[Towertalk] Top loading an HF2V

Pete Goudreau goudpj@mac.com
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 
difference either.

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)