Gene Fuller wrote:
. . .

[I wrote:]
One final note, regarding the NEC applied plane wave. My earlier
statement that the resulting field is twice the plane wave source
magnitude when a ground plane is present is true only when the plane
wave is applied over perfect ground at exactly grazing incidence
(zenith angle = 90 deg.). If applied from other angles the resulting
field strength will be different. If you apply a vertically polarized
wave over a ground plane, I believe the resulting field strength will
look like the pattern from a vertical radiator over a perfect ground
plane -- strongest when applied at the horizon, decreasing when
applied at higher angles, and dropping to zero if applied from
directly overhead. I haven't confirmed this, but believe it's
necessary in order to get a receiving pattern that's the same as the
transmitting pattern. So use it with caution when a ground plane is
present, and don't casually make assumptions about the resulting field.



I believe a better way to describe this situation is that the plane wave
field strength does not go to zero, but rather the effective aperture of
the antenna goes to zero as the plane wave is applied from overhead.
This does not change your conclusion with respect to antenna patterns.

I'm not sure either one of us quite has it right. In a model experiment,
I set up a short open circuited vertical dipole just above a perfect
ground plane, and applied a vertically polarized plane wave from the
horizon. Let's call the resulting voltage at the dipole center V1. Then
I changed the direction of the plane wave so it was coming from an
elevation angle of 45 degrees above the horizon, but with the same
amplitude. The dipole voltage was about 0.7 * V1, about what we'd expect
from the change in effective aperture of the vertical dipole due to the
different arrival elevation angle. But if I tilt the dipole back 45
degrees so it's parallel to the incident E field, the voltage drops to
about 0.5 * V1, another 3 dB. I believe this indicates that the field in
the vicinity of the dipole is oriented normal to the ground plane, and
it has a magnitude that's about 0.7 as great as it is when the same
amplitude wave is fired from a horizontal direction. A second check was
to tilt it 45 degrees the other way, so its end is pointing directly
toward the direction of the impinging wave. The result was again about
0.5 * V1, adding proof that the field in its vicinity is normal to the
ground plane and not tilted in the direction of the source. So the
antenna aperture is indeed changing as we change the orientation of the
antenna relative to the field in its vicinity. But that's not the same
as the orientation of the antenna relative to the direction from which
the plane wave originates. (They are of course the same if the ground
plane is absent.) The change in antenna output (in this case) when the
source direction is changed is due to the fact that the magnitude of the
field strength has changed, not because its orientation relative to the
antenna has changed.

When the direction of the plane wave is elevated 45 degrees, it has
equal horizontally and vertically polarized components. The horizontal
components cancel on reflection, while the vertical components reinforce
as before. This leaves only the vertical component in the vicinity of
the sense dipole, and it's 0.707 * the value when the same amplitude
wave is coming in horizontally. The dipole voltage is maximum when it's
oriented to be parallel with this field, that is, vertical.

At least I think this is a correct interpretation of what I'm seeing.
These effects are all tied in together, and I've spent so long looking
at the problem from the transmitting direction that I'm having some
trouble getting my thinking turned around. But I'm slowly getting there.

Roy Lewallen, W7EL