Charles H. Harpole wrote;
>Help me here, but if grnd screen has no effect on HF Yagis, then why is
>height above gnd important to Yagis ? de K4VUD
It does, but for all intent and purposes, is useless in practicality.
Here's why:
The reflection from ground that we are interested in takes place many
wavelengths away from the actual antenna. The lower the angle of radiation,
the farther away that occurs. This is due mostly to the normally radiated
free space "lower half" of the spherical radiation pattern. This reflection
from the ground is reflected with a phase relationship that may produce
either constructive interference or destructive interference, or somewhere
in between. In other words at some angles, will produce a null, and at
other angles produce signal enhancement. This is the angle of brewster, and
is of paramount importance for long haul dx. Since the angle of brewster
occurs so far away from the antenna, it is not practical to ground screen
30 wavelengths and farther away.
To say that a ground screen has NO effect is also in err. It DOES affect
the higher angle radiation lobes. Why? Simple, since the lower half of the
free space sphere is sent down at all angles to the earth, just as all
angles are radiated from the upper "radiated" sphere, the "underside"
(normally free space) sphere is reflected from ground, and induced back
into the antenna elements, producing different currents and therefore
impedances in the element than would occur at another height. Now, to get
to the point- just like the lower angles are reflected from earth at a far
distance, it makes sense then that the high angles would reflect from the
earth closer and closer to the actual antenna, therefore, incresing the
conductivity of the earth directly underneath will do more to suppress very
high angles, and make the nulls a little deeper than over real "dirt".
To say that a ground screen truly has "NO" effect is false. It is true,
however, that it does nothing for the angles that the radio amateur is
interested in.
Hope that clears it up,
73 es CUL
Shawn
VE6PV
lightfol@cadvision.com
hmmmmm....where does this door GOOOOOoooooooooooooooo.......
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