Hi Don,
> To work correctly, these rules need to be followed:
> A. Put on a metal tower with an HF size beam on top to serve as a
> "capacity hat".
That's because the tower is the "other half" of the antenna. Exactly
the same current leaves the feedline and flows into the tower as
flows out into the sloper antenna. If the tower does not present a
low impedance at the point of connection, current in the sloper will
drop.
A large yagi antenna(s) near the sloper will act as a counterpoise,
and keep the tower (and feedline, since for all practical purposes it
is part of the tower) below the sloper from carrying as much
current.
> B. Decouple the coax at the feedpoint with an RF choke (eg. 8
> turns, 8
> inches dia.)
What is the point in doing this, when the tower and shield are
directly connected??
A sloper is nothing more than a Marconi antenna with the tower
used as a counterpoise. The performance varies wildly with the
tower height and what is on the tower, because the tower is the
"other half" of the antenna.
The same amount of current flows into the tower as flows out into
the sloper.
While I agree with much of what you say, you've greatly undersold
verticals or inverted L's.
"Poorly working" Inverted L's or verticals almost always use small
ground systems. One, two, three or even four elevated radials will
NOT make a consistently good ground, and factually can never be
better than even a modest sized "real" ground system.
It is the "lazy-man's idea" of a good ground that causes efficiency
problem with most verticals.
While I certainly agree some slopers can work well (whatever "well"
means) they are very dependent on the tower height and loading
(what is mounted on the tower). If the tower is electrically short,
then considerable current will flow from the tower into the earth
causing ground losses. You are back to needing a ground system,
and might as well shunt feed the tower.
We often keep hoping, but there is no free lunch or magic. The
best we can hope for is an occasional cheap meal and blind luck.
73, Tom W8JI
w8ji@contesting.com
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