Frank wrote:

Not sure I really understand what is going on, but have been aware of your
previous postings, also on the NEC-list. What I should have said is that
the above program agrees with Reg's previous assumption -- but not with his
new program "grndwav4.exe". In any case, just to satisfy my curiosity, I
ran the following code, which is, in essence, almost identical to your
NEC-list post with 5.555.... kW input producing 1V/m peak at 1000m. The
following agrees exactly with Reg's new program.

CM Short Monopoles
CE
GW 1 50 0 0 1 0 0 0 0.000814
GW 2 50 1000 0 1 1000 0 0 0.000814
GS 0 0 1
GE 1
GN 1
EX 0 1 50 00 65698.12106 0.00000
LD 4 2 50 50 1.747 823.796
FR 0 3 0 0 19.9 0.1
RP 1 1 360 0000 0 0 1.00000 1.00000 1000
RP 0 181 1 1000 -90 45 1 1
EN

Noting the comments by others, obviously familiar with ATR measurement
techniques, this exercise with NEC is purely academic. There is no way you
could experimentally prove these results. Since I have never made
measurements on an "Open-air" test site it will be interesting to verify
Mac's assumptions, which I am sure are correct.

The confusions I have are now related to the fact that NEC results depend on
how the incident E-field is generated. I will check all previous posting by
Roy to see if I can figure out this anomaly. For some reason I have not
received any update concerning the NEC list postings.


I've just now finally gotten around to posting a response to the
NEC-list. It might help clarify things for you.

The essential point is that when you specify a plane wave source, it
acts like a plane wave of the specified amplitude coming from the
specified direction. That wave interacts with the ground plane just as
any other field would. When a ground plane is specified, the result is a
field strength -- and polarization -- which isn't generally the same as
that of the original wave. You can illustrate this by specifying a plane
wave which originates at an angle of 45 degrees above the horizon, and
looking at the current induced in a short circuited vertical wire or the
base voltage of an open circuited wire (the latter simulated by putting
a high impedance load at the base). Begin with the wire vertical, then
tilt the wire so the direction of the plane wave source is broadside to
the wire, and again so the direction of the source is in line with the
wire. You'll get the same result from the last two tests, and the
induced current or voltage in those two is less (by about 1/sqrt(2))
than when the wire is vertical. This shows that the field is purely
vertically polarized (normal to the ground plane) at the location of the
wire. (I think there's actually a small horizontal component except
exactly at the ground plane surface.) It does show conclusively that the
orientation of the field isn't the same as it was when it left the
source -- otherwise the induced current or voltage would be greatest
when the wire was tilted broadside to the plane wave source and zero
when tilted in the source direction.

So the interaction of the plane wave source's field with the ground
plane alters both the amplitude and the polarization of the field. When
the source is in the horizontal direction and the ground plane is
perfect, the field strength just above the ground plane is exactly twice
the amplitude of the plane wave source. So a 1 V/m plane wave source at
zero elevation angle (90 degree zenith angle) produces 2 V/m just above
the ground plane, which induces 1 V at the base of an open circuited
electrically short 1 m vertical wire.

Roy Lewallen, W7EL