Ron wrote:
Maybe I should restate my question. Assume a receiving antenna is in the
center of a sphere and the received signal is coming in equal amounts
from all points on the surface of the sphere. Which receiving antenna
would capture more power, an omni or a high gain beam? There are no
noise and no losses.

They'll intercept equal amounts, assuming both are lossless. The
directional antenna will intercept a larger fraction than the isotropic
antenna in the directions it favors, and less in others. The total will
be be the same.

In reverse, this is equivalent to calculating the average gain of the
antennas, which is the same for all lossless antennas.

Roy Lewallen, W7EL

Roy Lewallen, W7EL wrote:

They'll intercept equal amounts, assuming both are lossless. The
directional antenna will intercept a larger fraction than the isotropic
antenna in the directions it favors, and less in others. The total will
be be the same.

In reverse, this is equivalent to calculating the average gain of the
antennas, which is the same for all lossless antennas.


Roy Lewallen, W7EL



Hello to all, my name is Miguel, LU 6ETJ.

It is a pleasure to read this group. This topic is really an
interesting question...

Modestly I would want to point out the following thing:

Imagine an inner radiant spherical surface with a finite and uniform
density radiant energy.
Aim a directional antenna with a directivity of, for example, one
stereoradian on any direction.
How is it able to such an antenna to receive equal quantity of energy
of a smaller portion of the sphere than an antenna that is able to
receive the energy taken place by the entirety sphere?

73´s of Miguel Ghezzi (LU 6ETJ)

Untranslated text for reference (my written english is a little poor
;( ):

Hola a todos, mi nombre es Miguel LU 6ETJ.

Es un gusto leer este grupo. Este tópico es realmente una interesante
pregunta...

Modestamente desearía señalar lo siguiente:

Imagine la parte interior de una esfera radiante, con una densidad de
energia radiante finita y uniforme.
Apunte un antena direccional con una directividad de, por ejemplo, un
estéreo radian en cualquier dirección.
¿Cómo puede tal antena recibir igual cantidad de energía de una
porción menor de la superficie radiante, que una antena que es capaz
de recibir la energía producida por la totalidad de la esfera?

73´s de Miguel Ghezzi (LU 6ETJ)

On 30 Oct 2005 10:41:53 -0800, "lu6etj" wrote:

Imagine an inner radiant spherical surface with a finite and uniform
density radiant energy.
Aim a directional antenna with a directivity of, for example, one
stereoradian on any direction.
How is it able to such an antenna to receive equal quantity of energy
of a smaller portion of the sphere than an antenna that is able to
receive the energy taken place by the entirety sphere?

Hi Miguel,


You are quite right. Only an isotropic antenna can take all the
energy as only an isotropic could have transmitted it. The uniform
distribution and the spherical geometry force this solution even
though it is a practical impossibility.

The real question is, how did that energy get turned around to come
back?

73's
Richard Clark, KB7QHC

Richard Clark wrote:
The real question is, how did that energy get turned around to come
back?

A conductive Dyson's sphere?
--
73, Cecil http://www.qsl.net/w5dxp

Would it be possible that the question went by the following thing?

When we study directional or isotropic receiving antennas, we assume
for example:

Punctual sources generating spherical wave fronts (convex) or
infinitely far away punctual sources creating plane wave fronts for all
the practical effects.
Under these conditions the receiving antennas are "outer" of the
radiant sphere; this way, the effective area of a directional antenna
represents a bigger external surface and it intercepts more energy than
the corresponding to an isotropic antenna, then everything agrees with
what we have learned on the directivity of the antennas, but in this
example the conditions are inverted, now we don't have plane or convex
fronts, we have concave fronts. The solution under the new conditions
is different from the habitual one...

I think that the environment of the problem is similar that of the
Kirchhoff law of thermal radiation: "a small sphere inside a radiant
sphere".
I also think that the conditions of this problem could be similar (and
therefore taken place artificially) to those of light`s receiver inside
a luminous sphere.
In this case we proceed as when we study the entropía of an isolated
system, in such a system the entropía can diminish, although that is
not possible for the whole universe (I suppose this allows me to escape
elegantly of Richard's question... ; D

73´s for all, and thank you very much for your very interesting and
instructive habitual postings.

Miguel Ghezzi (LU 6ETJ)

Spanish text for reference (withouts my translation errors).

¿Sería posible que la cuestion pasara por lo siguiente?:

Cuando estudiamos antenas receptoras direccionales o isotropicas
asumimos por ejemplo:

Fuentes puntuales generando frentes de onda esféricos (convexos) o
fuentes puntuales infinitamente alejadas que producen frentes de onda
planos para todos los efectos practicos.
En estas condiciones las antenas receptoras están "fuera" de la esfera
radiante; así, el área efectiva de una antena direccional representa
una superficie exterior mayor e intercepta más energía que la
correspondiente a una antena isotrópica, entonces todo concuerda con
lo que hemos aprendido sobre la directividad de las antenas, pero en
este ejemplo las condiciones se invierten, ahora no tenemos frentes
planos o convexos, sino de frentes cóncavos. La solución en las
nuevas condiciones es diferente de la habitual...

Pienso que el entorno del problema es parecido al de la ley de
Kirchhoff de la radiación térmica: "una pequeña esfera dentro de una
esfera radiante".
También pienso que las condiciones de este problema pudieran ser
similares (y por lo tanto producidas artificialmente), a las de un
receptor de luz dentro de una esfera luminosa.
En este caso procedemos como cuando estudiamos la entropía de un
sistema aislado, en tal sistema la entropía puede disminuir, aunque
eso no sea posible para el universo entero (supongo que eso me permite
huir elegantemnnte de la pregunta de Richard ;D

73's para todos y muchas gracias por sus interesantes e instructivos
escritos habituales.

Miguel Ghezzi (LU 6ETJ)

On 30 Oct 2005 15:50:50 -0800, "lu6etj" wrote:

although that is
not possible for the whole universe (I suppose this allows me to escape
elegantly of Richard's question... ; D

73=B4s for all, and thank you very much for your very interesting and
instructive habitual postings.

Hi Miguel,

Can there be an escape? Ron's question was posed with an impossible
proposition. A collapsing sphere of electromagnetic energy? This has
so many so many fantastic presumptions built in.

I hope this translates well for you (it does not make sense in any
language).

73's
Richard Clark, KB7QHC

Hello Richard (my middle name is Ricardo = Richard. We are
"Tocayos"...)

Richard wrote:

Can there be an escape? Ron's question was posed with an impossible
proposition. A collapsing sphere of electromagnetic energy?

Ron wrote:

Assume an incoming rf signal has exactly the same strength in all 3
dimensions i.e., completely omnidirectional.

If I have understood well Ron's question...

What about a number tending (spreading?, going to? - a limit, as in
calculus ) to infinite, of coherent punctual electromagnetic identical
sources on the inner surface of a sphere with testing antennas in
center of it? (I think it isn't necessary neither coherence or
identicals sources. Noise sources fix well in my interpretation of the
concept that (I believe) Ron it wanted expose to us).

Can it these conditions to be thought?
Could it be simulated with an electromagnetic CAD as FEMLAB?
Are they agree with Ron question?

What do you say Ron?

Miguel Ghezzi (LU 6ETJ)

PS: I have another physical doubt, if you can help me.
Can a real DC current radiate electromagnetic energy?.
It is not captious or cheat question. I think yes, but I don't want to
condition your answers with my hypotesis.

Thank you in advance for your answers.
-------------------------------------------------------------------------------------------------------------------
Richard escribió:

Puede haber una salida?. La pregunta de Ron fue presentada con
proposicion imposible. Una esfera de energía electromagnetica colapsandose?

Ron escribio:

Asuma una señal de rf entrante que tiene exactamente la misma intensidad en las tres dimensioes, por ej, que es completamente omnidireccional.

Si he comprendido bien la pregunta de Ron...

Que tal un numero tendiendo a infinito de fuentes electromagneticas
puntuales identicas, situadas sobre la superficie interior de una
esfera y las antenas de prueba en el centro de la misma? (creo que no
es necesaria ni coherencia ni fuentes identicas, fuentes de ruido van
bien en mi interpretacion del concepto que (creo) que Ron quiso
presentarnos.

Pueden estas condiciones ser pensadas?
Podrian simularse en un CAD de electromagnetismo, tal como FEMLAB?
Estan ellas de acuerdo con la pregunta de Ron?

Que dices tu Ron?

Miguel Ghezzi (LU 6ETJ)

PS: Tengo otra duda física si ustedes pueden ayudarme. Puede una CC
real irradiar energía electromagnetica?
No es una pregunta capciosa ni una broma. Yo creo que si, pero no
quiero condicionar sus respuestas con mi hipotesis.

Agradezco sus respuestas por adelantado.

On 31 Oct 2005 09:35:09 -0800, "lu6etj" wrote:

PS: I have another physical doubt, if you can help me.
Can a real DC current radiate electromagnetic energy?.
It is not captious [capricious] or cheat [trick] question. I think yes, but I don't want to
condition your answers with my hypotesis.

Hi Miguel,

A "real" DC current?

Yes.

A "real" DC current (at some point in time) starts - and stops. It is
at each of these two points that the step change offers radiation. The
"time" it takes to go from one level to the other defines that
frequency, and its harmonics.

A "perfect" DC current has always been on, and will always be on. No
change, no radiation.

73's
Richard Clark, KB7QHC

Richard Clark wrote:

On 30 Oct 2005 15:50:50 -0800, "lu6etj" wrote:


although that is
not possible for the whole universe (I suppose this allows me to escape
elegantly of Richard's question... ; D

73=B4s for all, and thank you very much for your very interesting and
instructive habitual postings.


Hi Miguel,

Can there be an escape? Ron's question was posed with an impossible
proposition. A collapsing sphere of electromagnetic energy? This has
so many so many fantastic presumptions built in.

On all but the smallest of scales, the sky is quite uniform in its
luminosity. It can hardly be described as a "collapsing sphere"; not
even from the point of view of a geocentric model of the universe.

ac6xg

On Mon, 31 Oct 2005 10:16:07 -0800, Jim Kelley
wrote:

On all but the smallest of scales, the sky is quite uniform in its
luminosity. It can hardly be described as a "collapsing sphere"; not
even from the point of view of a geocentric model of the universe.

Hi Jim,

Perhaps not, but "quite uniform" is rather in the eye of the beholder.
When I take panagraphic photographs (a broad scale), it is quite
evident that the uniformity is not very uniform. Another variable is
that polarization is not very uniform either (which, photographically
may be saying the same thing).

The eye is a wonderful device, but not very precise.

73's
Richard Clark, KB7QHC