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

[Jim Brown]

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