Antenna gain question
 

This was only a mental exercise to help me visualize the concept of
gain. No resemblance to a real antenna or RF field was intended.
Thinking about it has helped me understand what antenna gain is
(assuming my conclusions are correct). And that's all it was supposed
to do. I hope it has helped someone else to do the same.

Ron

Jim Kelley wrote:


Ron wrote:

Question (repeated here for convenience):
--------------------------------------------------------------------
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.
---------------------------------------------------------------------

First, thanks for all the comments. They have helped me better
understand the answer. I am leaning toward the belief that the omni
(isotropic) antenna would capture more power and, as odd as it may
seem, would have more gain than a high gain beam (or any other
directional antenna for that matter). Here is my thinking:

This is a very unusual RF field. Usually the field is assumed to be
planar with coherent rays - then antennas behave as expected. But this
field originates uniformly from all points on the surface of a sphere.


Uniformly inward, outward, or both?

It does not spread but converges at the focal point of the sphere.


By focal point of the sphere do you mean the center of the sphere? How
big of a sphere are we talking about, and where is the antenna in
relation to the sphere?

An isotropic antenna placed at the focal point would collect all of
the rays whereas a directional antenna at would not.


Probably.

Therefore, in this particular situation, the isotropic would have
higher gain and capture more power than any directional antenna.


Not according to the accepted use of the term 'gain' in connection with
antennas.

Please correct me if I am wrong.

Ron, W4TQT


In the instance you describe, the antenna with gain will pick up less
signal than an antenna without gain. The gain antenna will be able to
sense signal arriving from only a fraction of the sphere, whereas the
isotropic antenna responds to signals arriving from the entire 4-pi
sphere. Therefore, the antenna with less gain produces the greater
signal level. But this should often be the case when a directional
antenna is pointed away from most of the signal. The omni, on the other
hand, is 'pointed toward' this particular signal in all directions.

Out of curiosity, what kind of signal source are you interested in?

ac6xg

Antenna gain question
 

Ron wrote:
Please correct me if I am wrong.

If the moon were made out of green cheese, then a
cow could jump over it.

That is a true statement, by definition.
--
73, Cecil http://www.qsl.net/w5dxp

Antenna gain question
 

Richard Clark wrote:
An orbital electron only radiates when it changes orbital levels to a
LOWER orbit.

His question was about a conductive loop RF antenna.
RF radiation from an antenna comes from free electrons. Their
associated photon energy levels are correlated to the frequency
of the excitation energy, not to changes in atomic orbits.
--
73, Cecil http://www.qsl.net/w5dxp

Antenna gain question
 

Hi Richard

Thank you for your answer.

You are quite right Richard, this is the great postulate of Bohr, but
from:

physics.indiana.edu/~sg/p641/chap2.ps

I have extracted this paragraph (rememember that I did study from books
in spanish, so I had to search the web to find a suitable english
references.)

"...Some physicists of the time did not at first believe in
Rutherford's model
of the atom because, classically, such a model is unstable. The
argument is as
follows: Electrons in orbits around a nucleus undergo acceleration. But
accelerating charged particles emit electromagnetic radiation, losing
energy. Therefore,
electrons orbiting a nucleus should lose energy and spiral into
nucleus.
Then in 1913 Bohr solved the problem by simply postulating that
electrons
move around the nucleus in certain stationary orbits without emitting
radiation.
According to Bohr¡s model electrons emit radiation as photons only
when making
a transition from one stationary orbit with a higher energy to another
stationary
orbit with a lower energy."

Bohr's postulate would not invalidate the postulate of the classic
electromagnetism that a accelerated charge irradiates energy in
classical conditions. Their postulate refers to an electron in an
atomic orbital, a typical quantum situation, the electrons in a
circular wire would be being part of a classic macroscopic system, (I
think).
In such a case, would not emit energy, according to that pointed out in
the cited Indiana's university text ?

I can think a more elaborated example of Direct Current to avoid
derived issues of the electronic movement in a complicated crystalline
structu An electron (or current of electrons) inside a magnetic
field in the vacuum, just as in a cyclotron...

Would it be rightfully an Direct Current?.
Does it fulfill the postulates of the classic physics?
Does it irradiate electromagnetic energy?

For these reasons, I think that DC irradiates electromagnetic energy (a
very, very, very little quantity, of course. This ponderings rises in
my mind from a friend's question about an eternal current in a
superconductor ring.

What do you think about?

Thank you very much for your corrections to my translations...

Miguel Ghezzi (LU 6ETJ)
---------------------------------------------------------------------------------------------------------

Hola Richard.

Gracias por tu respuesta.

Tienes razon Richard, ese es el gran postulado de Bohr, pero de:

physics.indiana.edu/~sg/p641/chap2.ps

Extraje este parrafo (recuerda que yo estudie de libros en espaniol de
manera que tuve que buscar en la web para encontrar referencias
adecuadas en ingles).

[Original english text not translated...]

El postulado de Bohr no invalidaría los postulados del
electromagnetismo clasico que una carga acelereada irradia energia en
condiciones clasicas. su postulado se refiere a un electron en un
orbital atomico, una típica situacion cuantica. Los electrones en un
conductor circular estarían formando parte de un sistema macroscopico
clasico, creo.
En tal caso, no emitirian energia de acuerdo a lo senialado en el texto
de la Universidad de Indiana citado?
Puedo pensar en un ejemplo mas elaborado de corriente continua para
evitar las cuestiones derivadas del movimiento electronico en una
complicada estructura cristalina: Un electron (o corriente de
electrones) dentro de un campo magnetico en el vacio, como en un
ciclotron.

Sería legitimamente una corriente conínua?
Cumple con los postulados de la fisica clasica?
Irradia energia electromagnetica?

Por estas razones creo que una CC irradia energia electromagnetica (una
muy, muy, muy pequenia cantidad, por supuesto). Estas cavilaciones
surgen en mi mente a partir de la pregunta de un amigo acerca de una
corriente eterna en un anillo superconductor.

Que piensas?

Miguel Ghezzi (LU 6ETJ)

Antenna gain question
 

On 31 Oct 2005 19:02:35 -0800, "lu6etj" wrote:
What do you think about [it]?

Thank you very much for your corrections to my translations...

Hi Miguel,

You do very well with English.

If you wish to be very, very, very specific, then yes DC will radiate.

Let's take something a little more practical than a cyclotron - the
monitor you are looking at to read this. The CRT is accelerating an
electron to strike a phosphor to illuminate a pixel. We will neglect
that light radiation as not being part of the discussion.

That electron will be accelerated by a field of some 20KV. What is
its frequency?
34.6 exaHertz
the wavelength of roughly 1/10 the distance of an electron orbit
around a Hydrogen atom.

Now, for something completely different. Does a car driving 40
KM/Hour down the road radiate? If you wish to be very, very, very
specific, then yes the car will radiate by the same principle at
5 zetta-yottaHertz

73's
Richard Clark, KB7QHC

Antenna gain question
 

One more thing: Before thinking about all this, I always thought that
since a high gain antenna has a narrower beam than a lower gain
antenna, the high gain antenna "sees" a smaller part of the incoming
field. I now believe this is wrong. The higher gain antenna sees a
larger field area. But as the antenna is rotated the sum of all the
rays decreases faster than if there were fewer of them. This is
probably due to the rays from the outer edge of the field causing a
faster decrease in the coherent summation of all rays than the closer
in rays. Of course, as the rotation is continued, many (but not as
many) of the rays add coherently again, giving rise to the side lobes.

Ron, W4TQT

Ron wrote:
This was only a mental exercise to help me visualize the concept of
gain. No resemblance to a real antenna or RF field was intended.
Thinking about it has helped me understand what antenna gain is
(assuming my conclusions are correct). And that's all it was supposed to
do. I hope it has helped someone else to do the same.

Ron

Antenna gain question
 

On Tue, 01 Nov 2005 16:09:22 GMT, Ron wrote:

I now believe this is wrong.

Hi Ron,

Such is the problem of your scenario if it lead you here.

73's
Richard Clark, KB7QHC

Antenna gain question
 

Hi Richard

Tanks for your last answer.

Hi hi, well, my friend was very, very very insistent also with his
superconducting and eternal current holder "golden ring"...

I will try learn more about [it] ;) (I have been hard working for
thirty years, and till now, I haven't any time to meditate in all these
beautiful things).

I am very pleased to know you, Richard (my other ham friends it isn't
"very interested" in this type of "exotic questions"... :)

I can see this usenet group always talk about very interesting topics,
with great knowledge and good "ham spirit".

73's

Miguel Ghezzi (LU 6ETJ)

Antenna gain question
 

Ron wrote:

This was only a mental exercise to help me visualize the concept of
gain. No resemblance to a real antenna or RF field was intended.
Thinking about it has helped me understand what antenna gain is
(assuming my conclusions are correct). And that's all it was supposed to
do. I hope it has helped someone else to do the same.

Ron

I think it was a good exercise, Ron - not unlike the kind seen in a good
text book. My response at the bottom presumed some things about the
nature of the sphere that were somewhat unclear in your message. I hope
I presumed correctly.

73, ac6xg

Jim Kelley wrote:



Ron wrote:

Question (repeated here for convenience):
--------------------------------------------------------------------
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.
---------------------------------------------------------------------

First, thanks for all the comments. They have helped me better
understand the answer. I am leaning toward the belief that the omni
(isotropic) antenna would capture more power and, as odd as it may
seem, would have more gain than a high gain beam (or any other
directional antenna for that matter). Here is my thinking:

This is a very unusual RF field. Usually the field is assumed to be
planar with coherent rays - then antennas behave as expected. But
this field originates uniformly from all points on the surface of a
sphere.



Uniformly inward, outward, or both?

It does not spread but converges at the focal point of the sphere.



By focal point of the sphere do you mean the center of the sphere?
How big of a sphere are we talking about, and where is the antenna in
relation to the sphere?

An isotropic antenna placed at the focal point would collect all of
the rays whereas a directional antenna at would not.



Probably.

Therefore, in this particular situation, the isotropic would have
higher gain and capture more power than any directional antenna.



Not according to the accepted use of the term 'gain' in connection
with antennas.

Please correct me if I am wrong.

Ron, W4TQT



In the instance you describe, the antenna with gain will pick up less
signal than an antenna without gain. The gain antenna will be able to
sense signal arriving from only a fraction of the sphere, whereas the
isotropic antenna responds to signals arriving from the entire 4-pi
sphere. Therefore, the antenna with less gain produces the greater
signal level. But this should often be the case when a directional
antenna is pointed away from most of the signal. The omni, on the
other hand, is 'pointed toward' this particular signal in all directions.

Out of curiosity, what kind of signal source are you interested in?

ac6xg

Antenna gain question
 

Ron wrote:

One more thing: Before thinking about all this, I always thought that
since a high gain antenna has a narrower beam than a lower gain antenna,
the high gain antenna "sees" a smaller part of the incoming field. I now
believe this is wrong. The higher gain antenna sees a larger field area.

Hopefully no one else was led to that belief by the exercise.

But as the antenna is rotated the sum of all the rays decreases faster
than if there were fewer of them. This is probably due to the rays from
the outer edge of the field causing a faster decrease in the coherent
summation of all rays than the closer in rays. Of course, as the
rotation is continued, many (but not as many) of the rays add coherently
again, giving rise to the side lobes.

Such a claim might be remotely plausible were it not for the fact that
rotating a directional antenna does not "coherently sum all the rays".
That's where the argument completely falls to the ground, as Monty
Python might say.

ac6xg

Ron, W4TQT

Ron wrote:

This was only a mental exercise to help me visualize the concept of
gain. No resemblance to a real antenna or RF field was intended.
Thinking about it has helped me understand what antenna gain is
(assuming my conclusions are correct). And that's all it was supposed
to do. I hope it has helped someone else to do the same.

Ron