On Wed, 17 Jan 2007 20:22:27 -0000, "David" nospam@nospam wrote:

With a vertical monopole antenna, the field emitted by the radials forms a
near field and interacts with the wave radiated by the vertical element. Is
it mainly in the vertical direction that the radial fields interact with the
field from vertical element?

Hi David,

No.

I would expect the antenna feedpoint impedance
to be formed from the length of antenna conductors and the interaction of
the fields.

Sure. There is nothing you've offered to suggest otherwise.

If a monopole is simulated in a NEC program above a perfect ground plane, is
it possible to see the reflection?

Yes. Even above an imperfect ground. Plane has nothing to do with it
except for the matter of geometry - radials even less so.

I would expect NEC program to show
reflection of waves from metallic surfaces e.g reflection from perfect
ground plane or parabolic dish.

You should then expect an NEC program to show that reflection in
relation to wavelength. They do this to as near to what is observed
as to show no difference, naturally.

Can a NEC program allow the user to see
whether the wave is reflected (as for perfect ground plane) or whether
effect is due to wave interaction (as for radials)?

NEC is not responsible for insight or interpretation. That only comes
with experience.

Can the user then vary a
finite size ground plane to see whether antenna impedance comes from
reflection or wave interaction?

The user might, but the proof of an argument is through a chain of
evidence.

A simple test of "reflection from radials" in a real implementation
(with ground):

1. Change the angle of the radials by 45°,
does the lobe angle change by 45°?
Explain why reflection does not follow convention.

2. As suggested by Roy, add a top hat with a structure that identical
to the radials,
does the lobe angle point into the ground?
Explain why reflection does not exist in this case.

3. Alternative: invert the radials/vertical, it is now upside down,
does the lobe angle invert with it?
Explain why radials are special only in the conventional case.

You might want to repeat these in real space, but then you have
removed the reflector, ground. Without it, what would you offer as a
proof of reflection that can be verified through any of the three
simple tests above?
Explain why those tests demonstrate reflection in free space when
those tests above do not.

73's
Richard Clark, KB7QHC