You should be able to find graphs of the radiation pattern versus
element length in many antenna books. 5/8 wave is the longest you can
make a single element and maintain a single lobe. Beyond that, the
pattern splits into two (and more, when the element is even longer)
lobes. To maintain a single lobe (a "flat pancake" pattern) with a
longer antenna, you need a way to divide the antenna into multiple
elements, and have the current in them all in-phase. You can simulate
a ground-plane antenna in programs like EZNec, and if you just look at
a vertical monopole over ground, or a center-fed dipole in freespace
that's twice as long, you should be able to use the free evaluation
version of EZNec. I'm not sure if the free version allows enough
wires/segments to do a ground plane with radials, but possibly. See
www.eznec.com.

Cheers,
Tom

"Marcello" wrote in message ...
Hello!
wich are the differences between two monoband ground plane vertical antenna
5/8 lambda and 3/4 lambda, with orizontal radilas?(90°).
There is a developing system software to view this difference?
Marcello