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Re: [TowerTalk] HFTA Disc....

To: towertalk@contesting.com
Subject: Re: [TowerTalk] HFTA Disc....
From: David Gilbert <xdavid@cis-broadband.com>
Date: Sat, 16 Jul 2016 20:42:40 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>

HFTA has a feature allowing you to actually plot your terrain file, and maybe that is indeed how you compared the two. I'd bet that there is a difference in the "choppiness" of the two plots. HFTA calculates dispersion and refraction of the various rays, as well as the secondary dispersion and reflection of those dispersed/refracted rays. The combinations can get pretty intricate. Sharper edges in the plots would give different results than smoother transitions ... perhaps significantly so if the transitions are appreciably sharp relative to a wavelength.

73,
Dave AB7E


On 7/16/2016 6:38 PM, N3AE wrote:

As an experiment this afternoon, I took a 30 meter resolution DEM data file and truncated 
it beyond 1470 meters so I could compare the results with 149 pairs of 10 meter 
resolution data that ended at 1490 meters from the tower. Once again, the two elevation 
profile plots were essentially on top of each other, differing by less than 2 ft at any 
range. But the takeoff angle plots still differed quite a bit. Not sure what's going on 
inside HFTA that causes this "butterfly effect" sensitivity to differences in 
terrain elevations, at least in the elevation profiles for my QTH.

Shawn
N3AE


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