K8GG wrote:
> A 3/8 wave inverted L is much much better than a GAP, but not as good as a
> 5/16 wave inverted L or shunt fed tower.
>
> The GAP is not top-loaded and therefore has a poor radiation efficiency.
Each Marconi's vertical antenna works against ground and if the ground is
bad its efficiency drops.
Short antennas suffers much more about this problem as Rr lowers and losses
require to be minimized, but the same formula totally apply also to full
size radiators.
A well designed short radiator, against an optimal ground plane, can
outperform a full size quarter wave radiator against a fair ground plane or
even a 1/2 wave tall radiator that 's mistakenly considered free of ground
plane needs.
A short radiator whose ohmic losses are considerably smaller than its Rr,
against an optimal ground plane is not so far from the efficiency of a
classic quarter wave vertical against an optimal ground plane.
The key point to make a vertical working is not in the shape or going beyond
a stated lenght but in keeping high the current flowing in the antenna,
keeping high the current in the vertical section, maximally at the base.
As much as the maximum current is infact raised along the vertical section,
the minimum radiation angle is cancelled (because of brewster diffraction)
and, if this has a minor impact on 160m (unless for ground propagation) a
raised current point shifts the current density return path away from the
antenna base and,
to reduce losses the radials have to be more and longer than they could.
Top loading is surely the best possible form of loading for short radiators
but alone it does not always fix all the efficiency problems.
73,
Mauri I4JMY
> A 3/8 wave inverted L is much much better than a GAP, but not as good as a
> 5/16 wave inverted L or shunt fed tower.
>
> The GAP is not top-loaded and therefore has a poor radiation effenciency.
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