[Towertalk] Top loading an HF2V
Mon, 8 Jul 2002 22:04:34 -0500
Being on a city lot, not much bigger than 60'x120', I plan to install
an HF2V ground mounted in the middle of the back yard, unfortunately
with an unavoidably limited radial field. But the soil is good here
in the DFW area so maybe the far field won't be so terribly bad and
I'll just have to suffer with whatever efficiency I can manage from
those losses in the induction field, I guess. Location is
32deg51'38"N by 96deg34'55"W if that helps.
What I'm concerned with is maximizing the radiation resistance so as
to maximize efficiency. Since I'll be using a tuner, I don't care
much if the antenna is too long on 40m but not so long as to exceed
the range of the AT-11MP tuner.
Anyway, the top loading kit for this antenna attaches eight feet down
from the tip of the antenna but wouldn't this shield the "whip"
formed by this top section and minimize its contribution to the far
field strength? It seems like a top hat that would move the current
up as far as possible would be better, I think. Not sure if the
following idea will work but if anyone has any comments, I'd be glad
for any help.
Take four lengths of FlexWeave antenna wire, about 13' long, and
clamp them all to the tip of the HF2V and then again eight foot down
from the tip. Set a thimble around each of these wires in turn and
using Dacron antenna rope (3/32" OD), make "guys" by tying off each
of four ropes around each of four thimbles. Run down to the ground
at an angle from the horizontal of about 35deg, each of the four
wires clamped to the antenna would then become the hypotenuse and
base of a right triangle under tension from their associated guy
rope. The vertical component of each right triangle would be the
uppermost eight foot section of the antenna itself. The resultant
"wireframe" shape would be that of right circular cone.
Haven't figured out a way to make a spreader of some kind at the tip
so the "wireframe" shape could be made a cylinder but it seems
possible. Not really certain either if that would make all that much
Regardless, it seems like a long way to go as opposed to the mfg's
top loading method but it also seems like it would move the radiating
current further up from the ground. But what I can't figure out is
whether it would be enough capacitance to eliminate the inductor in
the base loading that couples the LC "trap" at the bottom to the
upper section of the antenna and if not, just how much wire would be
necessary to do so. And I'm not certain if this can be modeled by
any of the lower cost programs or if any of these programs and then
give an idea of what the feedpoint Z of the antenna would be at 30m.
Building it and testing it with a CIA-HF just might be easier I
suppose but this sounds like it might be interesting to investigate
first to get a feel for whether it's worth the effort. Again, thanks
for any help anyone may bring to the problem and thanks for listening.
Pete, AD5HD (ex-WB5RKC)