Image theory is for a perfect groundplane e.g. large area metal sheet. The
wave emitted by the vertical radiating element is reflected by the ground
plane.

Image theory as I see it follows. Wave emitted by vertical element is the
incident wave that hits ground plane, inducing currents in the ground plane.
Currents flowing in skin depth of ground plane emit a wave of opposite
polarity to cancel out the wave at the boundary of the ground plane, thus
making the electric field in the ground plane zero. The wave of opposite
polarity is the reflected wave. The reflected wave appears to be coming from
an image antenna. Image theory is a mathematical model for solving antenna
simulations where there is a monopole over a ground plane.

How do the radials reflect the wave? If they are not a good enough ground
plane because of the gap, how do they reflect? I cannot see the transition
from ground plane to radials, when looking at image theory.

On Sun, 9 Jul 2006 23:53:40 +0100, "David" nospam@nospam wrote:
Image theory as I see it follows. Wave emitted by vertical element is the
incident wave that hits ground plane, inducing currents in the ground plane.

Hi David,

Well, given your repetition of "ground plane," be cautioned that is
not one-and-the-same meaning for radials (even if they are called part
of a ground plane antenna).

73's
Richard Clark, KB7QHC

David wrote:
Image theory is for a perfect groundplane e.g. large area metal sheet. The
wave emitted by the vertical radiating element is reflected by the ground
plane.

Image theory as I see it follows. Wave emitted by vertical element is the
incident wave that hits ground plane, inducing currents in the ground plane.
Currents flowing in skin depth of ground plane emit a wave of opposite
polarity to cancel out the wave at the boundary of the ground plane, thus
making the electric field in the ground plane zero. The wave of opposite
polarity is the reflected wave. The reflected wave appears to be coming from
an image antenna. Image theory is a mathematical model for solving antenna
simulations where there is a monopole over a ground plane.

How do the radials reflect the wave? If they are not a good enough ground
plane because of the gap, how do they reflect? I cannot see the transition
from ground plane to radials, when looking at image theory.

Picture a half wave disk of metal as the ground plane, producing the
inverted image of the vertical. Then imagine thin radial slots spread
around the vertical. Since these slots do not cross any current path
that is needed to produce the image, they have little effect on the
image. Widen those slots, and decrease the number of them, and
eventually you get to a ground radial system with only a few radials.
There has to be a transition point, where the radials are only a
poor approximation of the original disk. The question is, how well
must you approximate the disk to get a reasonable approximation of the
far field radiation pattern it would have helped produce?

John Popelish wrote:

Picture a half wave disk of metal as the ground plane, producing the
inverted image of the vertical. . .

It appears that what I've been writing the past few days either isn't
being read or isn't being believed. Among it is an explanation of why a
"ground plane" doesn't produce an "image" of the vertical.

Since you appear to continue to believe this, please explain the
mechanism by which you think a half wave disk produces an "image" of the
vertical.

Roy Lewallen, W7EL

Roy Lewallen wrote:
John Popelish wrote:


Picture a half wave disk of metal as the ground plane, producing the
inverted image of the vertical. . .


It appears that what I've been writing the past few days either isn't
being read or isn't being believed. Among it is an explanation of why a
"ground plane" doesn't produce an "image" of the vertical.

Since you appear to continue to believe this, please explain the
mechanism by which you think a half wave disk produces an "image" of the
vertical.

The disk forms an image by allowing the electric field lines to
terminate perpendicular to the "mirror" surface on exactly the same
lines as if they were heading toward a lower half of a dipole, while
the radial currents in the "mirror" allow the magnetic field lines to
encircle the monopole in the same pattern they would form if the
missing half of the dipole were in position.

This same pattern of electric and magnetic fields above the "mirror"
produces (half of the) photons that the full dipole would have
produced. A half wave diameter disk is about the minimum size
"mirror" that will keep the field patterns close enough to those of
the dipole to launch those photons. A larger disk would do better,
but not a lot better.

On Mon, 10 Jul 2006 14:27:05 -0400, John Popelish
wrote:

A larger disk would do better, but not a lot better.

Hi John,

In fact a larger disk will actually raise the launch angle - hardly a
satisfactory mirror analogy.

the "mirror" produces (half of the) photons that the
full dipole would have produced.

Photons? This is CecilBabble. Mirrors as "productive" sources of
photons demonstrates the failure of analogies.

73's
Richard Clark, KB7QHC

Richard Clark wrote:
On Mon, 10 Jul 2006 14:27:05 -0400, John Popelish
wrote:


A larger disk would do better, but not a lot better.


Hi John,

In fact a larger disk will actually raise the launch angle - hardly a
satisfactory mirror analogy.


the "mirror" produces (half of the) photons that the
full dipole would have produced.


Photons? This is CecilBabble. Mirrors as "productive" sources of
photons demonstrates the failure of analogies.

Do you deny the photonic nature of radio waves?

I just realized that the sentence you quoted s easily misinterpreted.
When I said "the "mirror" produces (half of the) photons that the
full dipole would have produced." I meant that half as many photons
are produced, compared to the full dipole antenna that produces the
same fields above the center line. I didn't mean that the mirror
produces half of the total photons that are radiated.

On Mon, 10 Jul 2006 17:57:42 -0400, John Popelish
wrote:

Hi John,

In fact a larger disk will actually raise the launch angle - hardly a
satisfactory mirror analogy.


the "mirror" produces (half of the) photons that the
full dipole would have produced.


Photons? This is CecilBabble. Mirrors as "productive" sources of
photons demonstrates the failure of analogies.

Do you deny the photonic nature of radio waves?

Hi John,

This last question is standard CecilBaiting at which he is a master.

I've made a career in photonics, so you will have to go some distance
to start offering a case that comes remotely close to their cross
application. Barring that, why introduce concepts that don't advance
the topic? The following is hardly any clearer by clinging to poor
metaphors:

I just realized that the sentence you quoted s easily misinterpreted.
When I said "the "mirror" produces (half of the) photons that the
full dipole would have produced." I meant that half as many photons
are produced, compared to the full dipole antenna that produces the
same fields above the center line. I didn't mean that the mirror
produces half of the total photons that are radiated.

73's
Richard Clark, KB7QHC

On Mon, 10 Jul 2006 17:57:42 -0400, John Popelish
wrote:

I meant that half as many photons
are produced, compared to the full dipole antenna that produces the
same fields above the center line.

Hi John,

So, proceeding along your avowed lines of Photons, one of several
questions:
Presuming 100W radiated, how many photons would that be so that we
can talk about them by halves.

Yes, that is perhaps unfair, however it demonstrates how easily the
discussion can tumble for lack of quantifiables such as that original
offering of 100W.

Should we discuss how infinitesimal the energy is in a 40M photon?
(Easily accounts for why so many are needed for that same 100W.)

No, I suppose not.

Want to get into the problems of diffraction with object lenses that
measure less than a wavelength of the photon?

Hard to escape, and makes a mess of describing mirrors too, especially
when they are skeletal approximations as well.

I can offer more thread-busters when it comes to photonics, but that
is a slam dunk. Get us rolling on one ace proposition, and I will get
back to you in a couple of hours.

73's
Richard Clark, KB7QHC