>
> would there be any advantage or even possible to use an umbrella load
>
less than at the top
> of the vert ?
The top loading tends to "win" in this case, and the part above the umbrella
doesn't do much radiating. The entire immediate vicinity of the whip above
the umbrella hat becomes pretty much at the same potential as its base, and
so there's not much current driven into it.
Whether or not there's an advantage really depends on how low you have to go
and how much you have to slope the wires down. This is what I found here.
If I remember correctly in my installation, there'd be a real advantage in
top loading a 60 footer at the tip, but I can't do that, I'd have to do it
at 50 feet. Then there'd be some worthwhile (to me) advantage up to a few
dB if I could go straight out (or even out and UP, like if I worked it out
with the neighbors next door to use their tree) with the top hat wires, but
even doing an inverted L sloping down from 50 to 40 feet (the tip of my
other Spiderbeam pole) would net me at most 2dB, probably more like 1.5dB.
Since I'm a low power station as of right now, I'd gain a lot more from just
adding an amplifier ;-)
There is some breakpoint between making it worthwhile to umbrella-hat your
vertical at a lower height and just leaving it a straight vertical of the
original height, but I think it depends a lot on the details of the
installation, and everything I just said comes from models which are not
very reliable (I don't really know my ground loss)
But I'd consider everything above the umbrella to basically get chopped off,
in the simplest terms.
73,
Dan
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