[TowerTalk] Stacking Tribanders

[Billy Cox]

Hi Paul,

Below is from pages 12 and 13 of Dean's document, perhaps
this might help to explain what you have observed there?

ACCURACY AND TESTING THE RESULTS

What would I estimate as the “accuracy” of HFTA elevation predictions? I would say that I would trust the results within plus/minus 3 dB. In other words, take HFTA results with a grain of salt. Don’t obsess with changing the height of your antenna by fractions of a foot to see what happens! 

Having said that, now I must state that it is a good idea to compare elevation patterns in intervals of perhaps 1 foot to assess whether HFTA
is generating reasonably smooth results. Often, the ¼steps used in the program don’t align exactly and artificial spikes (or holes) can be created. This is inherent in any ray-tracing program and can only be eliminated by using extremely small angular step increments —and doing so would slow down execution even more. 

After I do an evaluation for a particular antenna height, I will often specify an overlay of three heights separated by one foot each. For example, if you are interested in a single antenna at a height of 80 feet on 14.0 MHz for the K5MA-330.PRO terrain, you might first compare three heights of 79, 80 and 81 feet, bracketing that height. The three curves overlaid on each other look relatively smooth, except there is a 1.4-dB “bump” for the 79-foot height. 

Now, run three heights of 80, 79 and 78 feet. Now, the curves for 78 and 79 feet look smooth, but the 80-foot curve has a noticeable dip. This means that spurious artifacts of the ray-tracing process are occurring at 80 feet in the program —but these would not occur in the real world. The solution: don’t use the 80 foot point in the computer analysis, but you would mount your real antenna at that 80-foot height if you like the response at 79 or 81 feet.

Hope this help!

73, Billy AA4NU

> On 06/22/2022 1:09 PM Paul Christensen <w9ac at arrl.net> wrote:
> 
>  
> >"It's an interesting question."
> 
> And a good one for Dean Straw to answer.  When we conduct an HFTA analysis of a single antenna that results in extremely high ground reflection gain between 0 and 5 degrees elevation, minor changes in height, even as little as 12 inches, result in a significant reflection gain changes at the horizon.  So, I have to believe that a much more complex stacking model is required when using HFTA to get an accurate result.
> 
> Paul, W9AC   
> 
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