fluctuations in Z. At a greater distance, such
differences become trivial.

The local fields present a non-homogenous free-space media, some of
which is transparent, some of which is reflective, much of it
somewhere in between. The antenna distorts the medium it resides in
presenting much the same effect as gravity distorting the space-time
continuum. This is a leap of faith, certainly, but offers a
visualization that may be familiar. In optics it would be something
like dispersion where the structure is smaller than the wavelength
exciting it.

73's
Richard Clark, KB7QHC

EZNEC doesn't do the transformation you describe.

The following description is a very simplified version of how NEC works.
I believe the whole NEC-2 manual is available on the web, for anyone who
wants a deeper and surely more accurate explanation.

First, an impedance is calculated for each segment of each wire, and a
mutual impedance for every segment relative to every other segment. This
is done in a rather complex way by assuming that each segment has sine,
cosine, and constant currents, calculating the field from each segment
arriving at each other segment, and evaluating the current induced on
the other segment by it. These impedances are put into a matrix, then
the currents on each segment are found by solving Ohm's law in matrix
form, where the E is provided by the specified sources. Once the
currents are found, the impedance at each of the sources is known. The
field from each segment is computed from the known current and assumed
current distribution along the segment with an approximate integral
equation that's solved numerically. The impedance of the medium (fixed
at free space in NEC-2 but user selectable in NEC-4) is of course
involved in this calculation, as it is for the mutual impedance calculation.

The fields are summed to obtain the overall field (both E and H) at any
point the user specifies. Both are reported in a near field analysis
output. In a far field calculation, the distance of the observation
point to all segments is assumed to be the same, and only the E field is
calculated.

An excellent and easy to follow description of the method of moments can
be found in Kraus' _Antennas_, Second Ed. I assume it's in the third
edition also, but it's not in the first. The NEC-2 manual recommends
R.F. Harrington, _Field Computation by Moment Methods_ (McMillan, 1968)
but I haven't seen this book.

I've tried to point out on this thread that although the feedpoint
impedance is an impedance with the units of ohms, and the impedance of a
plane wave in free space also has the units of ohms, they're not the
same thing. Feedpoint impedance is the ratio of a current to a voltage.
Wave impedance, or the intrinsic impedance of a medium, is the ratio of
an E field to an H field -- it's also the square root of the ratio of
the medium's permeability to its permittivity. An antenna converts
currents and voltages to E and H fields, it doesn't just transform one
impedance to another. Hence my insistence on calling an antenna a
transducer rather than a transformer.

Any explanation of an antenna as a transformer will have to include
parasitic array elements, which have zero feedpoint impedance, and array
elements that have negative feepoint resistances.

The answer to your last question is beyond my ability to answer. It's
discussed in great detail in most electromagnetics and antenna texts.

Roy Lewallen, W7EL

William E. Sabin wrote:
William E. Sabin wrote:

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

Roy Lewallen wrote:

An excellent and easy to follow description of the method of moments can
be found in Kraus' _Antennas_, Second Ed. I assume it's in the third
edition also, but it's not in the first. The NEC-2 manual recommends
R.F. Harrington, _Field Computation by Moment Methods_ (McMillan, 1968)
but I haven't seen this book.


I'm looking for a text to help me increase my understanding of antennas
beyond what is contained in the ARRL Antenna Handbook. It looks like
"Antennas" by Kraus is it. Can anyone recommend any others?

Thanks and 73,

--
* Do NOT use Reply *
Reply only through ARRL forwarding service to K3TD

Tad, K3TD

tad danley wrote:
I'm looking for a text to help me increase my understanding of antennas
beyond what is contained in the ARRL Antenna Handbook. It looks like
"Antennas" by Kraus is it. Can anyone recommend any others?

_Antenna_Engineering_Handbook_, edited by Jasik, contributions by many.

_Antenna_Theory_Analysis_and_Design_, by Balanis
--
73, Cecil http://www.qsl.net/w5dxp



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tad danley wrote:
Roy Lewallen wrote:


An excellent and easy to follow description of the method of moments
can be found in Kraus' _Antennas_, Second Ed. I assume it's in the
third edition also, but it's not in the first. The NEC-2 manual
recommends R.F. Harrington, _Field Computation by Moment Methods_
(McMillan, 1968) but I haven't seen this book.



I'm looking for a text to help me increase my understanding of antennas
beyond what is contained in the ARRL Antenna Handbook. It looks like
"Antennas" by Kraus is it. Can anyone recommend any others?

Thanks and 73,


Kraus is not only an antenna expert, he is a
world-class authority on the entire field of
Electromagnetics, based on Maxwell's equations.
His mathematics is elegant.

Bill W0IYH

....and his writing is lucid. I read his first edition, a gift from my
Father, and knew where I wanted to go to grad school. He is also a very
fine person.
Buy and read his books.
73 Mac N8TT
--
J. Mc Laughlin - Michigan USA
Home:

"William E. Sabin" sabinw@mwci-news wrote in message
...
snip

Kraus is not only an antenna expert, he is a
world-class authority on the entire field of
Electromagnetics, based on Maxwell's equations.
His mathematics is elegant.

Bill W0IYH

Roy Lewallen wrote in message ...

I've tried to point out on this thread that although the feedpoint
impedance is an impedance with the units of ohms, and the impedance of a
plane wave in free space also has the units of ohms, they're not the
same thing. Feedpoint impedance is the ratio of a current to a voltage.
Wave impedance, or the intrinsic impedance of a medium, is the ratio of
an E field to an H field -- it's also the square root of the ratio of
the medium's permeability to its permittivity. An antenna converts
currents and voltages to E and H fields, it doesn't just transform one
impedance to another. Hence my insistence on calling an antenna a
transducer rather than a transformer.


I've agreed with you on the semantics of antennas as transducers,
but two transducers DO make a transformer.

Ohms are still always Ohms, regardless of what you are measuring.
And it's very interesting that the E and H fields have units of
Volts/meter and Ampere(turn)/meter, which when you divide one by the
other, you get basically Volts/ampere, just like you would in a
transmission line.

But I don't claim that a wave traveling in a transmission line is
the same as a wave traveling through free space.


Slick