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

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

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

Please correct me if I am wrong.

Ron, W4TQT

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

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

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

Please correct me if I am wrong.

Well, for one thing, your model assumes something which does not and
cannot exist. It assumes the existence of an actual isotropic
antenna. Such cannot actually be constructed - there's no way to get
a truly omnidirectional radiation pattern without violating Maxwell's
equations.

I suspect that you'll find the same problem existing, in the reverse
direction, if you try to construct the sort of RF field you're talking
about. If you try to specify the E-plane and H-plane field components
for a uniform, arriving-from-all-points-of-a-sphere field, I believe
that you'll find that you can't achieve your goal: there will always
be "seams" (abrupt discontinuities or cancellations) in the field
components in some directions.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

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

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

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

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

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

Jim Kelley wrote:
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".

Seems to me, a receiving Yagi causes constructive interference
in the forward direction and destructive interference in the
rearward direction.
--
73, Cecil http://www.qsl.net/w5dxp

Cecil Moore wrote:

Jim Kelley wrote:

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".


Seems to me, a receiving Yagi causes constructive interference
in the forward direction and destructive interference in the
rearward direction.

But does it seem the antenna causes destructive interference when the
forward direction of the radiation is toward the rearward direction of
the antenna, or does it seem like it causes constructive interference
when the forward direction of the radiation is away from the rearward
direction of the antenna....and if so, what does that have to do with
"coherently summing all the rays by rotating the antenna"? Just wondering.

ac6xg

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