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.

Ron

Ron wrote:

Assume an incoming rf signal has exactly the same strength in all 3
dimensions i.e., completely omnidirectional. Question: would an antenna
having gain capture any more signal power than a completely
omnidirectional antenna with no gain?

Ron, W4TQT

On Sat, 29 Oct 2005 00:21:02 GMT, Ron wrote:

Which receiving antenna would capture more power, an omni or a high gain beam?

An isotropic antenna would be the best, as the field described came
from the same emitter (such a field would be impossible otherwise).
Stepping out of this enigma (that the emitter and detector are
different, and the field which could only be generated by an isotropic
would then suddenly turn and come back) would answer the omni.

73's
Richard Clark, KB7QHC

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.

Are you trying to receive the background radiation left over
from the big bang? That's the only source outside of the
sphere that I know of that can accomplish your boundary
condition.
--
73, Cecil http://www.qsl.net/w5dxp

No, I'm just trying to understand antenna gain.

Ron

Cecil Moore wrote:
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.

Are you trying to receive the background radiation left over
from the big bang? That's the only source outside of the
sphere that I know of that can accomplish your boundary
condition.

I think Cecil was being facetious :-)

Ron wrote:
No, I'm just trying to understand antenna gain.

Ron

Cecil Moore wrote:

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.


Are you trying to receive the background radiation left over
from the big bang? That's the only source outside of the
sphere that I know of that can accomplish your boundary
condition.

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)