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

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

Ron, W4TQT wrote:
"How about a dish antenna?"

The parabolic reflector converts the spherical waves of its radiator at
the focus of the parabola into a plane wave of uniform phase across the
mouth or aperture of the parabola. Mouth ans aperture are syninymous
when applied to parabolic, lens, and horn antennas. Rays enter and exit
parallel but reflect through the focal point. Reciprocity rules and the
path through the antenna is the same, coming or going. The parabolic
reflector antenna sends and receives to and from a familiar spot on its
axis and at a distance. It is inoperative outside the spot and its path
of travel. The larger the parabola, the smaller the diameter of the
spot, and the higher the power gain.

The beamwidth of a large circular aperture such as a parabolic antenna
is inversely proportional to its diameter in wavelengths. The total
field radiated by a arabola is the vector sum of the fields generated by
the elementary areas making up the aperture or mouth of the parabola.
The directive gain of a parabola antenna is directly proportional to the
area of its mouth and inversely proportional to the wavelength squared.
See 1955 Terman page 899, equation (23-28) as pointed out at the bottom
of page 911.

Best regards, Richard Harrison, KB5WZI

On Fri, 28 Oct 2005 20:37:07 GMT, 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?

Hi All,

Well, it is time to discard the speculation and let modeling approach
this for an answer that at least offers more than swag.

First we strip away the sphere and solve this in two dimensions. To
do that we simply construct a ring of sources surrounding the
prospective antennas and let the winning design emerge.

EZNEC+ ver. 4.0

Dipole in Ring of Sources 11/2/2005 10:00:48 PM

--------------- LOAD DATA ---------------

Frequency = 70 MHz

Load 1 Voltage = 4.783 V. at 23.52 deg.
Current = 0.06643 A. at 23.52 deg.
Impedance = 72 + J 0 ohms
Power = 0.3177 watts

Total applied power = 2000 watts

Total load power = 0.3177 watts
Total load loss = 0.001 dB


EZNEC+ ver. 4.0

Vert Yagi in Ring of Sources 11/2/2005 10:21:32 PM

--------------- LOAD DATA ---------------

Frequency = 70 MHz

Load 1 Voltage = 1.418 V. at 25.9 deg.
Current = 0.1182 A. at 25.9 deg.
Impedance = 12 + J 0 ohms
Power = 0.1676 watts

Total applied power = 2000 watts

Total load power = 0.1676 watts
Total load loss = 0.0 dB


As the Bard would offer, there's many a slip between the cup and the
lip. For a first pass approximation, and for all the potential for
errors (which can now be routed out instead of gummed to death), it
appears that the low gain (directivity) dipole absorbs more power than
the high gain (directivity) yagi.

73's
Richard Clark, KB7QHC

Richard Clark wrote:

On Fri, 28 Oct 2005 20:37:07 GMT, 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?


Hi All,

Well, it is time to discard the speculation and let modeling approach
this for an answer that at least offers more than swag.

First we strip away the sphere and solve this in two dimensions. To
do that we simply construct a ring of sources surrounding the
prospective antennas and let the winning design emerge.

EZNEC+ ver. 4.0

Dipole in Ring of Sources 11/2/2005 10:00:48 PM

--------------- LOAD DATA ---------------

Frequency = 70 MHz

Load 1 Voltage = 4.783 V. at 23.52 deg.
Current = 0.06643 A. at 23.52 deg.
Impedance = 72 + J 0 ohms
Power = 0.3177 watts

Total applied power = 2000 watts

Total load power = 0.3177 watts
Total load loss = 0.001 dB


EZNEC+ ver. 4.0

Vert Yagi in Ring of Sources 11/2/2005 10:21:32 PM

--------------- LOAD DATA ---------------

Frequency = 70 MHz

Load 1 Voltage = 1.418 V. at 25.9 deg.
Current = 0.1182 A. at 25.9 deg.
Impedance = 12 + J 0 ohms
Power = 0.1676 watts

Total applied power = 2000 watts

Total load power = 0.1676 watts
Total load loss = 0.0 dB


As the Bard would offer, there's many a slip between the cup and the
lip. For a first pass approximation, and for all the potential for
errors (which can now be routed out instead of gummed to death), it
appears that the low gain (directivity) dipole absorbs more power than
the high gain (directivity) yagi.

73's
Richard Clark, KB7QHC

Hi Richard,

What is the plane of polarization of the ring of sources, and what is
the orientation of the dipole?

73, ac6xg

On Thu, 03 Nov 2005 11:47:21 -0800, Jim Kelley
wrote:

What is the plane of polarization of the ring of sources, and what is
the orientation of the dipole?

Hi Jim,

Vertical in free space (which, of course, has no direction, but we
know what Vertical implies). This also includes the yagi.

73's
Richard Clark, KB7QHC

Richard Clark wrote:

On Thu, 03 Nov 2005 11:47:21 -0800, Jim Kelley
wrote:


What is the plane of polarization of the ring of sources, and what is
the orientation of the dipole?


Hi Jim,

Vertical in free space (which, of course, has no direction, but we
know what Vertical implies). This also includes the yagi.

If you wouldn't mind, try moving your Yagi a half wave forward or reverse.

ac6xg