[TowerTalk] The Value of HFTA

[Bob Shohet, KQ2M]

There is another limitation as well....

While K6STI’s YO program allowed you to select from a list of “stock” antennas, or put in the actual measurements of your own, and THEN stack them, over ACTUAL terrain, 
HFTA does not allow you to specify an exact antenna – only a “generic” one.

That might not sound significant until you realize (by doing the actual modeling) that the “best” heights for a given wave angle on a particular band are actually DIFFERENT for different antennas.  And when you stack a pair or three of the same DIFFERENT antenna, the best heights for a given wave angle change even more and they have DIFFERENT lobes and patterns as the terrain changes from flat to something else.

My stack of Hygain HG105CA’s or HG155CA’s or HG205CA’s produces different patterns and lobes at different angles than a different 5L beam would.  Sometimes the differences are LARGE at a give wave angle.  Certainly this is different than one would expect without modeling them.  HFTA will not differentiate between different antennas – so what you see for lobes may be incorrect – better or worse.

Likewise, K6STI’s fabulous program did not have a graph or chart that showed the % of signal at a given wave angle.

So what to do?  The answer of course was simple – USE BOTH programs -  HFTA and K6STI’s suite of programs!  Each for their best and most useful attributes.

I learned a lot more and developed a better 3D understanding of different antennas, transmit and receive wave angles and the effects of stacking by using BOTH programs.  And by changing the direction of the Terrain file and replotting the actual heights about sea level in each direction for each antenna and stacking combination, I learned how even the smallest terrain changes could have ENORMOUS impacts on the best stacking heights and how the best stacking heights changed yet again depending on the SPECIFIC antenna that I was plotting.  Only through modeling and using BOTH programs did I discover all of these valuable and unusual characteristics and information that were essential to maximizing my 10, 15 and 20 meter signals.

The modeling programs are remarkably accurate and they have had profound influences on my level of understanding and the quality of my signal.  For me, they have been the most effective station additions that I could possibly buy!

For those of you who are still skeptical, I can say with complete confidence that there are virtually NO big contest or DX stations that have been built in the last several decades without extensive modeling of antennas and/or terrain for their qth.  It’s one of the reasons why they are LOUD!   

73

Bob  KQ2M



From: jimlux 
Sent: Monday, January 21, 2019 6:20 PM
To: towertalk at contesting.com 
Subject: Re: [TowerTalk] The Value of HFTA

On 1/21/19 11:33 AM, Jim Brown wrote:
> On 1/21/2019 11:23 AM, Edward Sawyer wrote:
>> One thing that HFTA does NOT do well is answer the question about 
>> stacking
>> from a "max gain" perspective.  EZNEC will tell you the best design for
>> stacking yagis for max gain in a direction however HFTA will tell you 
>> what
>> the right height for those yagis are or if the stack is even the 
>> strongest
>> signal under typical approach angles.
> 
> Right. HFTA models the performance of individual dipoles used alone, not 
> in combination. When you select a Yagi, it simply adds gain.
> 

Correct me if I'm wrong, but I think it also changes the effective 
vertical beamwidth of the point source radiator (in proportion to gain 
or sqrt(gain) or some rule).  that is a 2.15 dBi dipole has a vertical 
pattern that is uniform gain, but a 10 dBi Yagi has a vertical pattern 
with some significantly smaller vertical height.