Michael Tope wrote:
>  Original Message 
> From: "Dan Zimmerman N3OX" <n3ox@n3ox.net>
> To: <towertalk@contesting.com>
> Sent: Friday, October 19, 2007 9:47 PM
> Subject: Re: [TowerTalk] Top loading HF6V for 160
>
>
>> "Somewhere I have the modeling data which shows a
>> modest improvement in radiation resistance when the tophat wire is
>> horizontal (as compared to sloping), but it's not a make or break
>> difference.
>> "
>>
>> It's make or break for a 34 foot vertical on 160 I think, based on
>> another look at the model.
>>
>> 34 foot vertical with 4 straight top loading wires 60 feet long,
>> resonant 1.75 MHz, 5.6 ohms.
>>
>> 34 foot vertical with 4 60 foot sloped down 30deg is resonant around
>> 1.8MHz, 1.4 ohms, which is about the same as the radiation resistance
>> of the base loaded version.
>>
>> There's probably still a benefit over a very good ground system when
>> you consider the low loss nature of the cap hat wires vs. a coil of a
>> Q of a few hundred, which is going to have a loss resistance of a few
>> ohms (you need about +j920 to resonate)
>>
>> Dan
>
> Yes, I get similar results, Dan. The 60ft top hat wires sloping down at
> 30 degrees (relative to horizontal) are only about 1dB better than the
> base loaded 34' vertical assuming that Q=200 and a ground resistance
> of 10 ohms. The antenna with 60ft top hat wires perfectly horizontal is
> about 5dB better than the base loaded antenna with no tophat wires.
> If you shorten the top hat wires to 25 feet with the same 30 degree
> slope and center load the antenna (20 foot level) with 26uH, you end
> up only about 1.5 dB down from the case with 60ft horizontal top hat
> wires.
>
> These results are in line with my suspicion that for sloping tophat
> wires, there is some optimum wire length that produces an optimum
> balance between toploading, radiation cancellation, and inductor
> loss. The next step would be to come up with an optimizer whereby
> you put in the physical constraints (inductor Q, wire slope angle,
> radiator height, ground loss resistance) and then let the optimizer spit
> out an optimum position for the center loading inductor and a optimum
> length for the top hat wires.
>
4nec2 can probably do the optimization. If you can write algebraic
equations for the parameters in a NEC description in terms of a few
symbols, it can optimize..
For instance, I've done a bunch of models where I was looking at things
like inverted Vs, where I constrained the height of the center, the
height of the two end supports, but allowed the spacing of the end
supports to vary (effectively putting more or less nonconductive string
inbetween the end of the V and the support).
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