Roy Lewallen wrote in message
...
Chuck wrote:
[... ]

The extended thin wire kernel is invoked automatically when the model is
such that it's needed, according to the criteria given in the NEC
manual.

Which states: "Transmission lines interconnecting
parts of an antenna may be modeled either
explicitly by including the transmission wires in the
thin-wire model, OR implicitly by the method
described in the preceding section for nonradiating
networks." Then goes on to explain the implicit
model, followed by: "The implicit model is limited,
however, in that it neglects interaction between
the transmission line and its environment." (Which
is what's implied for a transmission line in the
EZNEC manual.)

When is the thin-wire model invoked?

What is your criteria for this 'automatic' choice?

Continuing: "This approximation is justified if the
currents in the line are balanced, i.e., in a log
periodic dipole antenna, and in general if the
transmission lies in an electric symmetry plane.
The balance can be upset, however, if the
transmission line is connected to an unbalanced
load or by unsymmetrical interactions. If the
unbalance is significant, the transmission line
can be modeled by including the wires in the
thin-wire model."

In the Raibeam design, the load is unbalanced
due to its "plumbers delight" construction and the
resultant matching networks. From this, one
would naturally assume the interconnecting
phasing line must be included in the thin-wire
model.

How does one determine if this is the case or
not?

Why is the user left unable to make this choice
independently?

And BTW, how does one model a receiving
antenna in EZNEC?

you leave me wondering if,
perhaps, some concepts simply lay
beyond your ability to visualize.

Indeed. I also have trouble with Chi, the healing power of crystals,
astrology, homeopathy, reflexology, phrenology, and water witching.

Does this include action-at-a-distance
as well?

Chuck, WA7RAI

Roy Lewallen, W7EL

On Mon, 18 Oct 2004 16:16:32 -0700, "Chuck"
wrote:

How does one determine if this is the case or
not?

Hi Chuck,

Model the structure explicitly.

73's
Richard Clark, KB7QHC

Chuck, WA7RAI wrote:
"wrote:
"And BTW, how does one model a revceiving antenna in EZNEC?"

No need. Antennas behave the same when receiving as when transmitting.
So if you know how an antenna behaves when transmitting, you also know
how it behaves when receiving.

Best regards, Richard Harrison, KB5WZI

"Richard Harrison"
Antennas behave the same when receiving
as when transmitting. So if you know how
an antenna behaves when transmitting, you
also know how it behaves when receiving.

A receiving antenna exposed to the far field, receives plane waves
(essentially flat wavefront). If you 'play the film backwards', does a
transmitting antenna emit plane waves?

;-)

A receiving antenna exposed to the far field, receives plane waves
(essentially flat wavefront). If you 'play the film backwards', does a
transmitting antenna emit plane waves?

;-)


They end up that way.

Reciprocity is a fundamental aspect of antenna theory.

73,
Chip N1IR

A receiving antenna exposed to the far field,
receives plane waves (essentially flat wavefront).
If you 'play the film backwards', does a
transmitting antenna emit plane waves?

"Fractenna"
They end up that way.

Soooooo... ...the wavefront in the nearfield is now flat smirk so that it
matches the flat wavefront in the receive case ?

Reciprocity is a fundamental aspect of antenna theory.

Only if someone can explain the above discrepancy.

It seems apparent that one might arrange an array to exploit the difference
between the flat (far field) incoming wavefront in the receiver mode and the
obviously-curved wavefront (near field) being emitted in the transmit case
to produce significantly different macro performance characteristics.

In other words, playing the film backwards doesn't work.

On Mon, 18 Oct 2004 21:40:32 -0300, "Bob McBeth"
wrote:
Only if someone can explain the above discrepancy.

It seems apparent that one might arrange an array to exploit the difference
between the flat (far field) incoming wavefront in the receiver mode and the
obviously-curved wavefront (near field) being emitted in the transmit case
to produce significantly different macro performance characteristics.

In other words, playing the film backwards doesn't work.

Does it say I am the walrus?

Hi Bob,

Obviously you've been subjected to a dumbed down attitude in response
to your question.

The fact of the matter is that the receive antenna does not sense a
flat wave approaching it because the antenna distorts the continuum
around it. The antenna and the medium out to several wavelengths is
NOT a characteristic Z of 377 Ohms as seen in an undisturbed field.
I've offered this treatment to Thierry, so it seems it would be useful
for you to observe as well at:
http://home.comcast.net/~kb7qhc/ante...elds/index.htm
with a specific example for a monopole at:
http://home.comcast.net/~kb7qhc/ante...ical/index.htm

As you may observe, the medium surrounding the antenna is anything but
consistent. In effect, the medium and the antenna present an RF lens;
and as you may well appreciate, a lens distorts paths to optimize for
a use. This distortion is like pressing into a bowl of jello, that
medium may have been consistent in the beginning, but with the applied
pressure, the near regions to the disturbance present new surfaces and
densities. Same goes for an antenna - transmitting or receiving.

73's
Richard Clark, KB7QHC

"Richard Clark"
Does it say I am the walrus?

No, although another infamous party to this thread bears a striking
resemblance (including precious 'ivory towers' [tusks], thick blubber and a
tough hide)...

http://images.google.com/images?q=Odobenus+rosmarus


To clarify things, lets go out 27.3 wavelengths from antenna A and mark an
X. When antenna A is in receiving mode, the wavefront at X is essentially
flat because antenna B is in the next county. When antenna A is in transmit
mode, the wavefront at X is noticeably curved with a radius of something
around 27.3 lambda. In other words, things are different even if the
pathloss is the same in either direction.

I believe that it would be possible to design an antenna system (an array, a
reflector system, or something with an RF lens) that took advantage of the
difference between 'flat' versus 'curved' to produce an antennas system that
had different pathloss in different directions (where antenna B was
something simpler).

No sense arguing about it. Someone has to produce an example.

The ball is NOT in your court.

A receiving antenna exposed to the far field,
receives plane waves (essentially flat wavefront).
If you 'play the film backwards', does a
transmitting antenna emit plane waves?

"Fractenna"
They end up that way.

Soooooo... ...the wavefront in the nearfield is now flat smirk so that it
matches the flat wavefront in the receive case ?

Reciprocity is a fundamental aspect of antenna theory.

Only if someone can explain the above discrepancy.

It seems apparent that one might arrange an array to exploit the difference
between the flat (far field) incoming wavefront in the receiver mode and the
obviously-curved wavefront (near field) being emitted in the transmit case
to produce significantly different macro performance characteristics.

In other words, playing the film backwards doesn't work.



Bob,

When I was a lil squirt, we had something called a 'ripple tank'. It showed how
the film works backwards.

73,
Chip N1IR

Chip,
N1IR wrote:
"They end up that way (as plane waves at a great distance from the
radiator). Reciprocity is a fundamental aspect of antenna theory."

Dr. Cohen is, of course, correct.

Dr. John D. Kraus says in his 3rd edition of "Antennas" on page 439:
"If an emf is applied to the terminals of antenna A and the current
measured at the terminals of another antenna B, then an equal current
(in both amplitude and phase) will be obtained at the terminals of
antenna A if the same emf is applied to the terminals of antenna B."

Best regards, Richard Harrison, KB5WZI