> Thomas KN4LF Says:
>
> The higher in elevation that you pull the highest current
> point above the grounds surface, the lower the capacitive
> coupling losses are and the fewer radials that you need.
While that may help somewhat, it is not a cure-all. WWVH had
to put radial fields under their VERTICAL DIPOLES (bottom ends
a foot or two above ground) because of the ground losses from
coupling high e-field end of the dipole - even though the maximum
current was 1/4 wave in the air. A "ground plane" with it's base
1/4 wave height would have been more efficient than the vertical
dipole without the ground system simply because the high e-field
would have been much farther away from the "lossy" earth.
Broadcast towers are generally MORE that 1/4 wave tall - I used
to have the rules by my desk - the required height approaches
.64 wave on the high end of the band. Even electrically "tall"
towers were required to use the full 120 radial ground system.
The real benefit of raising the maximum current is to increase
the total current in the vertical element. If the maximum
current is at the ground (or at the junction with the top
loading), the effective current (or current causing radiation)
is the integral of half the cosine function from zero to the
height of the vertical element in degrees. If the maximum
current in placed IN THE MIDDLE of the vertical element the
effective current is the integral of the cosine from -1/2
the electrical height to + 1/2 the electrical height. Simple
calculus will tell you that there is far more radiation in
the latter case than the former case.
The "integral of the current" is the principle behind top
loading in general - you always want the "fat part" of the
current distribution in the vertical element.
73,
... Joe, W4TV
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