[TowerTalk] More on Half Wave (and HFTA)

[jimlux]

On 1/19/19 11:08 PM, David Gilbert wrote:
> "Certainly the topography of adjacent radials affects the true 
> performance on one radial that won't show up in the HFTA model. No?"
> 
> I'm pretty certain that N6BV himself has pointed that out as a 
> limitation of HFTA.  It's a complicated world.

HFTA assumes that (for each direction) the profile is infinite in extent 
in the "cross range" direction.
It's a 2D ray tracer with N slices in aziumth.

It's sort of like how most folks do ionospheric models (e.g. VOACAP) and 
assume there's no cross range propagation, although there actually is.

One can do a full 3d model, but it's computationally expensive ( a lot 
more than just running radial at finer steps), and it's unclear if it 
would be useful for antenna planning.





> 
> 73,
> Dave   AB7E
> 
> 
> On 1/19/2019 8:33 PM, Paul Christensen wrote:
>>> "There are folks who claim that a few thousand meters or so is all 
>>> the further anyone should have to collected data points, but that's a 
>>> garbage generalization.  I have
>> another mountain range about 15 miles east of me and by editing the 
>> terrain file to either include it or not I can see definite differences."
>>
>> And that brings me to this point:  consider a case where a radial is 
>> analyzed in HFTA and extends to 14,000 ft.  At that distance, each 1 
>> deg radial is approx. 244 ft apart.
>>
>>  From what I can determine, HFTA is calculating the radiation profile 
>> from data on only one radial.  But since terrain often changes on 
>> adjacent radials, sometimes dramatically,  then doesn’t the model 
>> really need data that's spliced and factored from adjacent radials to 
>> get a true picture of the profile?  If the antenna beamwidth is 
>> say...40 degrees, then don’t we really need a more comprehensive 
>> analysis of radials within the swath just to determine the profile of 
>> one radial?
>>
>> Performance as actually measured on one radial could be undermined (or 
>> enhanced) by adverse terrain characteristics from adjacent radials.  
>> Certainly the topography of adjacent radials affects the true 
>> performance on one radial that won't show up in the HFTA model.  No?
>>
>> Paul, W9AC
>>
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