This can easily be done with the free EZNEC demo program available from
http://eznec.com. Here's a step-by-step for the demo or any other EZNEC
v. 4.0 program:

1. Open example file Vert1.EZ. (Click the Open button, enter Vert1, then
click Open in the file selection window.)
2. Click FF Plot to generate a 2D elevation plot of the vertical antenna.
3. In the 2D Plot Window, open the File menu, select Save Trace As,
enter a name in the blank box, and click Save. You can close the 2D Plot
Window to get it out of the way.
4. Click the View Ant button so you can see what you're doing.
5. Click on the Wires line in the large white window to open the Wires
Window.
6. In the Wires Window, open the Wires menu and select Rotate Wires.
(This feature isn't available in EZNEC v. 3.0 or earlier versions.)
7. In the Rotate Wires dialog box, make the following changes:
Rotation Amount: Enter the angle of the ground slope, and change the
direction to CW (to simulate ground sloping down to
the right)
Rotation Axis: Y
Then click Ok. You'll now see the tilted antenna. (The rotation will
seem to be CCW, but that's because we're looking at the Y axis in the
minus direction.)
8. Click FF Plot to generate a 2D plot.
9. In the 2D Plot window, open the File menu and select Add Trace. Enter
the name of the plot of the vertical you saved earlier and click Open.
You'll now see the plots of the vertical and tilted antenna superimposed.
10. If you'd like to see the dB difference between plots at any
elevation angle, click the name of the recalled trace near the upper
left corner of the plot window. The cursor will jump to the recalled
trace, and the difference in dB appears as the bottom line of the right
column in the data window below the plot. (This feature isn't available
in EZNEC v. 3.0 or earlier versions.) If you don't see the data window,
open the View menu and click Show Data. You can move the cursor by
dragging it with the mouse, or with the arrow keys.
11. To see what the patterns would look like when the ground is tilted,
print the plot (File/Print Plot), then rotate it clockwise by the angle
of the ground tilt. Note that this assumes that the ground is flat and
continues forever at the tilt angle, which of course can't be true. But
it'll give you a good general idea of the effect of tilting the antenna.

If you get confused about how the plot is oriented relative to the
antenna, go to the View Antenna display. Open the View menu, select
Objects, then check the 2D Pattern box. This will superimpose a
correctly oriented 2D pattern on the drawing of the antenna.

I did the experiment using a 30 degree tilt, and found a difference of
1.31 dB at an indicated elevation angle of 30 degrees. That would be at
the horizon, taking into account the ground tilt. At an indicated
elevation angle of 40 degrees (10 degrees above the horizon when ground
tilt is considered), the difference is 1.77 dB. You can modify the
ground conductivity and permittivity and repeat the experiment to see
how this changes with different ground types.

Roy Lewallen, W7EL

Frank wrote:

Since NEC cannot model a sloping ground, just try modeling a leaning
vertical, and see how it effects the pattern. I seriously doubt there would
be much difference. Certainly nothing you would notice.

Frank