-.-. --.- wrote:
Now, how i do treat a 5/4 wave phisical lenght monopole, in respect of
electrical lenght ??

The free demo version of EZNEC will answer your question.
1/4WL above the 5/4WL monopole's feedpoint, there is a
current node ('x' minimum point). Of course, there is another
current node at the top of the monopole. Looking at the
monopole with your head turned 90 degrees, the current
distribution looks something like this.
_ __
\ / \
fp----x------x------x
\ __ /

Since it is fed at a current maximum point, you need
a counterpoise/ground-plane for such an antenna.
You didn't ask, but the take-off-angle for a 5/4WL monopole
is about 58 degrees, not a good idea above 10 MHz.

The second question is about 1/4 wave vs 1/2 wave monopoles. I googled
around, but no one define exactly why "1/2 wave antenna don't need ground
plane".

A 1/4WL monopole is half of an antenna. The ground plane
is the other half. The feedpoint is at a current maximum
point which is a magnetic field maximum. The high intensity
magnetic field directly encountering the earth is a lossy
condition.

A 1/2WL monopole is a complete resonant antenna. The
feedpoint is at a voltage maximum point which is an
electric field maximum. A pure electric field encountering
earth ground is less lossy than a pure magnetic field
encountering earth ground. Since the feedpoint impedance
is very high, some hams simply use a ground rod for a
counterpoise.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com