 Original Message 
From: "Jim Lux" <jimlux@earthlink.net>
>
> Something along those lines...
>
> I thought about this a bit yesterday. 4nec2 can alter physical
> parameters (including the value of the loading inductor), but I'm not
> sure how to set it up to get what you want in one step.
>
> You've got a center loading inductor (or, more properly a middle loading
> inductor), but do you want the feedpoint impedance to be 50+j0, or, are
> you willing to let the input Z vary, and all you want to do is optimize
> the radiation efficiency?
I think you'll also want to optimize for minimum feedpoint reactance in
order to force the loading inductor to a realistic value. Otherwise,
depending on how sensitive your excitation source is to mismatch, the
optimizer may try to zero out the inductance to maximize efficiency.
.
> SO your optimization parameters would be the length of the top hat
> wires, and the angle you take them off at, the position of the center
> loading inductor, and its inductance. You'd optimize for field strength
> (for a constant excitation power) at some low elevation angle, I assume?
>
> You could run several optimizations with different tophat topologies
> (3,4,5 wires, whether or not they're spiderwebbed, etc.)
>
> This is an interesting optimization problem.. clarify what you're
> looking for and I'll grind on it a bit more.
For the portable operating scenario where you only have one support,
but lots of real estate, you could probably assume a fixed tophat angle
(say 60 degrees relative to the vertical). The vertical height would be
fixed as well. Too keep it simple, I would use a fixed number of tophat
wires (say 4). To summarize:
Fixed Parameters:
1) Radiator Height (34ft for the HF2V case)
2) TopHat Angle (60 degrees relative to vertical seems like a practical
number)
3) Ground Loss Resistance [I suggest running two separate cases 
very good ground ground system (2 ohms) and modest ground
system (10 ohms)]
4) Inductor Q (200 is a practical number)
Variable Parameters:
1) Inductor Position (0% = base load; 90% = close to tophat)
2) Inductor Value
3) TopHat Wire Length
Optimization Goals:
1) Minimize Feedpoint Reactance (this will force the inductor to a value
that will resonate the system). Don't worry about feedpoint resistance.
2) Maximize Low Angle Radiation (the angle of peak radiation will be
a function of the ground parameters and can easily be determined
with a few trial runs)
This is essentially what I was doing manually with EZNEC/Multinec
(very tedious). There may be a way to use Excel/Multinec to goal seek,
but I don't have the time right now to mess with it.
If you can get this to work, it would be worth writing up as it would be
a useful tool for folks comtemplating short lowband antennas.
73, Mike...........
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