You would have to create a ground plane that extends many wavelengths
from your antenna. The multiple lobe pattern is produced by reflections
from the ground which are far away -- the lower the angle, the farther
from the antenna.

Why don't you just lower the antenna to 1/2 wavelength if that's the
pattern you want? Then you'll have just the ground plane you need, the
right height below the antenna -- the ground.

Roy Lewallen, W7EL

Harbin wrote:
Howdy:
When a 1/2 wave vertical is about 1/2 wave off the ground you get
your best radiation pattern without extra lobes, but
if you were to put it on a tower, several wavelengths high, you get
multiple lobes. My question is how can you
keep a clean radiation pattern when you are several wavelengths high
with a 1/2 wave antenna? Can you put a ground
plane 1/2 wave below the antenna to keep the clean pattern?

--
SeeYaa Harbin Osteen KG6URO