"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

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

....Yuri will have to vent their frustrations
without me.
Roy Lewallen, W7EL

OK, we tried to have discussion about the current in loading coils, you
defended W8JI position, who used EZNEC model of coil with zero physical length
and "proved" that current is the same at both ends. You supported him by doing
"experiment" with toroid at the bottom of antenna and writing page about how
Yuri and Cecil are wrong.
We pointed out the fallacies in the way EZNEC models the coil, examples were
given how to get around using hairpin or coil modeled as a "helix" by breaking
turns into segments. Now this is incorporated in EZNEC 4.08 and surprise,
surprise - it reflects the current distribution properly, showing that there is
a difference of current in the loading coils as observed by me, measured by
W9UCW, explained by W5DXP and KB5WZI. I thanked you for implementing this
feature and pointed out another possible improvement in way the L is entered.
The result? W8JI still keeps the crap on his web site, your letter is included
there. Did we get thanks of heaven forbid apology? (You got better version,
made extra bucks as result of us being "stupid") Not that I care or want to
vent frustration. I mentioned before, I do ham radio as a hobby, when I see
crap I try to point it out for the benefit of others.
I was going to do an article, but fortunately I have a life besides radio, had
more pressing problems and this had to wait. I hope to do it soon, with some
examples using EZNEC.
I wish there was more attitude here trying to discuss the problems rather than
smart alec snotty attitude by some brother hams. If you screwed up or are
wrong, admit it, learn, give credit where is due and we will all benefit.
Nobody is perfect. But ridiculing or dumping on someone, because you (whoever)
is "superior" and you "know" better, just makes that egg in the face look more
yellow.

Thank you and good night!

Yuri, K3BU.us

Fractenna wrote:
I have the strong feeling that Chuck isn't nearly so interested in the
inner workings of EZNEC as he is in simply being as big a nuisance to me
as he can.


I see no evidence of this. Indeed, I see Chuck as being reasonable. I don't
always agree with him, nor, I presume, him with me, but so what?

Chuck is an honorable person and deserves respect. I do not view Chuck as a
competitor but as a colleague.

73,
Chip N1IR

Indeed. Which gives Chuck's claims the same legitimacy most of us
on this newsgroup ascribe to claims you make about fractal antennas.
73,
Tom Donaly, KA6RUH

"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.

Yuri wrote:
"True if---"

Good point. The fault is mine. Kraus followed his reciprocity statement
with a raft of qualifications, but I`m no typist, and Kraus` final book
deserves a place with every serious collector of antenna information.

Best regards, Richard Harrison, KB5WZI