Jim Kelley wrote:

Cecil Moore wrote:
Are we talking normal operation or receiving big
bang background radiation?

The source of radiation was not described; only its distribution. It
was like being surrounded isotropically by radio sources - not unlike
the 3 K background. But there are other sources which pretty well
surround us as well.

OK, sometimes I lose the context. If the radiation is arriving
isotopically, it doesn't matter which direction the Yagi is
pointed (as I inferred from what you said).

But arriving isotropic radiation would all converge at a point.
If a plumber's delight Yagi driven element is centered on that
point, it would receive all the radiation in a default-isotropic
mode.
--
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

Cecil Moore wrote:

Jim Kelley wrote:


Cecil Moore wrote:

Are we talking normal operation or receiving big
bang background radiation?


The source of radiation was not described; only its distribution. It
was like being surrounded isotropically by radio sources - not unlike
the 3 K background. But there are other sources which pretty well
surround us as well.


OK, sometimes I lose the context. If the radiation is arriving
isotopically, it doesn't matter which direction the Yagi is
pointed (as I inferred from what you said).

But arriving isotropic radiation would all converge at a point.
If a plumber's delight Yagi driven element is centered on that
point, it would receive all the radiation in a default-isotropic
mode.

I think Roy pretty well nailed the answer. An interesting result - both
antennas producing equal signals. I'd like to check that by comparing a
dipole to an isotropic in such a field. I assume the results would be
the same. Again, an interesting result. But you both bring some
interesting points.

ac6xg

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

Jim Kelley, AC6XG wrote:
"The question you have to ask yourself is, does it intercept all of the
energy reflected toward it, or only some fraction of it."

Nothing is perfect as Jim observes. "Imperfections" are sometimes
exploited to improve an antenna pattern. To a first approximation
though, we assume that all the parallel rays intercepted by a dish are
focused on the radiator and aid, adding in-phase. Received carrier power
excites the antenna and this causes a minimum of 50% of this power to be
re-radiated if the antenna is perfectly matched to to the receiver load.
The antenna`s radiation resistance in this case becomes the Thevenin`s
source resistance for the receiver load on the antenna. This requires a
conjugate match between the antenna and receiver input impedances.

50% of the received power to the receiver is the best that can be done
under optimum conditions, that is , with a perfect match. With a 100%
mismatch, a short-circuit, 100% of the intercepted power is re-radiated
by the antenna. If the antenna is open-circuited, it accepts none of the
power focused upon it.

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