Antenna gain question
 

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

Antenna gain question
 

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

Antenna gain question
 

Cecil Moore wrote:

Jim Kelley wrote:

... what does that have to do with "coherently summing all the rays by
rotating the antenna"? Just wondering.


"Coherently summing" certainly doesn't imply that interference
is only constructive. An antenna is "coherently summing" all
the rays it receives no matter what direction it is pointed.

And still, rotating the antenna has nothing to do with summing the
signals - coherently, or otherwise. Agreed?

ac6xg

Antenna gain question
 

Jim Kelley wrote:
... what does that have to do with
"coherently summing all the rays by rotating the antenna"? Just wondering.

"Coherently summing" certainly doesn't imply that interference
is only constructive. An antenna is "coherently summing" all
the rays it receives no matter what direction it is pointed.
--
73, Cecil http://www.qsl.net/w5dxp

Antenna gain question
 

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

Antenna gain question
 

On Wed, 02 Nov 2005 00:26:06 GMT, Cecil Moore wrote:

Jim Kelley wrote:
... what does that have to do with
"coherently summing all the rays by rotating the antenna"? Just wondering.

"Coherently summing" certainly doesn't imply that interference
is only constructive. An antenna is "coherently summing" all
the rays it receives no matter what direction it is pointed.

Has someone got a good definition of coherent.

I thought that it implied "same phase", as in a coherent source is one
where all rays, photons, whatever are in phase. An antenna may well
receive rays from a single source that are not in phase.

If that is the case, what is "coherent summing". Is it trying to refer
to a function that adds components algebraically, ie having regard for
the magnitude and phase?

Why is light a better vehicle for explanation of an antenna that radio
waves?

Owen
--

Antenna gain question
 

Owen Duffy wrote:

Has someone got a good definition of coherent.

I thought that it implied "same phase", as in a coherent source is one
where all rays, photons, whatever are in phase. An antenna may well
receive rays from a single source that are not in phase.
. . .

The way I've always seen it used in this context is meaning "exactly the
same frequency". They don't have to be in phase, but the same-frequency
requirement implies that the phase relationship wouldn't change with time.

This is consistent with the definition from _Merriam Webster's
Collegiate Dictionary_: "relating to or composed of waves having a
constant difference in phase ~light".

Roy Lewallen, W7EL

Antenna gain question
 

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

Antenna gain question
 

Jim Kelley wrote:
And still, rotating the antenna has nothing to do with summing the
signals - coherently, or otherwise. Agreed?

Are we talking normal operation or receiving big
bang background radiation?
--
73, Cecil http://www.qsl.net/w5dxp

Antenna gain question
 

Owen Duffy wrote:
Has someone got a good definition of coherent.

From the IEEE Dictionary: "coherent (1)(fiber optics) Characterized by
a fixed phase relationship between points on an electromagnetic wave ...
(2)(laser maser) A light beam is said to be coherent when the electric
vector at any point in it is related to that at any other point by a
definite, continuous sinusoidal function."

An antenna may well
receive rays from a single source that are not in phase.

I receive rays from WTAW that are sometimes in phase and sometimes not
over a period of mere seconds. Earth's atmosphere seems to be a
coherence killer.
--
73, Cecil http://www.qsl.net/w5dxp