Michael Tope wrote:
>
>  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...........
4nec2 can do this without too much trouble. One of the options is to
optimize for minimum mag(X) (i.e. drive to resonance, irrespective of
mag(Z))
OK if the inductor is modeled as a fixed physical length? Say, 3060 cm?
(That way, I can just make a segment that has inductive/resistive
loading with R = 0.005 X)
I assume something like a 12cm radius for the vertical mast and 1 mm
diameter for the top wires would work.
>
>
>
>
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