[TowerTalk] "experts" on loading towers on low bands

[David Gilbert]

Very well said, Jim.

73,
Dave   AB7E


On 9/11/2019 3:37 PM, Jim Brown wrote:
> On 9/10/2019 4:56 PM, Bob Shohet, KQ2M wrote:
>> Sure I did my homework ahead of time but my inclination was to build 
>> it and put it up anyway just to see how it performed.  I felt better 
>> about it after I modeled it even though I was using software which 
>> was not designed to provide a perfect representation of my qth with 
>> stacking heights over my wildly varying topography in each 
>> direction.   And while the antenna on 15 and stacking combos kicked 
>> butt at 109’, a similar stacking proposition for 10 meters with an 
>> antenna at 65’ stacked with others at 100’ and lower heights did NOT 
>> work as the model predicted.  In fact the 10 meter antenna at 65’ 
>> performed poorly in a stack with all other antenna heights and by 
>> itself in every direction BUT towards Europe.
>>
>> So “put it up” and try it had great results on 15 and mixed results 
>> on 10, whereas the model which predicted a great stacking pattern in 
>> all directions for both antennas on 10 and 15 also had mixed 
>> results.  Ultimately I removed  the sidemount @ 65’ for 10 – it was 
>> useless to rotate the antenna at that height for stacking purposes at 
>> my qth – it was never better than another antenna combo.  So, no 
>> model is foolproof.
>
> Not quite. This post, and the logic that goes with it, is a great 
> example of several important concepts/principles.
>
> 1) Nearly all real world problems are complex, whether technical, 
> personal, financial, legal, or political. Wise people know this, and 
> will reject input that tries to make things too simple.
>
> 2) The application of well known fundamental principles to solve a 
> problem will only be as good as the knowledge of, and the modeling of, 
> ALL of the variables.
>
> 3) Anyone who thinks that "there's a difference between theory and 
> practice" doesn't know enough about one or the other or both.
>
> 4) Nearly all commonly used design equations/formulas are simplified 
> versions of the full equation. The simplifications are based on 
> certain assumptions, and if the assumptions are satisfied, the formula 
> will give a good result.
>
> Example: Commonly used equations for Zo and VF of transmission lines 
> are simplified, based on the assumption that F is a high number, 
> leaving F out of the formula, yielding the same value for all 
> frequencies. But Zo and VF are NOT constant with frequency, and become 
> increasingly variable at low frequencies. Indeed, VF varies enough 
> that stubs for 40M and below cut using the simplified formula will 
> 1-2% off frequency.
>
> Example: NEC assumes flat earth and uniform ground characteristics, 
> and uses ground characteristics in the model. If the ground is not 
> flat, or if it is not uniform, or even if its characteristics are not 
> known, or are not entered correctly, NEC will yield some error.
>
> Example: Most hams have conductive elements within their direct field 
> that can act as parasitic elements of an antenna. Coax from horizontal 
> dipoles, towers, trees, even wiring in nearby homes can interact with 
> verticals. If these conductive elements are not known or not 
> considered, NEC will yield some error. I've posted here several times 
> the interaction between my tall tower and 160 verticals, and that I 
> add chokes along the coax from high dipoles to avoid interaction.
>
> Example: HFTA does NOT model interaction of stacked antennas, it only 
> sums their lobes, assuming that each is a dipole! When a given antenna 
> is selected (dipole, x-element Yagi) HFTA simply increases the gain. 
> And the result obtained from the model will only be as good as the 
> data entered. My terrain is quite irregular -- I don't know how good 
> the data is nor how closely spaced, and there are limits to the number 
> of data points.
>
> Example: HFTA can yield errors with some irregular terrain. N6BV has 
> always recommended running calcs at multiple closely different heights 
> and azimuths to expose these errors. When I used it to site my 
> antennas I ran in 10 ft increments to find "sweet spots," then +/- 5 
> ft. Likewise for azimuths -- I modeled in 5 degree increments to major 
> directions.
>
> Example: HFTA propagation data is statistical, over long periods of 
> time, so includes wide variations over the solar cycle(s). AND data 
> for some bands is interpolated from data for adjacent bands. I think I 
> recall Dean telling me that 160M data is interpolated.
>
> Example: The combined pattern of antennas in a stack will depend 
> entirely on the phase response of the combination, including ground 
> effects. The phase response of an antenna like a Yagi will vary not 
> only by brand name or generic design, but by every element of the 
> design, and it varies with angle in both the H and V plane. If we want 
> to model the behavior of a stack on something as simple as flat earth, 
> we must enter an accurate detailed model into our design software, and 
> the model must include accurate details of all matching elements, 
> phasing lines, and so on!
>
> 73, Jim K9YC
>
>
>
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