[TowerTalk] The Value of HFTA

[Bob K6UJ]

 Very good explanation Bob !

73,BobK6UJ




    On Sunday, January 20, 2019, 12:18:28 PM PST, Bob Shohet, KQ2M <kq2m at kq2m.com> wrote:  
 
 Speaking now about HFTA and K6STI’s YO programs, I won’t go into the shortcomings of HFTA but rather how they were BOTH useful to me.

HFTA was very useful for plotting the relative time of signals arriving at each wave angle for a given path and for a given qth and reference antenna.  This was expressed in a bar chart and a line above the chart which made it very easy to see and understand what arrival angles were most important for Europe, JA, etc. and then to see a plot of your reference antenna and qth below that.  It clearly pointed out where you might want to make changes in your antennas and stacking heights to better capture those arrival angles that had the tallest bars.

K6STI’s YO program allowed very specific tailoring of a given SPECIFIC antenna (HG205CA vs a 5L homebrew design) and specific stacking heights of that specific antenna over a very detailed distance from tower and height above sea level.  The more specific and detailed the heights/distances from the tower, the more accurate the pattern of the lobes produced  in the chart.  It was (and still is!) INVALUABLE!

It allowed me to find “magic heights” much lower down on the tower to place sidemounts to capture almost the same low angle paths as with the top most antennas.  Example:  By extensive modeling of the terrain at my qth, I was able to find that 37’ was a GREAT all around height on 10M – so useful that to the NW and W, the antenna at 37’ was almost always as good to JA and VK as the same 5L yagi @ 100’.  The plot showed that to be the cast and 20 years operating with it at that height also confirmed it!

Not only would 37’ be completely counter-intuitive as an almost optimum height for the JA/VK path on 10 compared to almost anything higher up, but by seeing the overall utility of that height, I realized that I should put a sidemount there and then I would be able to stack the 37’ antenna with the same antenna at different heights for additional stacking gain and flexibility.

In contrast, I discovered that a sidemount at 65’ was useless.  While the 65’ was good to EU and for stacking for a brief time, it was useless in any other direction.  After modeling it over my terrain, the plots confirmed that 65’ was almost like a null in almost every direction.  I removed the sidemount and rotator after that.  Had I modeled it first BEFORE putting it up, I could have saved myself quite a bit of wasted time and effort and expense.  I would not have seen that with HFTA, but thankfully I was using YO by K6STI where it was clear as day.  The main difference was my SPECIFIC terrain – which created nulls and enhancement in certain directions at specific heights which could not have been predicted without modeling.

Jim’s example is quite instructive.  There is no end to the value of good modeling programs and learning how to use them properly, especially those that allow you to model a specific antenna and/or stack of antennas over a specific terrain at a given qth.  $100 and a few hours spent to learn how to use a modeling program, can repay itself with a 100x or greater pay back in dollars and time saved by being able to avoid a less effective and/or more costly design/height.  There are few things more fun than enjoying optimal performance of the right antenna(s) at the best height(s) at your station AND also knowing that at the same time you avoided a costly mistake!

73


Bob KQ2M


From: Jim Brown 
Sent: Sunday, January 20, 2019 1:46 PM
To: towertalk at contesting.com 
Subject: [TowerTalk] The Value of HFTA

On 1/20/2019 10:23 AM, Wes wrote:
> Personally, I don't understand all of the fuss about HFTA.  In the 
> general case (sans 200' towers), aren't you pretty much stuck with 
> your environment with little to do about it?

My own station has complex topography, and Dean suggested that I take 
data out to 14 miles. I ran a LOT of radials at a lot of heights, and 
got pretty consistent results. They showed that the 120 ft tower I could 
build for my SteppIR was about as good as I could do to get over ridges 
to the north and east unless I wanted to add another 200 ft or so. :)  I 
consider that very useful and practical information!

73, Jim K9YC

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