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

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

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

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

Cecil Moore wrote:

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?

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.

ac6xg

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

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
--

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

Roy, W7EL wrote:
"They don`t have to be in phase, but the same frequency requirement
implies that the phase relationship wouldn`t change with time."

Definitions of terms like "coherent" change with time.

Slectromagnetic waves are described as "plane-polarized waves" because
variations in their electric and magnetic fields can be represented by
vectors that lie in a plane. It is proven that light waves and radio
waves are of the same sort but differ in frequency.

The first reference I ever saw of a coherent wave was a description of
light from a laser. It meant the waves started and stopped together
passing through zero at the same time.

Light waves are emitted by molecules or atoms that are excited by
thermal or electrical means. These molecules or atoms are randomly
positioned and so are the waves generated by the energy level changes
within them. Phase and polarization are thus random in light produced by
ordinary light sources. Light is coherent from a laser.

A photon can interact with an atom in a laser if its energy exactly
matches the energy difference (delta E) between two allowed (by Bohr)
energy states for the atom, it can cause a transition. This is called a
stimulated transition.

Best regards, Richard Harrison, KB5WZI