Ron wrote:
. . .

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.


Not sure what you mean by "focal point", but the best any antenna can do
is to intercept half the energy in some equivalent cross-sectional area
of an impinging field. It does this when connected to a matched load.

When an antenna intercepts one watt from a field having a power density
of one watt per square meter, it's said to have an "effective aperture"
or "capture area" of one square meter. The higher the gain of an antenna
in some particular direction, the larger its effective aperture in that
direction. Consequently, a high gain antenna would "capture" more power
from a wave arriving in its favored direction than an isotropic antenna
would. It would, of course, capture less from other directions, but
assuming equal efficiency, both antennas would capture equal amounts
overall.

The "capture area" isn't some physical region with boundaries -- it's
simply a way of expressing how much power is extracted from a field
having a given power density. In other words, it's just another way of
expressing antenna gain.

Roy Lewallen, W7EL

Roy Lewallen wrote:
Ron wrote:

. . .


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.


Not sure what you mean by "focal point", but the best any antenna can do
is to intercept half the energy in some equivalent cross-sectional area
of an impinging field. It does this when connected to a matched load.

I agree.

By "focal point" I meant the center of the sphere where the rays
converge and where the antenna would be located.

When an antenna intercepts one watt from a field having a power density
of one watt per square meter, it's said to have an "effective aperture"
or "capture area" of one square meter. The higher the gain of an antenna
in some particular direction, the larger its effective aperture in that
direction. Consequently, a high gain antenna would "capture" more power
from a wave arriving in its favored direction than an isotropic antenna
would. It would, of course, capture less from other directions, but
assuming equal efficiency, both antennas would capture equal amounts
overall.

In the unusual field defined in my example, the algebraic sum of all
the rays collected by the antenna would be higher in the isotropic
antenna than a high gain antenna. Think of the front to back ratio of
the high gain antenna which would result in very little output from
the rays behind and on the sides of the antenna. Therefore, the
isotropic would have a higher output which is indicative of higher gain.

I do not understand what you mean by "capture equal amounts overall".
Energy which may strike the antenna but does not result in any output
power isn't "captured".

The "capture area" isn't some physical region with boundaries -- it's
simply a way of expressing how much power is extracted from a field
having a given power density. In other words, it's just another way of
expressing antenna gain.

How about a dish antenna? Isn't the capture area proportional to the
physical area of the dish?

Ron, W4TQT

On Wed, 02 Nov 2005 03:58:56 GMT, Ron wrote:



In the unusual field defined in my example, the algebraic sum of all
the rays collected by the antenna would be higher in the isotropic
antenna than a high gain antenna. Think of the front to back ratio of
the high gain antenna which would result in very little output from
the rays behind and on the sides of the antenna. Therefore, the
isotropic would have a higher output which is indicative of higher gain.


Is this to rewrite the principle of reciprocity?

Owen
--

Ron wrote:
In the unusual field defined in my example, the algebraic sum of all the
rays collected by the antenna would be higher in the isotropic antenna
than a high gain antenna.

The same amount of energy is incident upon both antennas at
the center of the sphere. Maybe the high-gain antenna re-
radiates more energy than the isotropic?
--
73, Cecil http://www.qsl.net/w5dxp

Ron wrote:
. . .
By "focal point" I meant the center of the sphere where the rays
converge and where the antenna would be located.

I have to admit, I was looking at this a more of a problem of equal
signals arriving from all directions, rather than at the middle of some
sort of convergence. Of course, any rays reaching the center would
continue on through, Cecil's unique theories notwithstanding. I don't
have the spare time to contemplate what the end field distribution would
be like at the center of the antenna or its periphery.

When an antenna intercepts one watt from a field having a power density
of one watt per square meter, it's said to have an "effective aperture"
or "capture area" of one square meter. The higher the gain of an antenna
in some particular direction, the larger its effective aperture in that
direction. Consequently, a high gain antenna would "capture" more power
from a wave arriving in its favored direction than an isotropic antenna
would. It would, of course, capture less from other directions, but
assuming equal efficiency, both antennas would capture equal amounts
overall.


In the unusual field defined in my example, the algebraic sum of all the
rays collected by the antenna would be higher in the isotropic antenna
than a high gain antenna.

It's not obvious to me why that would be.

Think of the front to back ratio of the high
gain antenna which would result in very little output from the rays
behind and on the sides of the antenna.

That's true. But the output would be higher in reponse to the rays
arriving from the front. We call that "gain". Another way to express it
is that it intercepts a field from a larger area of the wave front.

Therefore, the isotropic would
have a higher output which is indicative of higher gain.

You're right that higher output means higher gain. I maintain that both
antennas have the same total gain, i.e., the same total interception of
power from all directions. This follows directly from the reciprocity
principle.

I do not understand what you mean by "capture equal amounts overall".
Energy which may strike the antenna but does not result in any output
power isn't "captured".

The field you're creating comes from something and goes somewhere. If
you subtract the total amount going from the total amount generated,
you'll get the amount dissipated in the load connected to the antenna.
That is the amount of energy "captured" or "intercepted" by the antenna.
And that's what I thought you were talking about all along.

The "capture area" isn't some physical region with boundaries -- it's
simply a way of expressing how much power is extracted from a field
having a given power density. In other words, it's just another way of
expressing antenna gain.


How about a dish antenna? Isn't the capture area proportional to the
physical area of the dish?

Indeed it is, in the front direction. But how about a dipole? The
capture area (or gain) broadside to an infinitesimal dipole is just
slightly less than that of a half wavelength dipole. And wire diameter
makes almost no difference.

Sorry, the theoretical construct is just a little too much like
Calvinball to hold my interest. I'll bow out now. Best luck in sorting
it out.

Roy Lewallen, W7EL

Thanks, Roy for your and everyone's participation. I think I will
bow out here also. Hope all this hasn't been a waste of space.
"Thinking" usually has some value.

Ron W4TQT

Roy Lewallen wrote:
Ron wrote:

. . .
By "focal point" I meant the center of the sphere where the rays
converge and where the antenna would be located.


I have to admit, I was looking at this a more of a problem of equal
signals arriving from all directions, rather than at the middle of some
sort of convergence. Of course, any rays reaching the center would
continue on through, Cecil's unique theories notwithstanding. I don't
have the spare time to contemplate what the end field distribution would
be like at the center of the antenna or its periphery.

When an antenna intercepts one watt from a field having a power density
of one watt per square meter, it's said to have an "effective aperture"
or "capture area" of one square meter. The higher the gain of an antenna
in some particular direction, the larger its effective aperture in that
direction. Consequently, a high gain antenna would "capture" more power
from a wave arriving in its favored direction than an isotropic antenna
would. It would, of course, capture less from other directions, but
assuming equal efficiency, both antennas would capture equal amounts
overall.



In the unusual field defined in my example, the algebraic sum of all
the rays collected by the antenna would be higher in the isotropic
antenna than a high gain antenna.


It's not obvious to me why that would be.

Think of the front to back ratio of the high gain antenna which would
result in very little output from the rays behind and on the sides of
the antenna.


That's true. But the output would be higher in reponse to the rays
arriving from the front. We call that "gain". Another way to express it
is that it intercepts a field from a larger area of the wave front.

Therefore, the isotropic would have a higher output which is
indicative of higher gain.


You're right that higher output means higher gain. I maintain that both
antennas have the same total gain, i.e., the same total interception of
power from all directions. This follows directly from the reciprocity
principle.

I do not understand what you mean by "capture equal amounts overall".
Energy which may strike the antenna but does not result in any output
power isn't "captured".


The field you're creating comes from something and goes somewhere. If
you subtract the total amount going from the total amount generated,
you'll get the amount dissipated in the load connected to the antenna.
That is the amount of energy "captured" or "intercepted" by the antenna.
And that's what I thought you were talking about all along.

The "capture area" isn't some physical region with boundaries -- it's
simply a way of expressing how much power is extracted from a field
having a given power density. In other words, it's just another way
of expressing antenna gain.



How about a dish antenna? Isn't the capture area proportional to the
physical area of the dish?


Indeed it is, in the front direction. But how about a dipole? The
capture area (or gain) broadside to an infinitesimal dipole is just
slightly less than that of a half wavelength dipole. And wire diameter
makes almost no difference.

Sorry, the theoretical construct is just a little too much like
Calvinball to hold my interest. I'll bow out now. Best luck in sorting
it out.

Roy Lewallen, W7EL

For your conceptual purposes, your question would be similar to this?:

In a deep focal point of parabolic dish two antenns are mounted...

Which of them it does pick up more energy?

An antenna with 180 degree beamwidht or an highly directional antenna
with 0,1 degree beamwidth (both pointed to dish, of course)?

(In focal point of dish there are convergent frontwaves also). (We
could think in a sperical dish, also).

73's

Miguel Ghezzi (LU 6ETJ)

Miguel Chezzi, LU6ETJ wrote:
"In a deep focal point of parabolic dish two antennas are mounted...

Which of them does pick up more energy?
An antenna with 180 degree beamwidth or a highly directional sntenna
with 0.1 degree beamwidth (both pointed to dish, of course)?"

I`ll risk being the fool. We sometimes test for illumination of a
reflector. We would not be concerned were it not advantageous to do so.

With 180-degree radiation, we fill the dish, using all its surface.
With 0.1-degree illumination, we might as well remove all but the
illuminated area. It would save dead load and wind loading.

My answer: The 180-degree radiation angle will receive a larger area of
the plane-wavefront and extract more watts from the wave with a given
number of watts per square area.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:

Miguel Chezzi, LU6ETJ wrote:
"In a deep focal point of parabolic dish two antennas are mounted...

Which of them does pick up more energy?
An antenna with 180 degree beamwidth or a highly directional sntenna
with 0.1 degree beamwidth (both pointed to dish, of course)?"

I`ll risk being the fool. We sometimes test for illumination of a
reflector. We would not be concerned were it not advantageous to do so.

With 180-degree radiation, we fill the dish, using all its surface.
With 0.1-degree illumination, we might as well remove all but the
illuminated area. It would save dead load and wind loading.

My answer: The 180-degree radiation angle will receive a larger area of
the plane-wavefront and extract more watts from the wave with a given
number of watts per square area.

Best regards, Richard Harrison, KB5WZI

The question you have to ask yourself is, does it intercept all of the
energy reflected toward it, or only some fraction of it.

We should always be cognizant of the limits imposed by the absence of a
free lunch.

ac6xg

Roy Lewallen wrote:
Of course, any rays reaching the center would
continue on through, Cecil's unique theories notwithstanding.

The way the incoming fields were defined, they all converge
at a point in the center of the sphere. Presumably, that's
where the isotropic antenna is located. Replacing the isotropic
with a Yagi whose feedpoint is logically located at the point
of convergence means that any part of the field that doesn't
encounter parts of the Yagi before the point of convergence will
converge at the feedpoint on the driven element of the Yagi in
a defaulting isotropic manner. Given the definition of the
spherical fields, there is no part of the fields that will not
encounter the Yagi.

Therefore, the isotropic and the Yagi receive the same amount of
energy, i.e. all that exists in the spherical fields. Any energy
not received by the Yagi beam elements is received in a default-
isotropic mode at the Yagi feedpoint.
--
73, Cecil http://www.qsl.net/w5dxp