[TowerTalk] Top loading HF-6V for 160

[Jim Lux]

Michael Tope wrote:
> ----- Original Message ----- 
> From: "Dan Zimmerman N3OX" <n3ox at n3ox.net>
> To: <towertalk at contesting.com>
> Sent: Friday, October 19, 2007 9:47 PM
> Subject: Re: [TowerTalk] Top loading HF-6V for 160
> 
> 
>> "Somewhere I have the modeling data which shows a
>> modest improvement in radiation resistance when the top-hat 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 top-hat 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 top-hat
> wires, there is some optimum wire length that produces an optimum 
> balance between top-loading, 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 non-conductive string 
inbetween the end of the V and the support).